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AN INTRODUCTION TO ANESTHESIA

This document is intended to provide information to an international audience outside of the US.


1 PREFACE

This booklet is designed as a basic introduction to the field of anesthesia for a variety ofinterested professional groups. These include healthcare and auxiliary staff, technicians,salespeople, service engineers, cleaners, instructors, and anyone else wishing to learn aboutthe fundamentals of anesthesiology. It is partly based on previous Maquet training materialthat has been revised and updated.

The information is intended to be simple and accessible to many different user groups andincludes diagrams, tables and graphs to illustrate the text and provide a better overview. Thepresentation is deliberately straightforward and readers interested in the more advancedaspects of anesthesia should refer to specialized textbooks, some of which are listed under"References".

No previous knowledge of anesthesia is required to understand this booklet, which is designedto serve as a useful key to a field that may sometimes appear bewildering to outsiders. Somefamiliarity with clinical concepts and environments is however an advantage since medicalterms are often used in the descriptions. As in many other specialized areas, the use of medicaljargon and abbreviations may confuse newcomers. A conscious effort has therefore beenmade to avoid complex terminology and "insider" shorthand. Readers will also find manyspecialized words and acronyms explained in the "Glossary" at the end of this booklet. Theseterms are highlighted in bold the first time they appear in the text.

After two brief introductory chapters, the next five chapters focus on what happens to thepatient during anesthesia. The perspective is more technical in remaining chapters where thefocus is on the machinery and equipment used. Optimal patient care requires familiarity withall these aspects, although different professional categories may wish to concentrate on theareas most relevant to their interests.

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2 ANESTHESIA - A BRANCH OF CRITICAL CARE MEDICINE

The specialized field of anesthesiology focuseson keeping patients (who may be seriously illor traumatized) stable and pain-free duringsurgery - a form of continuous intensive care.Traditionally, anesthesiologists have been atthe forefront of critical care medicine. Theirwork thus involves much more than simplyputting patients to sleep.

There is a considerable lack of consensuswhen it comes to how to refer to the variouspersonnel groups involved in anesthesia. Theproblem is that both anesthesia and intensivecare services are organized differently indifferent countries. So while it is theanesthetist/anesthesiologist who handles allpatient care in some countries, in others thenurse anesthetist may play a central role inuncomplicated anesthesia procedures. Toavoid confusion, the terms "anesthetist" and"anesthesiologist" are used somewhat looselyand interchangeably in the text below to referto the person in charge of the anesthesiaprocedure, whatever that person'sprofessional title, gender or role.

Anesthesia is generally associated with thehospital environment, particularly the operatingroom. It is however useful to remember thatanesthesia is nowadays also used in theradiology department and the emergencyroom, as well as during childbirth or MRIexaminations, for outpatients and on hospitalwards. Meanwhile, hospitalization times arealso decreasing steadily all the time. Inaddition, anesthesia is used for dental surgery,in cosmetic surgery clinics and psychiatricunits, as well as on the sites of accidents. The

OR, however, remains the most importantenvironment for the anesthesiologist and somefamiliarity with its layout is thereforenecessary.

2.1 SURGICAL WORKPLACES

Surgical workplaces will of course differ inlayout from one hospital to the next,depending on the specific uses to which theyare put. Some knowledge of concepts andfunctions routinely used in most suchworkplaces is however required.

Surgical workplaces can roughly be dividedinto three areas depending on the level ofcleanliness:

A general access area that is open to allpersonnel groups and often includes acentral reception desk or nursing station,along with a waiting area. Ordinary clothesare permitted here.

A "clean" area with more restrictive dress,requiring special clothes, includingheadgear, not worn outside the area.Routines nowadays generally also requirethe removal of all jewelry and watches,since they are known to be a potentialsource of infection. The area includescorridors and storage areas, as well as theanesthesiologist's workplace in the OR,where a surgical mask is generallyrecommended.

A "sterile" area that includes a large part ofthe operating room and its contents, inwhich a surgical mask must always be

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worn. Surgeons and their assistants will alsoput on sterile robes ("scrubs") and glovesafter the routine scrubbing-up anddisinfection procedures.

Individual operating rooms or theaters mayalso vary in design and layout depending onpracticalities and preferences.

The main components will however berecognizable almost anywhere and willgenerally include the following:

an operating table for the patient

an anesthesia machine

a stand or cart with work surface foranesthetic equipment and drugs

a pole for intravenous fluids

a suction device for removing mucus fromthe airways and clearing the stomach

surveillance monitors

stands for surgical instruments

a stand for compresses, etc.

a stand for surgical samples and specimens

a surgical suction device

a diathermy or surgical cautery unit

TV monitor/X-ray equipment

PC/laptop.

As is obvious from this list and the illustrationbelow, the operating room is often verycrowded and freedom of movement may beextremely restricted, particularly for the

anesthetist. Hospitals are therefore always onthe lookout for equipment that will save spaceas well as money. It should also be easy tohandle and ergonomically designed.

2.2 HISTORICAL OUTLINE

The following is a brief historical overview ofthe last two centuries of developments in theworld of anesthesia. It should of course beremembered that efforts to reduce and controlpain naturally predate this summarized outline.Examples include the use in many ancientcivilizations of methods such as acupuncture,and plant or herbal extracts such asmandragora, to relieve pain or induce sleep.

1799 - The analgesic effects of nitrous oxidewere discovered.

1818 - The anesthetic effect of ether vaporwas discovered.

1831 - Chloroform was discovered.

1842 - Dr C. W. Long first used diethyl etherfor surgical anesthesia.

1844 - Dr H. Wells used nitrous oxide fordental analgesia.

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1846 - Dr W.T. Morton made the first publicuse of ether for anesthesia.

1853 - Dr J. Snow used chloroform onQueen Victoria during the birth of PrinceLeopold.

1884 - Cocaine was first used for localanalgesia.

1917 - The face mask was introduced.

1920 - Tracheal tubes were introduced byMagill to deliver inhaled anesthetics.

1920 - Guedel published data on signs anddepths of ether anesthesia.

1930 - The circle system and CO2 absorber

were described by Dr Sword.

1934 - Thiopenthal induction was first usedby Dr Lundy.

1938 - A positive pressure respirator wasused during surgery.

1942 - Tubocurarine was used for surgicalmuscle relaxation.

1949 - Succinylcholine was used for musclerelaxation.

1956 - Halothane was used clinically byJohnson.

1960 - Xenon was introduced foranesthesia.

1972 - Enflurane was introduced in clinicaluse.

1981 - Isoflurane was used clinically.

1985 - Mallampati suggested a specialmethod for assessing intubation difficulty.

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1986 - The American Society ofAnesthesiologists, ASA, approved the firststandards of basic anesthesia monitoring.

1989 - Propofol came into clinical use.

1990 - Pulse oximetry was added to ASAstandards.

1992 - Desflurane was introduced in clinicaluse.

1994 - Sevoflurane came into clinical use.

1996 - etCO2 (end tidal carbon dioxide)

monitoring was added to ASA standards,complementing existing requirementsconcerning oxygen monitoring.

2005 - New ASA Standards for BasicAnesthetic Monitoring were published inthe same year in which ASA also celebrated100 years as a professional organization foranesthesiologists.

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3 ANESTHESIA - A BRIEF OVERVIEW

3.1 BASIC INFORMATION

The word anesthesia comes from a Greekterm meaning "without feeling", implying thatthe patient is not sensitive to external stimulisuch as pain. General anesthesia means thatthe whole body is anesthetized and involvessleep or unconsciousness (hypnosis),painlessness (analgesia), reduced orabolished reflexes and muscular relaxation. Infact, the three concepts are sometimesreferred to as "the triad of anesthesia".

Another important component of generalanesthesia is amnesia, which means that thepatient should have no recollection of anyevents after consciousness was lost.

Anesthesia is generally subdivided into threetypes described under the headings below.

3.1.1 LOCAL ANESTHESIA

As the name implies, agents are administeredtopically as creams, sprays or gels, or byinjection, to achieve a purely local effect. Thepatient is fully conscious, though sometimesmildly sedated by a suitable drug, generallyadministered orally.

Traditions may vary from one country to thenext and a variety of related substances maybe used. These include procaine (one of theearliest local anesthetics), lidocaine,etidocaine, prilocaine, mepivacaine andbupivacaine, as well as more recent additionssuch as ropivacaine and levobupivacaine.Local anesthesia will be familiar to anyone who

has been treated by a dentist. Like regionalanesthesia, discussed below, it may also serveas a useful complement to other forms ofanesthesia.

3.1.2 REGIONAL ANESTHESIA

Local anesthetic agents are injected/infusedto achieve a regional effect. Examples includespinal and epidural anesthesia, as well asperipheral nerve blocks. Once again, theadvantage is that the patient can be fullyconscious but feels no pain. Different agentsmay be used and the anesthetic drugs aresometimes complemented with opioidsadministered epidurally or intrathecally.Regional anesthesia can be used either aloneor in combination with other forms ofanesthesia to provide excellent operatingconditions and prolonged pain relief.

3.1.3 GENERAL ANESTHESIA

This involves a condition generallycharacterized by

sleep or unconsciousness

painlessness

reduced or abolished reflexes andsubsequent amnesia

when necessary, reduced muscular tonus.

General anesthesia is in turn subdivided intoseveral types depending on how theanesthetic agents are administered:

inhalation anesthesia, where the agent isadministered via inhalation;

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intravenous anesthesia, where the agent isgiven by intravenous injection or infusion;

balanced or combined anesthesia, whereboth of the above methods are used. Thisoffers an opportunity of combining the bestqualities of different agents, enablingdosages to be adjusted and potentialsavings to be made, while also allowinganesthetic methods to be adapted tospecific needs.

Since the patient under general anesthetic isunconscious, it is imperative for the anesthetistto monitor a number of vital parameters withgreat care. All life-sustaining care revolvesaround the airways, breathing and circulation.In the context of anesthesia, this may involvethe use of intubation to ensure free airways,manual or machine assisted ventilation to helpthe patient breathe, and infusion fluids toensure patient hydration, as well as carefullyselected drugs to optimize circulation.

In addition, the anesthetist must monitor andcontrol the level of the patient's consciousnessand ensure that he/she remains insensitive topain with the help of intravenous or volatile,i.e. inhalation, pharmaceuticals. These aspectsare of course interrelated and have an impacton each other as well on patient status.Anesthesia is thus a complex and specializedbranch of medicine, and different patientsrequire the anesthetist to focus on differentaspects. Each procedure therefore needs tobe tailored to suit individual needs andprevailing conditions.

3.2 GENERAL ANESTHESIAROUTINES

Although every patient case is unique andindividual hospitals and doctors have differentlocal routines, it is possible to identify a patterncommon to most forms of general anesthesia.The section below outlines the various stagesinvolved, while coming chapters will focusmore on some of their details.

3.2.1 PREOPERATIVE ASSESSMENT

This is a chance for the anesthetist to meetthe patient, inform him/her about the plannedprocedures, perform a physical examinationand discuss any questions. General healthstatus, laboratory values, drug therapy andplanned type of surgery are all taken intoaccount in selecting the anesthesia method.

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3.2.2 PREANESTHETIC PREPARATIONS

Preparations may vary depending on localtradition. Fasting is often recommended,although patients in some countries are nowallowed clear liquids up to 2 hours beforeplanned surgery. Sometimes, carbohydratedrinks may even be given up to 2 hours beforeinduction to prevent post-operative insulinresistance. There are also variations inpremedication routines. These involveadministering drugs for sedation and anxietyrelief either before or after transportation tothe operating room (OR). In the OR or apreparatory room, the patient is greeted bythe anesthesiologist or anesthetist nurse, whochecks lab values and ID. The patient is thenmoved to the operating table and carefullypositioned to avoid discomfort or injury duringsurgery. For some types of surgery it may benecessary to reposition the patient after he/shehas been put to sleep (see chapter 12).

3.2.3 MINUTES BEFORE ANESTHESIA

Routines for the order in which the varioussteps are taken may vary from one clinic tothe next. As a general rule, however, allwatches and jewelry should have beenremoved before the patient arrives in the OR,although it is a good idea always to check thatthis has been done. (These days, it may alsobe a good idea to ask about any piercings.) Ifan intravenous line is not already present, oneshould be arranged. Oxygen (usually 80-100%)is given via a face mask to increase theamount of oxygen in the lungs beforeinduction (preoxygenation). The anesthetistwill also attempt to reassure nervous patientsand help them to relax.

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3.2.4 INDUCTION

To put the patient to sleep, a short-actinghypnotic agent is given intravenously,although sleep is sometimes also induced byinhalation of an anesthetic agent via a facemask. Whichever method is preferred, a facemask is used to assist patient breathing anda check is made to ensure patent airways. Ifmuscle relaxation is needed for surgery orintubation, muscle relaxants are given. In thiscase, some form of analgesia (generally anopioid) may also be given to avoid pain duringintubation. A laryngeal mask or anendotracheal tube is fitted to administer amixture of oxygen (30-40%) and nitrous oxide(60-70%), although medical air may also beused for ventilation.

3.2.5 MAINTENANCE

Anesthesia is maintained by continuouslysupplying the patient with anesthetic agentvia inhalation or intravenous infusion. Thedosage is adapted to individual needs. Carefulmonitoring and/or observation are essential.The parameters generally include ventilation,gas concentration, circulation (pulse, bloodpressure, ECG), temperature (continuous orintermittent), degree of muscle relaxation, fluidbalance (fluids given and lost), metabolism(blood glucose, acid-base balance), andsometimes depth of anesthesia. Sophisticatedequipment is not always necessary. A greatdeal of information can be obtained throughcareful observation and alertness to changesin status.

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3.2.6 ELIMINATION

Anesthetic agents are eliminated via the lungsor broken down by the body and removed asmetabolites. Antidotes may be given tocounteract specific agents, although this isnot common. Once the patient is awake, thelaryngeal mask or endotracheal tube isremoved (extubation) and pure oxygen isgenerally given via nasal prongs.

3.2.7 RECOVERY

The patient is taken to the recovery room orpostoperative ward for recovery andmonitoring of vital functions. Once thepatient's condition is stable, he/she istransported back to the ward or possibly evensent home, depending on the type and extentof the surgery performed.

In principle, all procedures involving general anesthesia will follow a pattern similar to the onedescribed above.

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4 PATIENT ASSESSMENT AND PREPARATIONS

Anesthetic procedures are nowadayscommonplace, safe and relatively simple aslong as appropriate training has beenprovided. In fact, the level of risk associatedwith modern anesthesia is now so low (whenthe patient's only medical problem is whatbrought him or her to surgery in the first place)that it is virtually impossible to performrandomized studies to evaluate the risksassociated with new procedures or equipment.This is due largely to developments over thelast twenty years, particularly the introductionof reliable equipment and careful monitoringby qualified personnel.

4.1 THE PATIENT'S CONDITIONBEFORE ANESTHESIA

Thorough knowledge of the patient's casehistory, current health status, drug therapy,laboratory values and indications for surgeryhelp prevent unnecessary problems and selectthe best anesthetic method. Preoperativeassessment should be part of hospital routinesfor all patient categories. The extent and levelof detail will however vary depending on thepatient's health status, clinical condition,intended surgery and prevailingcircumstances. The most important elementis the taking of a careful case history.

4.1.1 PATIENT RECORDS

The case history provides details of thepatient's medical history, including surgicaloperations and types of anesthesia, as wellas allergic tendencies, coagulation disorders,pregnancy, muscular disorders, rheumatic or

endocrine disease, neurological or cardiacdisorders, hypertension, lung disease andhereditary disorders, such as a predispositionto develop malignant hyperthermia.

4.1.2 PATIENT INTERVIEW ANDEXAMINATION

The interview is an opportunity for theanesthetist to ask questions and for the patientto find out more about the plannedprocedures. The assessment should includegeneral state of health, degree of mobility(particularly mouth and neck), blood pressure,functional heart and lung status, assessmentof potential intubation difficulties (see"Endotracheal intubation" in chapter 5), dentalstatus, body height and weight.

Depending on the patient and the intendedsurgery, it may be necessary to obtain an ECGand a laboratory status. The latter will generallyincludes hemoglobin, serum electrolytes andserum creatinine, although it may be extendedto cover liver status, coagulation andhemostasis status and blood glucose. It isimportant to discuss current drug therapy,particularly drugs used to treat heart disease,anticoagulants, vasoactive agents,psychotherapeutic drugs and steroids. Alcoholor drug abuse may have adverse effects onanesthesia and the anesthetist should askabout such problems and bear them in mind,even if patient answers may sometimes beunreliable. Inquiries should also be madeabout bleeding disorders, both hereditary andacquired. The patient's blood group shouldalso be routinely ascertained or checkedahead of major surgery.

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4.1.3 THE ASA SYSTEM

Patients undergoing surgery and anesthesiamay differ tremendously in terms of age,constitution and health status. It is thereforeuseful to assess and categorize them in astandardized manner so as to avoidmisunderstandings and improve patientoutcome.

When assessing patient status prior toanesthesia, the anesthetist interviews patientsand generally groups them in accordance withthe globally accepted ASA (American Societyof Anesthesiologists) classification outlinesbelow.

ASA Class 1: No systemic or psychiatricdisease, the patient is healthy and normal.

ASA Class 2: Moderate systemic diseasethat does not limit the patient's activitiesand is often under good medical control,e.g. a patient with well controlledhypertension or diabetes without medicalcomplications.

ASA Class 3: Moderate or severe systemicdisease that does limit the patient'sactivities and may be under incompletemedical control, e.g. stable anginapectoris, diabetes with medicalcomplications, previous myocardialinfarction or emphysema.

ASA Class 4: One or more severe systemicdiseases that are life-threatening. e.g.severe congestive heart failure, advancedrenal insufficiency, liver or lung disease,recent myocardial infarction.

ASA Class 5: Severe illness in a patientrunning a substantial risk of death within 24hours with or without surgery, although it isconsidered "best to try".

The E designation: Emergency status. Inaddition to indicating underlying ASA status(1-5 above), any patient undergoing anemergency procedure is given the suffix"E". A fundamentally healthy patient, forexample, undergoing emergency surgerysuch as an appendectomy is thus classifiedas 1-E. The E designation is not used whensurgery is planned.

ASA Class 6: Organ donors who havealready been pronounced clinically deadbut are waiting for surgery to be performed.

This information may be included in theanesthesia record chart under preoperativeassessment (see chapter 7).

4.2 PREANESTHETICPREPARATIONS

Different hospitals and anesthetists have theirown routines although these tend more or lessto follow the outline below.

4.2.1 INFORMATION

Most patients are unnerved at the prospect ofanesthesia and surgery, although anxietylevels may vary considerably. It is therefore agood idea to inform the patient thoroughlyabout the procedure and what to expect.

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4.2.2 ENSURING THAT THE STOMACH ISEMPTY

The traditional view is that the stomach shouldbe empty before anesthesia and surgery, sincevomiting is a complication which may belife-threatening if aspiration to the lungsoccurs. Fasting (both food and drink) istherefore often recommended for 4-5 hoursbefore anesthesia, or longer if patients arepregnant or overweight. Variations in thisroutine do however occur, particularly withregard to fluid intake (see under "Preanestheticpreparations" in chapter 3 above). In case ofemergency that precludes fasting, a rapidsequence induction (RSI) is used.

Fasting may cause dehydration, which is whya preoperative infusion is often given. Attitudesto the use of larger quantities of ínfusion fluidshave however recently become morerestrictive.

4.2.3 DRUG THERAPY

The patient's normal drug therapy may requirechanges due to surgery and anesthesia. Somedrugs, such as peroral anticoagulants andaspirin, should be avoided before all types ofsurgery.

Others, such as insulin treatment,corticosteroids, epilepsy medication andhypertension and heart medication, must becarefully assessed and sometimes adjusted.

4.2.4 PREMEDICATION

If premedication is given, its purpose isgenerally fourfold:

to raise the pain threshold

to sedate the patient and relieve anxiety

to reduce reflex central nervous activity

to counteract any side effects of anesthesiaand surgery.

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5 MANAGING THE AIRWAYS

Airway management is crucial in all forms ofgeneral anesthesia, and both the agents andthe equipment used for this purpose are keptclose to the patient. Even when patients arebreathing spontaneously, the anesthetist mustalways be ready to take control of ventilation.Ensuring free airways and adequate ventilationis essential to safe anesthesia.

5.1 FREE AIRWAYS AND ASSISTEDBREATHING

5.1.1 FACE MASK

When surgery is elective and requires generalanesthesia, the patient is generally ventilatedvia a face mask during induction andsometimes after extubation. During theprocedure, the patient may either breathespontaneously or with the anesthetist'sassistance in the form of manual or controlledventilation. For emergency procedures,routines are somewhat different.

Previously, only short operations wereperformed with the patient breathingspontaneously. Recent developments, suchas the laryngeal mask, however, now enablespontaneous breathing during longerprocedures as well.

5.1.2 LARYNGEAL MASK

The LMA (for laryngeal mask airway) isbecoming increasingly popular as analternative to the face mask, since it frees theanesthetist's hands and simplifies the job ofensuring free airways.

The laryngeal mask is also widely used as analternative to endotracheal intubation, sincethe latter may sometimes give rise tocomplications and problems.

The mask is positioned so that the tip coversthe upper esophageal sphincter, ensuring freepassage of air to the trachea. Once the maskis in place, the rim is inflated

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It is primarily used during spontaneous orsupported breathing, but is also gaining morewidespread acceptance for use in combinationwith controlled breathing modes.

There is now a growing range of LMAs, bothfor one-time use and re-use, available in themarket in a variety of sizes and designs and

with different accessories. Some have alsobeen adapted to allow for the insertion of agastric tube. These developments contributeto their increasing popularity.

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5.1.3 ENDOTRACHEAL INTUBATION

Certain types of surgery or specific patientpositions on the operating table may make itnecessary to intubate the patient.

This is done with the help of a laryngoscope,which enables the intubator to visualize thelarynx. The blades of this instrument areavailable in different sizes to suit a range ofpatients.

The endotracheal tube (ETT) is then insertedand positioned so that the tube's cuff is belowthe vocal cords. The cuff is then inflated toprevent aspiration and leakage. The tube isalso available in a variety of sizes and may beinserted via either the nose or the mouth. It isalmost always disposable.

As mentioned in connection with preanestheticevaluation, potential difficulties associatedwith intubating patients should be assessedbefore the procedure. This generally involvesassessment of mouth opening and neckextension, as well as vocal cord visualization.Sometimes a special classification system isused, the most widely accepted of which isthe Mallampati system.

In cases where major difficulties areenvisaged, it may sometimes be necessary toresort to a fiber optic bronchoscope (FOB).All intubation is of course only performed bytrained medical personnel and care is alwaystaken not to damage the teeth, mucousmembranes, vocal cords or surroundingtissues.

When intubating infants, no cuff is used onthe endotracheal tube due to the sensitivity oftheir mucous membranes, since any swelling

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may lead to serious breathing problems afterextubation. Allowances must therefore bemade in pediatric anesthesia for leakagearound the tube.

5.2 PATIENT VENTILATION

The following is a brief outline of gasadministration and breathing systems. For amore detailed description of various breathingcircuits and of the anesthesia machine as awhole, the reader is referred to chapters 8 and9.

5.2.1 GAS ADMINISTRATION

Oxygen, nitrous oxide and medical air areprovided via pipes from cylinders or thehospital's central supply. Gas pressure isreduced for medical purposes. The flow rateand concentration of the gases are individuallyset and altered during anesthesia.Traditionally, the anesthesia machine'sflowmeters allow the anesthetist to set thedosage.

The gases then pass through a vaporizer,when used, containing a volatile agent usedfor inhalation anesthesia. If such a vaporizeris not in use, the gas mixture is suppliedstraight to the breathing system.

5.2.2 BREATHING SYSTEMS

Inhalation agents are given via the breathingsystem, which may differ from one machineto the next with regard to level ofsophistication and the degree to which itincorporates rebreathing. At the rudimentarylevel, most systems will include the following:

patient mask

patient tubing

fresh gas inlet

bag for manual ventilation

ventilator

unidirectional valves and/or pop-off or reliefvalves

CO2 absorbent canister when rebreathing

systems are used.

Developments have led to substitutions andadditions to this basic breathing apparatus.These are also described in greater detail inchapter 9.

5.3 MANUAL VENTILATION

It must always be possible for the anesthetistto support patient ventilation or control itcompletely. On most machines, access to abag for manual ventilation is thereforestandard. It should be checked at regularintervals to ensure that it is fully functional.

Both bags and tubing come in a variety ofsizes for different patients and uses.

5.4 MECHANICAL VENTILATION

In general, the ventilator on the anesthesiamachine is used when surgery necessitatesmuscle relaxation or is due to last an hour ormore. However, a certain amount of flexibilityis often necessary depending on the patient,the surgery to be performed and local routinesand preferences. Mechanical ventilation isdiscussed in chapter 8.

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6 ANESTHESIOLOGICAL METHODS AND PHARMACEUTICALS

The preoperative assessment helps theanesthetist select the appropriate method touse based on the patient's health status andwishes, the type of surgery to be undertakenor environment in which it takes place and theexamination results, etc. The followingprovides a brief guide to the most commonanesthetic methods in use today.

6.1 REGIONAL ANESTHESIA

There are several forms of regional anesthesia(see chapter 3), of which those involving acentral block of the spinal cord or the nervesleaving it are perhaps the most widely knownand used. They may be used alone or incombination with other forms of anesthesiaand are briefly described below.

6.1.1 SPINAL ANESTHESIA

The anesthetic is injected into thecerebrospinal fluid (CSF) that bathes the spinalcord and brain. The needle is generallyinserted just below the intervertebral spacebetween the first and second lumbarvertebrae, known as the L1-L2 level, althoughthis may vary with the type of surgery.

A combination of a local anesthetic and anopioid is often used. The extent of the blockdepends on the level reached by the

anesthetic in the cerebrospinal fluid, which isin turn governed largely by the doseadministered. This method is common duringoperations performed below the waist andlasting less than two hours, for example hipsurgery and elective caesarean section.

6.1.2 EPIDURAL ANESTHESIA

Again, a local anesthetic is injected in thelumbar (or thoracic) region, but the needlestops in the epidural space surrounding themembrane (or dura) around the cerebrospinalfluid. Normally, a fine catheter is left in placewhich can be used for post-operative painrelief (epidural analgesia). The anestheticblocks the nerves that pass through theepidural space as they leave the spinal cord.

Epidural analgesia is perhaps best known forits use during childbirth, although it does haveother areas of application, such as foranalgesia after major abdominal surgery.

6.1.3 CONTRAINDICATIONS

The main contraindications for regionalanesthesia used alone are as follows:

allergy to local anesthetics

patient choice

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uncooperative or restless patients

bleeding disorders

ongoing anticoagulant therapy

topical infections.

6.2 GENERAL ANESTHESIA

This common form of anesthesia is generallydivided into the following categories:

intravenous anesthesia

inhalation anesthesia

combined (or balanced) anesthesia.

6.2.1 INTRAVENOUS ANESTHESIA

Anesthetic agents that are given intravenouslyare rapidly distributed. Their actions andpossible side-effects are immediate in patientswith normal circulation. With inhalationanesthesia, however, it can take severalminutes to achieve a specific anesthesia levelthat only takes seconds to achieveintravenously.

DOSAGE

By combining different drugs, the specificbenefits of each can be utilized to tailoranesthesia to the individual patient. Effectiveanalgesics can be carefully dosed andcombined with sleep inducing drugs toanesthetize the patient.

METABOLISM AND ELIMINATION

The elimination of intravenous agents dependsmainly on liver metabolism and renal functionand generally takes longer than that of inhaledagents. Remifentanil, however, which is arelatively recently developed fast-acting opiod,is metabolized by blood and tissue enzymes.so its elimination is not dependent on the liverand kidneys.

FREQUENCY OF USE

Total intravenous anesthesia (TIVA) is lesscommon than combined anesthesia, in whichseveral administration routes are usedsimultaneously. It is however preferred in someparts of the world or for special purposes, andsome experts see a trend towards increasinguse.

6.2.2 INHALATION ANESTHESIA

Inhalation anesthesia is a popular methodbecause the gases and agents used are easyto measure and control, with the lungs actingas a kind of "buffer". Less appealing is the factthat we still do not know exactly how inhaledanesthetic agents work.

These agents are volatile liquids that arevaporized and administered to the patient ina gas mixture, generally oxygen-nitrous oxideor oxygen-air. General laws governing gaspressure, volume, temperature, flow andconcentration are thus applicable here. It hasbeen suggested that the agents act on theproteins in the cell's lipid membrane and theirpotency is thus related to their solubility infat. This also makes sense when oneconsiders that their target organ, the brain, islargely composed of fat.

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It also explains why halothane, which used tobe a very popular anesthetic agent and ishighly lipid soluble, is the most potent of theanesthetic agents, while nitrous oxide, whichis comparatively insoluble in fat, has the lowestpotency of the agents in clinical use.

Lipid solubility is represented by the oil/gaspartition coefficient (OGPC), although inclinical practice, potency is generallyassociated with the Minimum AlveolarConcentration (MAC). The pharmacokineticsof the agent (how fast it reaches and leavesthe brain) must also be considered whendiscussing inhalation anesthesia. Theimportant factors are the concentration ofanesthetic agent in the inspired gas mixture,the choice of gas mixture, alveolar ventilation,the blood flow to the lungs (or cardiac output),the gradient between venous and arterialblood and agent solubility in blood and othertissues. These aspects are all discussed inmore detail below.

MAC - MINIMUM ALVEOLARCONCENTRATION

MAC is an index of the anesthetic potency andthus the pharmacological effect of aninhalation agent. It is defined as the agent'salveolar concentration (in percent), as reflectedby the expired end-tidal concentration, atwhich 50% of patients will not move inresponse to a surgical stimulus (skin incision).1.3-1.4 MAC is the alveolar concentrationneeded for surgical anesthesia.

It should be noted that MAC is higher in infantsand lower in the elderly, and that both opioidsand premedication allow surgery to beperformed at a lower MAC. Another importantaspect is that if more than one anesthetic

agent is used, the MAC values are additive.Thus, 0.6 MAC of nitrous oxide combined with0.4 MAC of isoflurane is equivalent to 1.0 MACof isoflurane. Two volatile agents, such assevoflurane and desflurane, will also haveadditive effects. Several other concepts areoccasionally used in relation to MAC, such as"MAC 95%", "MAC awake" and "MAC aware".They refer to different MAC levels ("MACaware", for example, is about 0.3 MAC).

UPTAKE OF INHALATION AGENTS

During inhalation anesthesia, it is crucial toensure a rapid and adequate concentration ofanesthetic agent in the brain. Since the agent'spartial pressure in all tissues eventuallyapproaches that in the alveoli, the alveolarpartial pressure at steady state reflects thepartial pressure in the brain. It should howeverbe noted that anesthetists (and anesthesiamachines) generally measure end-tidalconcentrations as volume fractions in percent,rather than as partial pressures, since MAC isalso a percentage value.

A high fresh gas flow and high agentconcentration in the gas mixture enable theanesthetist to increase the agent's alveolarpartial pressure rapidly. These two factors arethus the major determinants of how rapidlythe agent is delivered to the brain.

Agent uptake is best described by the rate atwhich alveolar concentration (FA) rises in

relation to inspired concentration (FI). If

cardiac output is constant, the increase inthe FA/FI ratio over time reflects the degree

of solubility of the agent in blood.

23


0 10 20 30Minutes

1.0

0.8

0.6

0.4

FA/FI N2O

Sevoflurane

Desflurane

Enflurane

Halothane

Ether

Isoflurane

Wash-in curve for inhalation agents (rate of vapor uptake)

SOLUBILITY OF INHALATION AGENTS INBLOOD

The solubility of an inhalation anestheticdetermines how fast it both works and wearsoff. The less soluble the drug, the faster itworks. The term also denotes the amount ofagent that dissolves in the blood in relation tothe amount present in the alveolar gas at thesame partial pressure. Thus if one liter of bloodis exposed to one atmosphere of isoflurane,then 1.4 liters of the agent will have enteredthe blood once steady state is reached.

The solubility of an agent is specified by itsblood/gas partition coefficient (BGPC).

A highly soluble anesthetic, such as ether, willthus dissolve to a far greater extent in theblood, thereby successively decreasing theamount in the alveoli (if no further agent issupplied). Nitrous oxide, on the other hand, ishighly insoluble. It will therefore remain in thealveoli, with only a small amount (0.47 liters if

the same experiment as above is conducted)passing into the blood. Its alveolar partialpressure thus remains high, which means thatthe partial pressure in the brain will also behigh, as explained above. This results in ashort induction time.

GRADIENT BETWEEN VENOUS ANDARTERIAL BLOOD

The gradient between mixed venous bloodand arterial blood depends on the uptake indifferent tissues. Anesthetic agents havedifferent partition coefficients in differenttissues and this naturally affects their uptake.Potent agents (those with low MAC values)that have a high solubility in fat dissolve to agreater extent in fat-rich tissues.

There will thus be a larger partial pressuregradient between venous and arterial bloodin fat-rich tissues, such as the brain. The maincoefficient of interest here is the oil/gaspartition coefficient. An agent with a highcoefficient, such as halothane, will have

24


greater potency at the site of action than anagent with a low coefficient, such asdesflurane. In addition, organs with a high rateof perfusion (such as the brain, kidneys, liverand heart) equilibrate more rapidly with thealveolar partial pressure than organs with alower rate of perfusion.

CARDIAC OUTPUT

Higher cardiac output means higher pulmonaryblood flow. The agent is therefore more rapidlydistributed from the alveoli, reducing alveolarpartial pressure. Reduced cardiac output, onthe other hand, means decreased uptake ofthe agent and the alveolar concentration maythen be much higher than the concentrationin the inspired gas mixture. The net effectdepends on the agent used and its solubility,as explained above.

THE CONCENTRATION AND SECOND GASEFFECTS

If a gas with a low potency (or high MAC), suchas nitrous oxide, is part of the inspired gasmixture, two phenomena known as theconcentration effect and the second gas effectwill occur. These are explained below.

Since the required concentration of nitrousoxide is high, its uptake into the blood will berapid. The expired nitrous oxide volume willbe lower than the inspired volume, whichmeans that the oxygen and any secondanesthetic agent will be diluted in a smallervolume, leading to the alveolar concentrationthat the effect is named after.

The alveolar partial pressure of anyaccompanying gas will also rise more quickly,which means that induction with an inhalationanesthetic is more rapid in the presence ofnitrous oxide. This is known as the secondgas effect.

6.2.3 PRACTICAL EXAMPLE

The main steps involved in anesthetizing apatient with the help of inhalation agents aredescribed below. As noted previously,combined or balanced anesthesia, in whichintravenous agents are used together withvolatile inhalation agents, remains the mostwidely used method.

INDUCTION

INHALATION INDUCTION

This method makes use of a face maskthrough which a mix of oxygen and nitrousoxide or oxygen and air is initially given, afterwhich an inhalation anesthetic, such assevoflurane, is introduced by the anesthetist.An alternative is the single-breath techniquefor patients able to cooperate, which givesrapid induction within 20-30 seconds.Inhalation induction is used for young childrenand patients with upper and lower airwayobstruction.

INTRAVENOUS INDUCTION

A drug such as propofol or a fast-actingbarbiturate is often used during induction toput the patient to sleep. It is administeredintravenously while oxygen is given via themask. This is the most common inductionmethod and is especially appropriate forpatients undergoing emergency surgery where

25


there is a risk of regurgitation.The method isthen referred to as "rapid sequence induction"(RSI) and is described below (see under"Anesthesia in emergency conditions" inchapter 13).

RAPID SEQUENCE INDUCTION

Tilt the table and ensure that suction isready. A trained assistant must be presentand IV access must be secured.Sometimes, gastric volume is firstminimized via a nasogastric tube andoccasionally gastric acidity is reducedpharmacologically.

Preoxygenate the patient for about 3minutes, if possible.

Give an appropriate sleep dose of theinduction agent.

Apply cricoid pressure (this involvespressing against the cricoid cartilage topush it backwards, compressing theesophagus. Moderate pressure may beapplied before loss of consciousness, andfirmer pressure maintained until the cuff ofthe tracheal tube is inflated.)

Give suxamethonium, 1 mg/kg, andproceed with intubation, checking theposition of the tube before releasing thecricoid. Secure the tube.

INHALATION TO STEADY STATE

Regardless of induction method, an inhalationagent is now added in fairly highconcentrations to an oxygen-nitrousoxide/oxygen-air mixture until steady state isachieved.

During the first part of induction, largeamounts of the agent are taken up by thecirculation until steady state is reached. Muchhigher concentrations are therefore used forthis phase of induction than for maintenance.

MAINTENANCE

The concentration of the gas is then loweredto the desired level for anesthesiamaintenance.

ELIMINATION AND RECOVERY

Modern inhalation agents are mainlyeliminated via the lungs since they are onlymetabolized to a very limited extent. Toxicityis largely dependent on the amountmetabolized. Agents that are metabolized toa lesser extent are eliminated almost solelyvia the lungs.

Recovery times are similar to induction times,in that less soluble agents have quickerrecovery times. It should however be notedthat lengthy periods of anesthesia requiremuch longer recovery due to the accumulationof anesthetic agent in fat-rich tissues.

26


One example of the process described aboveis illustrated in the simplified four-part drawingbelow.

Induction (IV), sleepO2 + air + drug

Induction to steady stateO2 + N2O + inhalational agent

O2 + N2O + inhalational agent

Maintenance

Elimination andrecoveryO2 + air

27


6.2.4 NITROUS OXIDE, N2O

NITROUS OXIDE, N2O

Light blueColor code (cylinder)0.47Blood/gas coefficient1.4Oil/gas coefficientWeak, sweetSmell

1.0 MAC in(105 = theoretical value)* 100% O2

-* 30% O2 + 70% N2O

-% metabolized

The low solubility of this gas means that itprovides rapid analgesia, although relativelyhigh concentrations are required. It thus servesas a useful complement to other anestheticagents.

During the first minute of induction, uptake isaround 1-1.5 liters when the inspired fraction(or FI) is 70%, after which it falls quickly to

reach around 100 ml after an hour.

The high concentrations of nitrous oxiderequired affect the uptake of other agentsgiven at the same time (concentration andsecond gas effects), although only during theinitial rapid uptake phase.

EFFECT ON BREATHING

N2O is non-irritant and does not cause

bronchospasm. It slightly decreases tidalvolume, although this is offset by an increasein respiratory rate. It may cause diffusionhypoxia at the end of surgery. It expandsair-filled cavities because it is 40 times assoluble as nitrogen, passing from the bloodinto the cavity faster than nitrogen can diffuseout. This can double the size of apneumothorax in 10 minutes at aconcentration of 70%.

CARDIAC AND CIRCULATORY EFFECTS

In experiments, nitrous oxide decreases thecontractility of the myocardium, althoughmean arterial pressure is in practice usuallywell maintained by a reflex increase inperipheral vascular resistance. For patientswho are unable to increase their sympatheticdrive, however, the direct myocardialdepressant effects may reduce cardiac output.Nitrous oxide does not sensitize the heart tocatecholamines.


Due to its low solubility in blood and tissues,nitrous oxide is only minimally metabolized. Itis eliminated via the lungs.

CONTRAINDICATIONS

N2O should not be used for patients with

bowel obstruction, pneumothorax or middleear and sinus disease. Some cliniciansmaintain that nitrous oxide use should berestricted during pregnancy because of effectson DNA production and evidence of unwantedreproductive outcomes, but its use remainsvery popular.

Severe vitamin B12 and folic acid deficienciesalso constitute contraindications for nitrousoxide use, and anesthesiologists thereforealso need to consider the nutritional status ofvegans.

On the whole, there is a slow downward trendin the use of nitrous oxide, although prevailingroutines, traditions and preferences may differfrom place to place. Among the negativeeffects of nitrous oxide, frequent reference ismade to post-operative nausea, while itseffects on the environment, both in general

28


and at work, are also increasingly cited. It ishowever still a reliable and useful drug that isrelatively inexpensive and has few side-effects.It therefore remains widely used.

6.2.5 SEVOFLURANE

SEVOFLURANEYellowUniversal color code

(bottle and adapter onfilling system)

0.6Blood/gas coefficient47Oil/gas coefficientWeakSmell

1.0 MAC in* 100% O2

2.05Adults2.4-3.3Infants & children

* 30% O2 + 70% N2O

0.66Adultsunknown - 2Infants & children< 5% metabolized

Sevoflurane's solubility in blood is quite low,making it a rapidly acting agent that is easyto adjust during administration.

EFFECT ON BREATHING

Sevoflurane produces dose-related respiratorydepression, although recovery from suchdepression is generally rapid due tosevoflurane's elimination characteristics. Itcauses an increase in respiratory rate,although minute volume remains unchanged,and a decreased response to hypoxia andhypercapnia. It relaxes bronchial smoothmuscle and does not irritate the airways, andits effects are very rapid. These aspects makeit ideally suited to inhalation induction andanesthesia maintenance in adults and children.On the other hand, sevoflurane alsopotentiates the action of depolarizing andnon-depolarizing muscle relaxants to a greaterextent than either enflurane or halothane


Sevoflurane causes a decrease in myocardialcontractility and mean arterial pressure. It haslittle effect on the heart rate and does notsensitize the myocardium to circulatingcatecholamines. It does not cause ‘coronarysteal’ although dose-related hypotension hasbeen noted. For patients undergoing cardiacsurgery, recent research suggests thatsevoflurane (and desflurane) even has acardioprotective effect and could help preventmyocardial infarction.


Elimination is rapid due to low solubility. Over95% is eliminated via the lungs, predominantlyunchanged, while less than 5% is metabolized.Sevoflurane is however unstable in thepresence of soda lime, producing smallamounts of a a degradation product knownas "Compound A". In rats, this has beenshown to damage the kidneys. In humans,however, no effect on renal function has beenseen. Low flows, high temperatures,desiccated absorbent containers andpotassium based absorbents increase theproduction of Compound A.

CONTRAINDICATIONS

Sevoflurane, like isoflurane, is a trigger agentfor malignant hyperthermia. No inhalationagent should be used on patients with aknown or suspected genetic tendency todevelop malignant hyperthermia.

29


DOSAGE

Dosage is age-related. For children, thedosage during induction is higher than thatgenerally given to adults. The concentrationgenerally required for maintenance is between0.5-0.8 and 3%.

6.2.6 ISOFLURANE

ISOFLURANEPurpleUniversal color code


1.4Blood/gas coefficient98Oil/gas coefficientWeakSmell

1.0 MAC in1.15* 100% O2

0.56* 30% O2 + 70% N2O

0.2% metabolized

The effects of isoflurane are very similar tothose of enflurane. Nowadays, sevoflurane isoften preferred to isoflurane because of itslower solubility, which makes both inductionand recovery quicker.

EFFECT ON BREATHING

An increased incidence of "airway problems"has been reported during induction withisoflurane compared with sevoflurane.Isoflurane is a respiratory depressant anddecreases tidal volume, while having littleeffect on respiratory rate. It causes adecreased response to hypoxia andhypercapnia and is very irritant to therespiratory tract. It also causesbronchodilation.


Arrhythmias are uncommon with isoflurane,although blood pressure may decrease in adose-related manner and a reflex tachycardiamay occur. Isoflurane has a mild negativeeffect on the heart and circulation, causingvascular resistance to decrease and meanarterial pressure to fall. It has been suggestedthat it causes ‘coronary steal’.


Isoflurane is mainly eliminated via the lungs(95%), with only approximately 0.2% beingmetabolized.

CONTRAINDICATIONS

Isoflurane, like sevoflurane, is a trigger agentfor malignant hyperthermia. No inhalationagent should be used on patients with aknown or suspected genetic tendency todevelop malignant hyperthermia.

DOSAGE

Dosage is age-related (see MAC explanationabove and values in table). During the first partof induction, the required isofluraneconcentration is around 3-4%, while formaintenance, it is between 0.5 and 3%.

30


6.2.7 DESFLURANE

DESFLURANEBlueUniversal color code


0.42Blood/gas coefficient19Oil/gas coefficientSweetSmell

1.0 MAC in* 100% O2

5.75-7.25Adults7.20-10.65Infants & children

* 30% O2 + 70% N2O

1.75-4.25Adults5.15-7.75Infants & children0.02% metabolized

Desflurane's solubility in blood is very low,making it a very rapidly acting agent that iseasy to adjust during administration. It is fairlyexpensive but is often the preferred drug whenanesthetizing patients with a high BMI, sinceit has a low oil/gas coefficient. It does howeverrequire a special vaporizer (see section onvaporizers in chapter 8).

EFFECT ON BREATHING

Airway problems may occur during inductionand an intravenous agent is thereforerecommended to put the patient to sleep. Adose-related reduction in breathing has beenreported for desflurane.


Desflurane causes a decrease in myocardialcontractility, although sympathetic tone ispreserved. It does not sensitize the heart tocirculating catecholamines or cause 'coronarysteal'. A dose-related decrease in bloodpressure has been noted, as well astachycardia caused by an indirect autonomic

effect, generally when agent concentration isincreased. This makes the physiologicalresponse to this agent somewhat differentfrom reactions to other anesthetic agents.


Desflurane is hardly metabolized at all (only0.02%).

CONTRAINDICATIONS

Desflurane is a trigger agent for malignanthyperthermia. No inhalation agent should beused on patients with a known or suspectedgenetic tendency to develop malignanthyperthermia. In addition, desflurane is notsuitable for induction due to an increasedtendency towards airway instability andlaryngospasm during induction.

DOSAGE

Dosage is age-related. The concentrationusually required for maintenance is between2 and 6%.

6.2.8 ENFLURANE

ENFLURANEOrangeUniversal color code


1.9Blood/gas coefficient98Oil/gas coefficientEther-likeSmell

1.0 MAC in1.8* 100% O2

0.57* 30% O2 + 70% N2O

2.4% metabolized

31


The effects of enflurane are very similar tothose of isoflurane (see above). Enflurane ishowever more soluble, resulting in slowerinduction and recovery.

EFFECT ON BREATHING

Breathing is affected even during the earlystages of enflurane anesthesia. The breathingpattern typically displays small tidal volumesand high breathing frequency.


Enflurane is characterized by peripheralvasodilation and a tendency to bradycardia,leading to a drop in blood pressure. Thedeeper the anesthesia, the more pronouncedthe effect.


Enflurane is metabolized to a lesser degreethan halothane. 2.4% is metabolized in theliver and eliminated in the urine, while the restis eliminated via the lungs.

CONTRAINDICATIONS

No inhalation agent should be given to patientswith a known or suspected genetic tendencyto develop malignant hyperthermia.

EEG changes may occur when higherconcentrations of enflurane are used. It istherefore not used on patients with epilepsy.The metabolites of enflurane may cause renaldamage in higher concentrations. Enfluraneis therefore not used on patients with knownor suspected renal insufficiency.

DOSAGE

Enflurane is not used for gas induction. In anoxygen and nitrous oxide mixture (30-40%:60-70%), the maintenance concentration isusually between 0.6 and 3%.

6.2.9 HALOTHANE

HALOTHANERedUniversal color code


2.4Blood/gas coefficient225Oil/gas coefficientSweetSmell

1.0 MAC in0.75* 100% O2

0.29* 30% O2 + 70% N2O

15-20% metabolized

Halothane is a very potent hypnotic agent(see MAC values) although it has a muchweaker analgesic effect. It is one of the earlyinhalation anesthetics and is not widely usedtoday due to its serious side-effects and thedevelopment of new and better agents. Insome countries, it is not even registered forsale.

EFFECT ON BREATHING

Similar to the effect of enflurane.

CARDIAC AND CIRCULATORY EFFECT

Similar to the effects of enflurane, althoughhalothane causes more arrhythmias.

32



15-20% is metabolized in the liver andeliminated in the urine.The metabolites areimmunologically active and may cause a fatalform of hepatitis. Halothane has thereforebeen withdrawn from most markets.

CONTRAINDICATIONS

When and if halothane is still used (see above),it should be avoided when patients suffer fromliver disease or have previously been exposedto halothane and afterwards shown signs ofabnormal liver function (tiredness, fever,icterus, known jointly as "halothane hepatitis").

As always, no inhalation agent should be givento patients with a known or suspected genetictendency to develop malignant hyperthermia.

6.2.10 OTHER AGENTS

XENON

Xenon is a noble (or inert) gas with even lowersolubility than nitrous oxide and high potency(MAC = 71%). With no major depressanteffects on the cardiovascular system or irritanteffects on the airways, it has great appeal asan anesthetic agent. It is however extremelyexpensive and relatively complicated for theanesthetist to control. It is not currently usedin the US, so most data comes from Europe,where it is believed that it will be increasinglyused in the future despite its high costs.

HELIUM AND OXYGEN/HELIUM MIXTURES

Helium is a light noble gas present in air andnatural gas from which it is extracted. It issupplied either as Heliox (21% O2, 79% He)

in cylinders marked with the colors used

locally for helium (brown) and oxygen (e.g.white in Sweden, green in the USA), or as100% helium in brown cylinders. Helium hasa lower density than oxygen, nitrogen and air.During turbulent flow, velocity is higher whenHeliox is used. This reduces work of breathingin patients with upper airway obstruction, suchas a tumor. Its use has also been proposed inpatients with severe asthma and other lowerairway disease. For anesthesia purposes,Heliox may in future improve gas flow inpatients with chronic obstructive pulmonarydisease (COPD), pulmonary hypertension orsevere hypoxia.

6.3 COMBINED ANESTHESIA

This is a common form of anesthesia in whichboth inhalational and intravenous anestheticagents are administered in an attempt tocombine the best qualities of different agentsto anesthetize and relax the patient. Thecomponent agents are selected to suit thespecific patient's status and the surgery to beperformed.

6.4 MUSCLE RELAXANTS

Muscle relaxants affect only the skeletalmuscles and have no anesthetic or analgesiceffect. They are used only when the patient isasleep. They paralyze the respiratory muscles,so the anesthetist must be able to ventilatethe patient before administering them.

Muscle relaxants are frequently given tofacilitate endotracheal intubation. In addition,abdominal surgeons often require musclerelaxation in their patients to enable them to

33


operate. Deep anesthesia will generally ensuremuscle relaxation, but it is most common touse muscle relaxants to avoid any risks thismight entail.

There are two different kinds of musclerelaxants:

non-depolarizing.

depolarizing

In simple terms, the depolarizing agents mimicthe appearance and effect of the body'snormal neurotransmitter, acetylcholine, bindingto its receptors at the neuromuscular junctionand causing prolonged depolarization of themuscle, thereby making it unreceptive to newimpulses. The muscle is effectively paralyzed.

The most common depolarizing agent issuccinylcholine (suxamethonium) and it ismainly used prior to intubation.

Its most common side-effect (apart frompost-operative muscular pain caused by initialmuscular contraction) is bradycardia,especially if more than one dose is given. Thiscan be prevented by the prior administrationof atropine. Children develop this complicationmore commonly than adults. It may also causeraised potassium levels and may even triggerthe onset of malignant hyperthermia in patientswith this genetic disorder.

Non-depolarizing agents, meanwhile, aresimilar in appearance to acetylcholine, but notin effect. They block its receptors at theneuromuscular synapse and prevent anydepolarization and subsequent musclecontraction. They are water soluble, polarmolecules and thus do not cross theblood-brain barrier and have no effect on the

central nervous system. All non-depolarizingdrugs should be used with care in patientssuspected to be suffering from myastheniagravis (a neuromuscular disease) ormyasthenic syndrome, since these patientsare extremely sensitive to their effects.

A large number of non-depolarizing agentshave been developed in recent decades basedon the earliest drug, pancuronium. Theyinclude vecuronium and rocuronium, as wellas atracurium and its more recent derivatives,with their slightly modified effect profiles.

They are initially used in fairly large dosessupplemented as and when necessary withsmaller maintenance doses. Cardiovasculareffects are minimal, although some may renderthe patient vulnerable to bradycardia duringanaesthesia.

To assess the degree of neuromuscular block,a number of monitoring tools arerecommended. These are discussed later (seechapter 10).

6.5 OUTLINE OF A TYPICALANESTHESIA PROCEDURE

There are certain basic steps that are the samein all anesthesia procedures. They are brieflyoutlined in the text and diagrams below.

6.5.1 PREOXYGENATION

A face mask is used to give the patient80-100% oxygen for a few minutes beforeinduction to ensure high oxygen saturationand prevent complications.

34


6.5.2 INDUCTION

For IV induction, a drug such as propofol orthiopental is given intravenously to put thepatient to sleep. The alternative is inhalationinduction (see discussion under section 6.2.3).

A check is performed to ensure free airwaysand make sure that manual ventilation ispossible. Once deep sleep is achieved, musclerelaxants are given if required.

Intubation is then performed, if necessary, ora laryngeal mask is fitted. Analgesics and/orinhalation agents are administered via thetube/mask. Careful observation and monitoringare important throughout the procedure.

6.5.3 MAINTENANCE

The gas mixture is administered as needed,and adjustments are made using inhalationagents and/or intravenous injections.

Careful monitoring is crucial to ensure patientsafety, but the value of clinical observationshould not be underestimated. Changes in thepatient's condition are generally perceptibleto an alert anesthetist or nurse.

When surgery has been completed, theanesthetic agent ceases to be given andextubation is performed when the patient isawake and breathing spontaneously and themuscle relaxant wears off or is reversed.

6.5.4 RECOVERY

Oxygen is given via a mask to preventcomplications. Nasal prongs can also bepositioned to supply the patient with oxygen.

Once the patient is breathing normally, it istime for transport to the recoveryroom/post-operative ward.

6.6 EFFECTS OF ANESTHESIA

Both breathing and circulation are alwaysaffected by general anesthesia. In some cases,regional anesthesia, particularly when it is"high", may also cause a blood pressure drop,as well as affecting breathing.

6.6.1 BREATHING

NORMAL REGULATION OF BREATHING

The volume and frequency of breathing arecontrolled by impulses from a cluster of nervesin the brain stem called the respiratory center.These impulses are governed by informationfrom different receptors in the body: centralreceptors close to the respiratory center andperipheral receptors in the carotid arteries.

The respiratory center and central receptors

Peripheral receptors

The impulses from the central receptorsdepend mainly on the carbon dioxide level inthe blood, which also affects the pH value inthe fluid surrounding the brain and spinal cord(cerebrospinal fluid, CSF).

35


The pH value of the cerebrospinal fluid has adirect effect on the respiratory center, since alow pH (high CO2 level) stimulates breathing,

and a high pH (low CO2 level) causes a

decrease in breathing activity.

The peripheral receptors are also affected bythe pH value of the blood, since low blood pHstimulates breathing.

The Physiology of Respiration (TrainingMaterial Workbook) provides a simplesummary of respiratory anatomy and asomewhat more detailed summary ofrespiratory physiology.

CHANGES IN BREATHING

Certain inhalation agents may cause adecrease in breathing, leading to alveolarhypoventilation.

COMPRESSIBLE VOLUME

Controlled ventilation always entails a certainvolume of gas being compressed in theapparatus and patient tubing, etc.Compressible volume should therefore becalculated and added to minute volume if ithas not already been automaticallycompensated for by the machine.Compressible volume is described in moredetail in chapter 8.

DEAD SPACE

Dead space is defined as the volume in whichno gas exchange takes place. It is generallydivided into anatomic, physiological andapparatus dead space.

All apparatus naturally contributes toincreasing total dead space: a face mask maydouble it, for example (more apparatus deadspace), while endotracheal intubation causesa smaller increase. Dead space is an importantaspect to consider when assessing patientventilation and is also described in more detailin chapter 8.

COMPLIANCE

General anesthesia may alter the elasticity ofthe chest, reducing functional compliance inproportion to the depth of anesthesia. Drugs,type of surgery and body position on theoperating table all affect functional complianceand breathing.

6.6.2 CIRCULATION

Blood pressure drops are common when manyanesthetic agents are administered. Tocounteract such falls, fluids are infused andsometimes vasoactive drugs may also be usedto normalize patient blood pressure.

VAGAL REFLEXES

During general anesthesia, pressure from thesurgeon on certain abdominal organs orstimulation of the airways may cause vagalreflexes, slowing the heart rate, reducingblood pressure and causing a drop in strokevolume.

ARRHYTHMIA

Some inhalation agents affect the heart ratealthough arrhythmia may also be the result ofhypo- or hypervolemia.

36


BRADYCARDIA

This is caused by many drugs, especiallysuccinylcholine, fentanyl, alfentanil andremifentanil.

TACHYCARDIA

Combined with an increase in blood pressure,this may be a sign of poor pain control,insufficient depth of anesthesia or evenawareness.

Tachycardia and low blood pressure, on theother hand, indicate blood loss or incipientshock.

BLOOD LOSS

This is monitored by the anesthetist on anongoing basis and compensated whennecessary with transfusions. Significant lossesare signalled by tachycardia, low bloodpressure and peripheral coldness. If untreated,there may be a further fall in blood pressure.Such losses must be compensated to preventthe development of shock.

37


Agents used in general anesthesia

Sedation/amnesia: benzodiazepines, propofol

Hypnosis during induction: propofol, thiopenthal.

Analgesia: opiods, both traditional (fentanyl) and new faster-acting drugs (alfentanil, remifentabil,sufentanil)

Muscle relaxation: non-depolarizing/depolarizing agents

Amnesia, hypnosis, analgesia (+ some muscle relaxation): inhalation agents

Effects on essential anesthesia parameters ("the triad of anesthesia") of drugs routinely used duringbalanced anesthesia

RelaxationAnalgesiaHypnosis

0++++Nitrous oxide

++++++Volatile inhalationagents

00+++IV barbiturates

+++00Muscle relaxants

0++++Opioids

38


7 KEEPING A RECORD

A record is kept of each patient case on an anesthesia record chart. This documents everythingthat has happened to the patient in the OR.

Records are still by and large paper-based, although computerized systems are now beingintroduced. They have not yet however gained general acceptance. The latest ASA poll (2006)showed that they were not used in more than around 20% of cases in the USA.

The layout of the patient record may differ from one hospital and country to the next, but thechart generally includes the following points:

patient details

date

preoperative assessment (including ASAclassification)

(premedication and preoperative fluids, ifany)

(certain laboratory values)

position of patient on table

peripheral cannulation

choice of airway management

record of gases used

breathing system used

ventilator used

drugs administered

infusion fluids and blood transfusions

blood pressure (BP) every 5 minutes

pulse rate and oxygen saturation every fiveminutes

neuromuscular transmission

ventilatory status (etCO2, respiratory rate,

tidal volume)

temperature monitoring

39

| Keeping a record | 7 |

clinical notes, such as excessive bleeding,allergic reactions, etc.

estimated fluid loss

estimated blood loss

time of anesthesia induction

time at which surgery began

time at which surgery ended

time of anesthesia termination

prescriptions.

40

| 7 | Keeping a record |

41

| Keeping a record | 7 |

8 THE ANESTHESIA MACHINE

Anesthesia machines may differ inappearance, size and degree of sophisticationbut generally speaking, they consist ofsections for

ventilation

source of gases (pipes and/or cylinders)-

- flowmeters/mixer for gas dosage

- vaporizers for storing and dosinginhalation agents

- patient breathing system

- manual ventilation bag and ventilator

- suction device

- gas evacuation

space for monitoring equipment

accessories

storage space

worktop.

It is extremely important to test the anesthesiamachine every time it is used. This involvestesting many of the individual parts beforeeach use, as well as the functioning of thesystem as a whole before connection to thepatient.

Different manufacturers issue differentrecommendations concerning checkoutprocedures, both prior to use and betweenpatients. The same applies to the authorities

in different countries (such as the FDA in theUS and the relevant regulatory authorities inthe EU) and the routines of different hospitalsand healthcare institutions.

42

| 8 | The anesthesia machine |

It is therefore crucial for personnel tofamiliarize themselves with the requisiteprocedures in their own workplaces.

There is a great deal of information for theanesthetist/nurse to monitor throughoutanesthesia, including oxygenation, ventilation,blood pressure, heart rate, muscle relaxation,fluid status, acid-base balance, anesthesiadepth, etc.

8.1 CART OR COLUMN

The cart or column should preferably betailored to the environment of the OR. Thismay involve making it mobile, ceiling-mountedor wall-mounted and equipping it to suitspecific anesthetist or hospital requirements.Space is restricted and it is also an advantageif the machine is lightweight and easy tohandle. On the other hand, the anesthetistrequires a comfortable and ergonomicallyadapted workplace with space for all theequipment needed. The design thereforeinvolves striking a balance between what isdesirable and what is practically feasible.

8.2 SOURCE OF GASES

The various gases used (oxygen, nitrous oxideand medical air) may be piped via the wallfrom central storage cylinders or come fromsmaller cylinders in the OR or on the cart itself.There may also be extra cylinders on the cartin case of failure of the usual source.

Oxygen is stored in cylinders as a compressedgas at a pressure of 20000 kPa. The volumeof decompressed oxygen is thereforeproportional to the pressure indicated on thepressure gauge. Nitrous oxide, on the otherhand, is a liquid that is stored at a pressurelevel of around 5170 kPa or 750 psi, so that itvaporizes in the cylinder. This means that thepressure level indicated on the pressure gaugeis not relevant to the amount of gas left in thecylinder.

Because of the high internal pressure,pressure regulating valves have to be used onthe cylinders.

8.2.1 IDENTIFICATION OF GASES

Correct identification of the gas being suppliedto the anesthesia machine is clearly vital ifpotentially lethal accidents are to be avoided.A number of different measures are taken toensure this:

Color coding of cylinders provides a rapidmeans of identifying their contents.However, the definitive indicator of thecontents is always the label.

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| The anesthesia machine | 8 |

The gas tubing and connectors are colorcoded and the connectors are usuallydesign coded (see below) for specific gasesto avoid any confusion or incorrectconnection.

A gas-specific pin-index system is providedon small cylinders: pins on the yoke of themachine mate with holes drilled in specificpositions on the valve of the cylinder toprovide a mechanical means of preventingincorrect connection. Especially when smallindividual cylinders are used, theseprecautions should be included in thedesign of the flowmeters and/or theventilator.

Gas-specific connectors are used on largecylinders that make it impossible to attacha regulator or fitting to the wrong cylinder.

8.2.2 OTHER SAFETY ASPECTS

Built-in safety systems that automatically shutdown the nitrous oxide supply in case of lowoxygen levels or total oxygen failure arecrucial. They are known as hypoxia guardsand are designed to prevent hypoxic gasmixtures from ever being administered to thepatient even in the unlikely event of theanesthetist mistakenly setting a fresh gas flowof, for example, 100% nitrous oxide.

8.2.3 GAS ANALYZER/OXYGENMEASUREMENT

It is now mandatory for the anesthesiamachine to provide the anesthetist with ameans of sampling the oxygen administeredto the patient. This is the most reliable methodof ensuring that the patient has an adequateoxygen supply and that no errors have been

made in the gas connections. In addition tousing a gas analyzer, it is recommended touse pulse oximetry to monitor the patient'soxygen saturation levels.

8.3 FLOWMETERS

The flowmeter allows the operator to controland know the flow rate of each gas, usually inliters or subunits of liters per minute.

Traditionally, the flow rate and concentrationof the gases are individually set for eachpatient and are often altered duringanesthesia.

The conventional flowmeter is a vertical glasstube that is wider at the top and contains alight float. Each is calibrated for a specific gas.

When the gas is turned on, the float stabilizesand rotates at a height inside the tube wherethe force of the gas flow is equal to the weightof the float. The gases from each flowmeterare then mixed.

Newer flowmeter units have built-in safetyfeatures, including shutting down the N2O

supply and sounding an alarm if the oxygen

44


supply should fail (see above). Many also havea preset minimum oxygen concentration whichvaries from one manufacturer to another,although it never falls below 21%.

Flowmeters are typically pneumatic, butelectromagnetic digital flowmeters are nowgaining popularity.

8.4 VAPORIZERS FOR INHALATIONAGENTS

A vapor formed from the volatile liquidanesthetic agent is added to the gas mixtureby the anesthesia machine's vaporizer, whosefunction is to deliver a safe, reliableconcentration of volatile agent to the patient.The output of older vaporizers is affected bythe flow rate, the ambient temperature andthe amount of inhalation agent, although moremodern vaporizers compensate automaticallyfor variations in these parameters.

In terms of safety, there are unique colorcodes for each agent, a special key fillingsystem with specific design coded adaptersto prevent the vaporizer from being filled withthe wrong liquid, and anti-spill mechanisms.

The vaporizers also undergo regular overhaulsto ensure that the set concentrations do notvary, although in more traditional types ofequipment, the concentrations delivered mayvary at low fresh gas flows. Nowadays, themonitoring of anesthetic agent concentrationin the patient breathing system is thereforestandard, especially during low flowanesthesia.

It is useful to have some understanding of thebasic principles of anesthetic vaporizers,including the principles that affect vaporizeroutput and how they influence vaporizer

design. Generally speaking, there aretraditional simple vaporizers, often of thedrawover type, and more modern precisionvaporizers. The latter are common indeveloped countries and are generally flowand temperature compensated, as well asbeing unaffected by positive pressureventilation. Drawover vaporizers are basic androbust, have a low resistance to flow and sodo not require pressurized gases. Becausetheir performance is variable, accuratecalibration is more or less impossible. Theyare common in countries that have fewerresources to invest in equipment.

Precision vaporizers have developed in recentyears and newer vaporizer designs enablecontrol of the vaporizer by a central processingunit in the machine. The concentration ofvapor is then monitored on an ongoing basisand adjusted by altering the fresh gas flowthrough the vaporizer. Some of the differenttypes on offer now include:

the plenum vaporizer, where the incominggas is accurately split into two streams. Onepasses straight through the vaporizer in thebypass channel, while the other is divertedinto the vaporizing chamber. Gas in thevaporizing chamber becomes fully saturatedwith volatile anesthetic vapor. This gas isthen mixed with the gas in the bypasschannel before leaving the vaporizer.Modern vaporizers of this type are notsensitive to variations in temperature andpressure;

the injection principle vaporizer, where thegas flow is throttled, causing a pressuredifference between the liquid reservoir andvaporizer outlet. The difference isproportional to the degree of throttle and is

45


used to inject the desired amount of agentinto the gas flow. The use of the injectionprinciple allows accuracy to be maintainedregardless of patient tidal volume;

the electronic vaporizer, where electronicallycontrolled valves ensure higher accuracy.

There is also an agent-specific vaporizerconstructed for use with desflurane. Becauseof its unique physical properties, desfluranerequires a special vaporizer in which the agentis heated, pressurized and added directly tothe gas stream.

Below is a diagram of one recent development- an electronic vaporizer with injector system.

Filter

8.5 USING A VENTILATOR

Traditional ventilators used for anesthesia havegenerally been simple, since they were onlyused to replace manual ventilation when theanesthetist needed his/her hands free. Surgeryis rapidly becoming more complex andgrowing numbers of patients, who are botholder and more severely ill than ever before,are now eligible for surgical interventions. Theneed for more advanced anesthesia machineswith ventilators that offer additional and moresophisticated ventilation modes has thereforeincreased.

Ventilators are generally pressure, flow andtime (cycle) regulated. This refers to themechanism used to switch from inhalation toexhalation.

The text and figure below have been adaptedfrom a number of relevant articles listed under"References".

Most recent ventilators used for anesthesiainclude specific design characteristics thathave been more or less standard for all acrossmanufacturers (see figure below). They havea bellows that delivers inspiratory gas whenpressurized by an external gas source(bag-in-bottle design). Exhalation is passiveand there is a circuit to reuse variable amountsof exhaled gas in the inspiratory limb. A freshgas flow (FGF) also contributes to the flowdelivered to the patient, while a CO2 absorberis used to remove CO2 from the gas deliveredto the patient.

This design has remained the same for manyyears, although anesthesia ventilators are nowchanging to meet new demands (see chapter9).

46


CO2

Drive gas

Fresh gas flow

Inspiratorylimb

Expiratorylimb

Ventilator

In terms of safety and alarms, a minimumrequirement for all simple ventilators is thatthey should alert the user in case ofdisconnection or if the upper pressure limit isreached. It should be noted that for infantsand children, special equipment is used.

8.5.1 COMPRESSIBLE VOLUME

Part of the inspiratory minute volume isneeded to compress the gas in the apparatusand tubing and thus fails to reach the patient.This means that the choice of breathingsystem, tubing and ventilator affects thecompressible volume. If the tubing,connectors, valves and canister have a largeinternal volume, a higher fresh gas flow will berequired.

To lower the compressible volume, it is a goodidea, at the machine design level, to positionthe manual ventilation bag or ventilator asclose to the patient as possible. The

compressible volume should be calculatedand added to the minute volume if theventilator does not compensate automaticallyfor it.

8.5.2 DEAD SPACE

Dead space is by definition the volume in theairways where no gas exchange takes place.The alveolar ventilation volume is thus tidalvolume minus dead space.

Anatomic dead space is proportional to thesize of the patient and includes the volumefrom nostrils and mouth down to the upperbronchioles. It is generally around 2 ml/kg ofbody weight and is decreased by endotrachealintubation and tracheostomy.

Apparatus dead space includes many of theaccessories associated with the Y-piece thatare necessary to ventilate a sleeping patient,such as a Y-piece, connection tubes and thehumidifier unit. It is often around 50 ml.

Physiological dead space (which is alveolarplus anatomic dead space) is a dynamicconcept and depends on the efficiency of gasexchange. Poor diffusion capacity increasesphysiological dead space, as do some formsof controlled ventilation and some pathologicalprocesses.

8.6 MONITORING

The monitoring systems used to follow thepatient's vital signs and inspired and expiredgas concentrations are described more fullyin a separate chapter below.

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8.7 GAS EVACUATION ORSCAVENGING SYSTEM

From the point of view of both ecology andthe work environment, gas evacuation is animportant standard feature of most anesthesiamachines.

In the diagram below, the green arrow showsgas coming from the patient (left)and movingtowards the gas evacuation system (right),while the orange arrow (right) shows the inflowof room air when needed.

There are passive through-the-wall systemsor active systems with a central vacuum oractive ducts. The systems also include built-inprotections against subjecting the patient tonegative pressure.

8.8 SUCTION DEVICE

The suction device is a crucial part of theequipment, since all anesthesia entails a riskof aspiration and the consequences for the

patient may be serious. The suction deviceforms an integral part of the equipmentneeded to ensure free airways and managethe patient safely.

The device must therefore have both highcapacity in case of vomiting or regurgitation,and offer lower capacity for simply clearingthe airways. Like all other parts of theequipment, it must be carefully checked beforeuse on each new patient.

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9 DIFFERENT BREATHING CIRCUITS

The function of a breathing circuit is to deliveroxygen and anesthetic agent to the patientwhile at the same time removing carbondioxide from the circuit.

There are several ways of classifying thebreathing circuits or systems used inanesthesia machines. Over time, a wide rangeof names has been used to describe them. Inorder to understand the often confusingnomenclature surrounding these systems, ashort review of the historical background isappropriate. Many of the older classificationsare no longer adequate, particularly now thatgas analyzers have become an essentialfeature of the anesthesia machine. Thishistorical review will be followed by asuggestion for a more suitable practicalclassification of breathing systems.

The selection of a specific system naturallyinvolves assessing the clinical situation (typeof surgery and anesthesia), although it is alsoinevitably connected with personalpreferences, financial priorities and potentialbenefits. This chapter focuses less on theseaspects than on classifying the systems inpractical and functional terms.

9.1 OLDER CLASSIFICATION

Historically speaking, breathing systems havebeen classified in terms of their degree ofopenness to the surrounding atmosphere. Thismeans that they are in principle either open orclosed, with systems in between that arevariously and often confusingly described as

semi-open and semi-closed. The list below isbased on this old-fashioned and nowredundant nomenclature, dividing breathingsystems as follows:

Open systems in which the anesthetic isgiven via the atmosphere (through a maskand/or cloth). This was the first methodused to anesthetize patients.

Inhaled Exhaled

The classic if somewhat outdated exampleis ether administered via a cloth insertedinto an open mask. Totally open systemsare thus primarily of historical interest.

Closed systems in which the anesthetic gasis given via tubing from a reservoir. Thecarbon dioxide in the system is eliminatedchemically and only the amount of oxygenand anesthetic agent that is actuallyconsumed is supplied. Such a system isreminiscent of the breathing circuits usedby certain military divers, for example.

Finally, the terms semi-open andsemi-closed, described by Moyers in 1953,tend to confuse rather than clarify the issueand are therefore best avoided. Moyershimself referred to a system with a reservoirbut without rebreathing as semi-open, while

49

| Different breathing circuits | 9 |

a circuit with a reservoir and partialrebreathing was semi-closed. Differentauthors, however, have meant differentthings when using these terms.

Due to this lack of consistency, thisclassification will not be applied to thepresentation below. In the modern circlesystems presented there, it is the fresh gasflow that determines the degree of openness.

9.2 FUNCTIONAL CLASSIFICATION

A more practical classification dividesanesthetic breathing systems as follows:

non-rebreathing systems, the classicexample of which is the breathing circuit ofan ICU ventilator (see survey below)

rebreathing systems, which are in turndivided into:

- partial rebreathing systems without CO2

absorption, where the fresh gas flowregulates the elimination of carbondioxide

- circle systems with CO2 absorption,

which are by far the most widely usedsystems in clinical practice today.Normally, a circle system is a partialrebreathing system with CO2 absorption.

The fresh gas flow generally plays no partin the elimination of carbon dioxide. Itdoes, however, determine the degree ofrebreathing of oxygen and anestheticagent. It should be noted that a very highfresh gas flow (higher than the inspiratorypeak flow) will transform the circlesystem into a non-rebreathing system.On the other hand, modern gas analyzertechnology has enabled the fresh gas

flow to be minimized without jeopardizingpatient safety. This has resulted in morewidespread use of low flow anesthesia(see chapter 11), which is as close as onegets in clinical practice to a closedsystem (although it is now in fact possibleto construct a truly closed circle systemas well).

The various systems are described andillustrated in the breathing system survey insection 9.3 below.

9.3 BRIEF SURVEY OFCONTEMPORARY BREATHINGCIRCUITS

9.3.1 NON-REBREATHING SYSTEMS

In these systems, only fresh gas is suppliedto the patient and no gas or anesthetic agentis recycled. Valves are used to prevent exhaledgas from entering the inspiratory limb andmixing with inhaled gas. The gas mixture givento the patient usually needs to be heated andhumidified to prevent damage to the airways.

Fresh gas

Exhaled gas

A traditional ICU ventilator is one example ofa non-rebreathing system. Anesthesiamachines using non-rebreathing circuits of

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| 9 | Different breathing circuits |

this type have a "fresh gas flow" equal to theminute volume. This means a highconsumption of anesthetic agents sincenothing is recycled

On the other hand, the ventilation performanceof these systems is often good and they maytherefore be preferred when anesthetizingpatients with severe pulmonary disease andchildren, where the fresh gas flow per se islow.

9.3.2 REBREATHING SYSTEMS - PARTIALREBREATHING WITHOUT CO2

ABSORPTION

These systems use both fresh and recycledgas. Exhaled gas heats and humidifies theinspired gas. The systems have a surplus valveand are generally referred to as Maplesonsystems, where each variant has a differentletter (Mapleson A to F) depending on theposition of the valve and the fresh gas inlet.

These systems require relatively high fresh gasflows, which generally entails a highconsumption of anesthetic agents.

Fresh gas

Re-used gas

Excess gas

One Mapleson variant that is still used is theBain system shown below, which is amodification with an inner tube for fresh gas,The inspired gas is heated by and mixed withexhaled gas close to the mask or tube.Although somewhat old-fashioned and notparticularly sophisticated, the Bain system hasremained popular in many parts of the worldlargely because of its simplicity and ease ofuse, and because the patient receives thesame gas mixture (of fresh and recycled gas)during spontaneous and controlled ventilation.

Fresh gas

Re-used gasExcess gas

There is also the very simple Ayre's T-piecesystem shown below, where the fresh gas flowonce again determines the level of rebreathing.This system is mainly used to ventilate smallerinfants.

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Fresh gas

Re-used gas

Excess gas

Another example of partial rebreathing withoutCO2 absorption is the modification of the

Ayre's T-piece, the Jackson-Rees system. Ituses a reservoir bag for gas and requires afresh gas flow that is 3-5 times higher than theminute volume.

Fresh gas

Re-used gas

Excess gas

9.3.3 REBREATHING SYSTEMS - CIRCLESYSTEMS WITH CO2 ABSORPTION

These systems were designed to use a lowfresh gas flow to economize on the use ofagents and avoiding side-effects. As leakagelessened due to more sophisticatedequipment, anesthetists were quick to see thebenefits of lower flows. The systems use twounidirectional valves, a relief valve, a CO2

absorber, a manual ventilation bag and tubingto connect all the parts, resulting in a simplecircle system that is not totally closed to thesurrounding environment. One example is thecircle system with a fresh gas flow higher thanthe patient's needs. Benefits relate to bothrecycling of unused gas and inhalation agentand automatic gas humidification and heating

A circle system reuses a specific amount ofexhaled gas, passing it through a CO2

absorbent. This eliminates the carbon dioxideand simultaneously heats and humidifies thegas. The latter still contains nitrous oxide andanesthetic agent not taken up by the patientand is mixed with fresh gas as it returns to thepatient.

Fresh gas

Non return valve

Reliefvalve

Excess gas

Absorber

Re-used gas

Manual ventilation

As mentioned above, the circle system can infact be used not only as a partial rebreathingsystem, but also as a non-rebreathing or totalrebreathing system. The amount of rebreathingdepends on the fresh gas flow, although atrates above 4 liters/minute, most of theeconomic advantages of the system are lost.

Using low flows in a circle system doeshowever mean that it takes a while for thealtered settings to take effect. When rapidchanges in fresh gas or inhalation agentconcentrations are required, a high fresh gasflow or a significant increase in anestheticagent concentration is needed.

As mentioned above, the circle system canalso be used for total rebreathing, whichminimizes the amount of exhaled gas thatescapes from the system. This maximizes

52


efficiency with regard to use of gases andvolatile agents, since once steady state isreached, the amounts of fresh gas requiredare very low (see chapter 11).

Re-used gasFresh gas

Absorber

Interest in these systems has grown due toimprovements in the accuracy of gasconcentration measurement and dosage, theelimination of leakage, the increasing costs ofgases and anesthetic agents and growingenvironmental awareness. On the other hand,there are certain safety concerns with a closedsystem. These relate to smaller safety marginsand potential risks connected with theproduction of Compound A (see discussionof sevoflurane in chapter 6).

Fresh gas

Non return valve

Absorber

Manual ventilation

Re-used gas

9.4 THE CARBON DIOXIDEABSORBER

A canister is used to hold the absorbentsubstance, often a mixture known as sodalime. The CO2 reacts with the absorbent,

producing both heat and water to humidifyand warm the rebreathing gas.

There is also an indicator that changes coloras the soda lime becomes depleted. Thecarbon dioxide absorbent canister is illustratedbelow.

9.5 RECENT MODIFICATIONS ANDINNOVATIONS

9.5.1 ADVANCED VENTILATORS WITHBREATHING SYSTEMS

Modern anesthesia machines have improvedconsiderably in terms of their usability in bothoutpatient and intensive care. They have alsobecome increasingly sophisticated. Much ofthe discussion below is based on the relevantarticles listed in the "References".

Today's anesthesia machine ventilators are infact approaching the performance ofventilators used in the ICU. Anesthesiamachines need to deliver a gas mixture of

53


known composition and now also recirculateexhaled gases into the system. Most thereforeinclude a CO2 absorber and a bellows-style

or piston driven ventilator.

This design implies a large internal volume,which leads to progressive decreases in tidalvolume delivery when airway pressuresincrease. Modern ICU ventilators, on the otherhand have much smaller internal volumes.They are thus less vulnerable to high airwaypressure. Accordingly, new anesthesiamachines are now being designed to offerbetter performance to suit critically ill patients.The reduction in internal volume has alsoenabled more precise ventilation of infants andchildren. These technological advances haveled to greater flexibility in perioperative care.

These advances also extend the range ofpatients that can be supported by ananesthesia machine and make new ventilationmodes available to the anesthetist. Thesecombine patient- and ventilator-initiatedbreaths and may enable spontaneousbreathing techniques to be used for longerperiods and facilitate rapid changes inanesthetic depth.

9.5.2 REFLECTOR SYSTEMS

Another new concept is to use a reflectorsystem, which may allow the anesthetic agentto be reused more efficiently. Different typesof reflector system are mentioned in thesediscussions.

One example of such an agent conservingdevice is the volume reflector. It uses areservoir tube in which the anesthetic agentcontained in the exhaled gas is temporarily

stored and recycled. It is based on a verysimple idea that is similar to a bellows, and itenables rebreathing of all gases, includingnitrous oxide.

The idea of reflector systems is to economizeon the use of expensive agents and enhancethe efficiency of the anesthesia process.

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10 MONITORING

During anesthesia, it is obligatory to monitora number of parameters and vital functions toensure patient safety. The type of surgery tobe performed and the health status of thepatient determine how sophisticated themonitoring needs to be. Technologicaladvances have also increased the number ofvariables that can be monitored andanesthesia machines therefore offer a moreor less extensive range of possibilities. Thecrucial aspect here is to provide theanesthetist/nurse with the information he/sherequires, presented in a clear and reliable way.

It is however important to stress that theno-tech or low-tech approach should neverbe forgotten: observe the patient, feel thepulse, watch and listen for signs of distress orother clues to status, even if you have accessto highly sophisticated apparatus.

10.1 OXYGENATION

10.1.1 MONITORING INSPIRED OXYGENLEVELS IN THE BREATHING CIRCUIT

A sample of the contents of the breathingtubes is drawn sidestream into a gas analyzer,which uses O2 sensors based either on a

galvanic fuel cell (inspired O2) or on magnetic

or magnetoacoustic technology to monitoroxygen levels. The sampled air is passedthrough a sensor before being returned to thecircuit. Special care should be taken withinfants and small children, since the analyzer"steals" a large part of the tidal volume fromthe patient.

10.1.2 PULSE OXIMETRY

This non-invasive method of monitoringoxygen saturation is now standard almosteverywhere. Normal SpO2 values are between

95 and 100% in adults, but lower for neonatesand the elderly.

A sensor is applied to the finger, for example,sending pulses of red and infrared lightthrough the tissue. This enables the calculationof the percentage of oxygen-carryinghemoglobin molecules, known as oxygensaturation. The oximeter will also display apulse curve known as a plethysmograph,where the amplitude reflects peripheralperfusion and the pulse rate can also bedisplayed.

Other suitable sites for application of a sensorinclude the ear lobe and foot.

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| Monitoring | 10 |

The reliability of the oximeter is limited inpatients with poor peripheral circulation (e.g.patients with hypothermia or heart disease),while carbon monoxide poisoning may leadto falsely high values.

10.2 VENTILATION

10.2.1 CAPNOGRAPHY(ETCO2)/CAPNOMETRY

Capnometry enables a number of importantparameters, such as end-tidal carbon dioxidelevels, inspired CO2 levels and respiration rate,

to be monitored. Capnography reflects aslightly delayed or real-time CO2 wave form,

while the capnometer presents only values.CO2 levels are affected by both ventilation and

metabolism, making capnography a usefulmethod of following the respiratory process.

Capnography/capnometry is an important toolwhen intubating patients, since any carbondioxide response detected shows that the tubeis correctly positioned in the trachea.

The most common method is based on theuse of infrared sensors that detect infraredlight that is passed through a sample of airwaygas. An example of a normal capnogram isprovided below.

Exhalation Inspiration Exhalation

The sample may be taken using eithermainstream or sidestream technique. Theformer uses a sensor placed in line with theventilator tubing (e.g. between the Y-piece

and the tubing). In the sidestream methoddescribed above, on the other hand, thesampled air is removed and passed througha sensor before being returned to the circuit.

10.2.2 RESPIRATORY RATE

When the patient is breathing spontaneously,the respiratory rate is used to estimate bothventilation efficiency and depth of anesthesia,with a slower rate indicating deeperanesthesia.

There are various techniques for measuringrespiration rate. Impedance pneumography isan older method used in conjunction with ECGmonitoring. The newer method used inconjunction with capnography (see above)calculates the rate on the basis of thecapnogram wave form and is less prone toartifact-induced error caused by movements.

10.3 BLOOD PRESSURE ANDPULSE

10.3.1 NIBP (NON-INVASIVE BLOODPRESSURE)

This is the traditional method of checking apatient's blood pressure. It is routine tomonitor it during anesthesia, generally via acuff on the arm. Normal systolic bloodpressure is between 90 and 150 mm Hg, whilediastolic is between 60 and 80 mm Hg. MeanBP should be in the interval 70-90 mm Hg. Inaddition to the standard manual systems,there are automatic systems for measuring BPwith the help of oscillometry.

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| 10 | Monitoring |

10.3.2 IBP (INVASIVE BLOOD PRESSURE)

This is common during lengthy surgery orwhen the patient's health is poor. It is morerapid than NIBP. It is often used on patientswith heart disease, as well as on critically ill orhemodynamically unstable patients.

The measurements are obtained from acatheter positioned in the patient's vascularsystem, e.g. the radial artery in the wrist formonitoring peripheral arterial blood pressure.Central venous pressure may be monitoredas well (mainly as a reflection of volumestatus), although it is nowadays often used incombination with other information.

10.3.3 SPO2 AND ECG

Pulse oximetry, discussed above, is also auseful method of following the patient's pulse,as is the electrocardiogram, discussed below..

10.4 ECG

The electrocardiogram provides informationabout the electrical activity of the heart and ismainly used to identify various types ofarrhythmia. It is the oldest monitoringparameter and is a standard feature of mostanesthesia machines, although its contributionto minimizing morbidity during anesthesia isrelatively small.

The events recorded by the ECG are the de-and repolarization of the heart. First theelectrical stimulation originating from thesinoatrial node in the right atrium spreadsthrough the atria, causing them to contract(the P wave), then through the ventricles(whose contraction is reflected in the QRSwave). This is followed by the repolarizationprocess (reflected in the final T-wave).

10.4.1 S-T MONITORING

The S-T segment of the ECG represents thephase in the cardiac cycle between the endof depolarization (i.e. contraction of theventricles) and repolarization (i.e. relaxationand refilling of the ventricles).

The segment is monitored to detect elevationsor depression, since these may be early signsof myocardial ischemia (insufficient bloodsupply).

10.4.2 TEE (TRANS-ESOPHAGEALECHOCARDIOGRAPHY)

This is a method used to monitor cardiacfunction, particularly contractility and filling.By following the movements and volume ofthe various chambers of the heart, it enablesthe clinician to treat the patient before anyserious problems arise. In many situations, ithas replaced the use of the more invasivepulmonary artery catheter.

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| Monitoring | 10 |

10.5 MULTIGASANALYZER/ANESTHESIA GASES

The gas monitoring system measures inspiredand expired values of oxygen, carbon dioxide,nitrous oxide and the most commonanesthetic agents (see section on methodsand agents above).

A sample of the contents of the breathingtubes is drawn using the sidestream technique(see above). It is then fed into the gas analyzer,which uses infrared light of differentwavelengths to determine the levels of CO2,

NO2 and inhalation agents. O2, on the other

hand, is monitored using O2 sensors (see

10.1.1 above).

The development of accurate gas monitoringsystems has enabled safe and widespreaduse of low flow anesthesia (see chapter 11).

10.6 BODY TEMPERATURE

The maintenance of normal body temperatureis important in many patients, since it has beenshown to have a positive effect on coagulationand diminish the risk of wound infections.Close monitoring of body temperature isparticularly important in children and patientswhere there is reason to suspect a risk ofeither hypothermia or malignant hyperthermia.

It is best measured in highly perfused tissues(oral, rectal, in the ear) since this is morereliable than measurements taken at the skinsurface. Recent technical innovations alsoallow temperature to be measured either inthe bladder via a specially adapted catheter,or in the esophagus.

10.7 NEUROMUSCULARTRANSMISSION (NMT)/MUSCULARRELAXATION

The monitoring of neuromuscular transmission,NMT, is particularly important whenadministering and titrating muscle relaxantsand reversing their effects.

The degree of muscle relaxation is animportant parameter and there are severalmethods available nowadays for monitoringit.

The method most commonly used is Train ofFour, or TOF, monitoring, in which themagnitude and type of neuromuscularblockade are measured. Four electricalcurrents are applied for 2 seconds to aperipheral motor nerve and the ratio of theamplitude of the fourth evoked mechanicalresponse to the first one is observed. If theresponse decreases over time, the patient isstill under the influence of muscle relaxants.

10.8 BIS/ENTROPY/AUDITORYEVOKED POTENTIAL (AEP)

Electroencephalography (EEG) measurescortical activity in the brain and the patternschange during sleep and anesthesia. It cantherefore be used as an indicator of the depthof anesthesia.

Several methods can also be used to processthe information obtained from an EEG so asto make interpretation easier, the mostsuccessful of which is described below.

The bispectral index (BIS) monitor provides adimensionless number obtained fromautomatic analysis of EEG waveforms. Thisnumber ranges from 0 (EEG silence) to 100

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| 10 | Monitoring |

(fully awake adult), while numbers between 40and 60 are expected during generalanesthesia. BIS monitors the depth ofanesthesia, enabling the appropriate titrationof anesthetic drugs. This makes for fasterwake-ups and better recovery, and reducesintraoperative awareness.

Consciousness toUnconsciousness

Unconsciousness to Consciousness

100

80

60

40

20

0

BIS

Time

BIS measurements can help healthcareprofessionals tailor the type and dosage ofanesthetic or sedative medication to the needsof each patient. The method is however notyet widely used. There are several sources oferror and the BIS monitor follows trends ratherthan absolute values.

10.9 SVO2

SvO2 is a form of invasive pulmonary artery

oximetry that measures venous oxygencontent, or saturation, in the blood returningto the heart. This gives an indication of oxygendemand and consumption, with normal valuesbetween 60 and 80%.

To obtain these measurements, a very fine(Swan-Ganz) catheter is inserted into a centralvein and led into the pulmonary artery.Spectrophotometry is used to estimate theoxygen content in the blood. This method alsoprovides an opportunity to monitor cardiacoutput (see below).

ScvO2 (where "c" stands for cava) is a form of

invasive central venous oximetry that involvesusing a central venous catheter placed in the

vena cava. This type of catheter is increasinglyused in place of a pulmonary artery catheter,since the pressures measured using bothmethods are considered to be approximatelyequal.

10.10 CARDIAC OUTPUT (CO ANDCCO)

This is a measure of the amount of bloodpumped by the heart and is generally statedin liters per minute. Normal values lie between4 and 8 liters per minute for healthy adults andare affected by vascular resistance, heart rateand contractility.

Thermodilution is the method generally usedto measure cardiac output with the help of apulmonary artery catheter, often in conjunctionwith SvO2. It results in a curve from which the

desired volume may be calculated. A morerecent technique measures the outputcontinuously (CCO).

Non-invasive and less invasive methods ofmonitoring cardiac output and otherhemodynamic parameters by using specialalgorithms have now been developed to avoidinsertion of a Swan-Ganz catheter in thepulmonary artery.

10.11 URINE OUTPUT

Urine output is measured to help monitor fluidbalance and circulation status. This mayinvolve the insertion of a catheter. Urinevolume is the most important parameter tonote. Ultrasound is now also a commonmethod of monitoring residual urine volumein many recovery rooms.

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10.12 BLOOD LOSS

This may be either measured or estimated andmay be actively compensated with the helpof transfusions, if necessary, to raise bloodpressure, enhance oxygenation and preventvascular collapse. Even small losses aregenerally compensated in children, althoughthe attitude to compensation is nowadayssomewhat more restrictive in adults.

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11 LOW FLOW ANESTHESIA

Low flow anesthesia has gained ground inrecent years and has become increasinglyaccepted as the preferred mode of anesthesia.It requires a reliable gas monitoring systemand a rebreathing system with a CO2 absorber.

It is as close as one can get in clinical practiceto a closed system. Although the extent ofrebreathing and the fresh gas flow used mayvary (low flow, minimal flow and closed systemanesthesia), the basic principle remains thesame.

The benefits of low flow anesthesia are bothfinancial (cutting costs of increasinglyexpensive anesthetic agents) andenvironmental. During low flow anesthesia, itis crucial to analyze the oxygen, carbondioxide and anesthetic agent in the breathingsystem. Now that reliable gas analyzers areavailable, low flow anesthesia is as safe asanesthesia using a non-rebreathing system.

11.1 PRINCIPLES OF LOW FLOWANESTHESIA

Low flow anesthesia involves the use of bothintravenous and volatile agents. The preciselimits of low flow anesthesia are not clearlydefined, but a fresh gas flow below 2 liters isgenerally considered low flow in the US, whilein the EU, low flow is generally below 1 liter.

The technique is based on the patient'sphysiological needs. The proportion ofrebreathing and desired fresh gas flow settingare adjusted to the patient's status (oxygenconsumption and CO2 production, etc.)

The uptake of N2O and other inhalation agents

is high in the first few minutes of anesthesia,falling once the tissues become saturated.

When uptake falls, the concentration at thevaporizer may be lowered to ensure that theexhaled amount does not rise. One alternativehere is to allow the concentration differencebetween inspired and expired gas to decreasethe fresh gas flow. The exhaled gas is usedfor rebreathing once it has passed a CO2

absorber. This means that both fresh gas flowand the quantities of volatile agents used canbe decreased.

Accurate monitoring is essential to avoidcomplications and help the anesthetist tailorthe anesthesia to the needs of the patient.

11.2 LOW FLOW TECHNIQUE

While there are no general rules foradministering low flow anesthesia, onepossible procedure is outlined below.

11.2.1 PREOXYGENATION

Extra oxygen is administered for some 3-5minutes, usually via a face mask. Thoroughoxygenation is still the best safeguard againstcomplications. The patient is generally put tosleep with an intravenous agent during thisphase and if necessary intubated.

11.2.2 HIGH FLOW - INDUCTION TOSTEADY STATE

A fresh gas flow using standard flows ofoxygen and nitrous oxide/air is given for some6-10 minutes. Nitrogen from the body's ownnitrogen stores is gradually eliminated from

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the body and replaced by nitrous oxide.Although nitrous oxide is still used relativelyoften, the current trend is pointing towards adecline in its use.

11.2.3 MAINTENANCE

When the uptake of gases stabilizes and thepatient is comfortably asleep, oxygen is alwaysadministered as required by the patient.

The nitrous oxide flow depends on uptake andthe inspired oxygen fraction, as showncontinuously on the monitor.

With regard to inhalation agents, the vaporizeris initially used in the traditional manner (i.e.high flow) for 6-10 minutes. As the patient'svolatile agent requirement decreases, the flowcan be set to a lower value for maintenancepurposes.

The patient is carefully monitored throughoutanesthesia, and the various values can beadjusted to suit changing patient needs.

Maintenance - low flow

Steady state - high flow

Induction - high flow

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11.2.4 ELIMINATION AND RECOVERY

High flow and increased oxygenconcentrations are once again administeredfor 5-10 minutes to eliminate the anestheticagent. The nitrous oxide has already beenturned off and the extra oxygen supply shouldbe maintained for at least 5-10 minutes afterrecovery to avoid diffusion hypoxia.

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12 SAFETY CONSIDERATIONS

In anesthesia, the patient is dependent on theskill and experience of healthcareprofessionals and on high standards of safety,as well as built-in precautions in the equipmentthey use. Here, as elsewhere, prevention ofpatient injury is crucial.

The anesthesia machine is always checkedbefore use, including the patient breathingsystem and suction device. As mentionedabove, it is crucial for personnel to bethoroughly familiar with the relevant routinesgoverning safety and checkout procedures.All members of staff should be updated on thelatest developments on an ongoing basis.Warnings, cautions, recommendations andinstructions concerning intended use shouldalso be followed at all times

Although incidents are rare and complicationsunusual, there are a number of risksassociated with anesthesia of any kind andboth personnel and patients should be awareof these. While every effort is made tominimize them, anesthesia can never be totallyrisk-free.

12.1 RESPIRATORYCOMPLICATIONS

Respiratory complications may arise duringthe induction and recovery stages of generalanesthesia. Obstruction of the upper airways,laryngospasm and bronchospasm, may occurand an assessment of the risk of airwayproblems should therefore be performedbefore anesthesia.

Anesthesia has an effect on spontaneousbreathing, which is normally stimulatedprimarily by an increase in carbon dioxide

levels. This regulatory system becomesincreasingly insensitive to raised carbondioxide levels as anesthesia deepens. Thismay lead to carbon dioxide retention. Theregulatory system also gradually becomesinsensitive to lack of oxygen as depth ofanesthesia increases.

External apparatus dead space must alwaysbe considered, since face masks may doublethis dead space, while intubation decreasesit.

One reason for reduced compliance may bethe use of surgical retractors and abdominalpacks that press the diaphragm upwardsduring open abdominal surgery. Even theposition of the patient (such as theTrendelenburg position) may have a negativeimpact on breathing, as may some of thedrugs used during anesthesia. The use of largeamounts of intraabdominal carbon dioxideduring laparoscopy may also affect bothrespiration and circulation.

12.2 CARDIOVASCULARCOMPLICATIONS

Falls in blood pressure are primarily treatedby increasing the fluids infused or tipping thepatient to lower the head below the level ofthe rest of the body (Trendelenburg position).If these procedures should fail, patient bloodpressure may also be raisedpharmacologically.

Vagal reflexes are most common at thebeginning and end of general anesthesia.Sensory stimulation normally leads to anincrease in the heart rate. Vagal stimulation ofvarious kinds may however cause bradycardia

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and a lowering of stroke volume and bloodpressure. Pressure on certain abdominalorgans during surgery may also cause vagalreflex activity.

Tachycardia accompanied by high bloodpressure may also be a sign of inadequateanesthesia.

Rapid changes in the concentration ofdesflurane may also cause tachycardia andduring isoflurane or desflurane anesthesia,tachycardia may cause coronary ischemia.

12.3 NERVE AND OTHER INJURIES- THE IMPORTANCE OF PATIENTPOSITIONING

Anesthesia depresses the autonomic nervoussystem and thus the capacity to make thephysiological adaptations needed whenchanging body position. Nerve injuries mayresult from pressure or strain or be due to thefact that the patient has been anesthetized.Anesthetists and surgeons should thus beaware of the effects of the most commonsurgical positions, the areas vulnerable toinjuries, and the precautions needed toprevent them.

The main positions of interest are:

the supine or horizontal position, in whichmost patients are anesthetized and thenrepositioned if necessary;

the head-down position, including thefamous but now little-used Trendelenburgposition with the head and body tilteddownwards 45 degrees from horizontal.Most head-down positions involve a tilt of10-15 degrees in combination with variousanglings of the lower body;

the lithotomy position, with a 90 degreeflexion of both hips and knees andabduction of the hips, mainly used for pelvicand perineal surgery;

the prone position, which involves lying onthe stomach;

the lateral position, with the patient lying oneither the right or the left side;

a number of more specialized positions,including, for example, the modified sittingposition and the lumbar spinal surgeryposition, sometimes modified as theknee-elbow position, which may involve theuse of a special operating table for spinalsurgery.

Safe positioning of patients involves teamwork between anesthesiologist, surgeon andnurse. All aspects of positioning should beplanned in advance and tasks assigned, andthe accessories needed should be checkedbeforehand.

Positions associated with major physiologicalchanges (like the modified sitting position)should be achieved in a stepwise fashion,checking hemodynamic and other parametersand adjusting anesthesia depth. The positionof the endotracheal tube or laryngeal maskshould also be checked. Padded cushionsshould be kept under areas vulnerable fornerve compression. Bony areas may be left incontact with the mattress. However, if thesurgical time is likely to be prolonged, or if thepatient is likely to be hypothermic orhypotensive, they also need to be wellpadded. It is preferable to have all the jointsin the body (except the ankle) in minimalflexion. During anesthesia, the eyes should be

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kept closed and the ears may be protectedwith the help of ear plugs. Finally, the patientshould be covered as much as possible sothat heat loss is minimized, except in the caseof artificially induced hypothermia (used foropen heart surgery, for example).

12.4 VOMITING ANDREGURGITATION

Anesthetized patients always run the risk ofvomiting and regurgitation, particularly duringinduction if the airway is not free and gas ispumped into the stomach.

Endotracheal intubation diminishes the risk ofaspiration, although the laryngeal mask doesnot. There is also a higher risk of vomiting afteranesthesia and surgery, although the patientis then conscious and generally able to moveand communicate.

12.5 SHIVERING

Shivering may occur after all generalanesthesia and is not necessarily due to heatloss.

Shivering greatly increases oxygenconsumption, causing hypoxia, hypercapniaand even acidosis. Moreover, oxygentransportation often decreases duringshivering. The treatment involvesadministration of pure oxygen and sometimesvery small doses (10-20 mg) of pethidine aregiven intravenously with almost immediateeffect.

12.6 AWARENESS

This may occur if the patient is not fullyunconscious and hears sounds or words, oreven feels pain, without being able to

communicate his or her condition. It isgenerally associated with the use of musclerelaxants, while technical failures rarely causeawareness.

Inhalation anesthesia without the use ofmuscle relaxants minimizes the risk ofawareness, as long as the induction phase isadequately completed and sufficient time isallowed for the agent to equilibrate andprovide full anesthetic effect.

12.7 MALIGNANT HYPERTHERMIA

Some patients have a rare geneticpredisposition to develop malignanthyperthermia (or hyperpyrexia). When volatileagents are used, body temperature andmetabolism both rise dramatically in thesepatients. Increased metabolism is reflected inhigher etCO2 values. Suxamthonium may also

be implicated in malignant hyperthermia.

All inhalation agents appear to produce theseside-effects, which may, if they go untreated,ultimately cause death. The complication isvery rare and runs in certain families.

12.8 GAS LEAKAGE

Anesthesia machines may occasionally haveproblems with leakage, which may occur fromvaporizers, flowmeters, patient tubing,connectors or any of the other componentsof the apparatus. All leakage should beavoided to maintain safety in the OR andensure environmental friendliness. The use oflow flow anesthesia has been shown to reduceoccupational exposure to anesthetic agents.

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By performing a checkout procedure prior tolow flow anesthesia, leakage can be detected.Low flow anesthesia may not be used ifleakage is higher than 100 ml.

12.9 AIR POLLUTION

In theory, nitrous oxide has a negative impacton the environment, both at the OR level vialeakage and at the global level, since it is apotent greenhouse gas. The effect of nitrousoxide emissions from hospitals is howevernegligible when compared with emissionscaused by other human activities. There is stillinsufficient research into the long-term effectsof lengthy exposure to volatile anestheticagents, although regulations involving verylow occupational limits are in place so as toprotect personnel from all unnecessaryexposure.

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12.10 ELECTRICAL HAZARDS

The most common source of malfunctionscaused by electric current is surgicaldiathermy, which may cause serious failuresin medical devices and interrupt theirfunctioning.

There is a also a general recommendation toavoid the use of cell phones in the vicinity ofmedical devices as the signals may interferewith the way the equipment functions.Routines may however vary from one countryor hospital to another. Modern mobiletelecommunication systems also carry a muchlower risk of interference than older systems.

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13 SPECIAL APPLICATIONS

Anesthesia is used in the OR not only forroutine surgery, but also for special types ofsurgery that involve special anesthesiologicalrequirements. Pediatric care, neurosurgery,transplantation and cardiovascular surgery arejust a few examples. Anesthesia is also usedin other parts of the hospital, such as forsedation within intensive care (the ICU) or inMR environments. This section presents someof these special applications, although readerslooking for more in-depth information shouldconsult specialized works on anesthesia (seeReferences).

13.1 PEDIATRIC ANESTHESIA

It is not just in size that children and infantsdiffer from adults. The following sections coverthe main points to remember whenanesthetizing children.

13.1.1 AIRWAY ANATOMY

The mucous membranes are more sensitiveto trauma in children. In addition, theappearance of the larynx differs, with thenarrowest point located just below the vocalcords rather than at their level.

This means that when endotracheal tubes areused, the cuff is not inflated so as to avoid therisk of trauma and swelling. However, this mayincrease the risk of leakage, although this isnot generally a problem in practice.

13.1.2 DEAD SPACE

In relative terms, there is more dead spacewhen a child is anesthetized than when anadult is. The space should therefore be limitedwherever possible and compensation shouldbe made for it.

13.1.3 BREATHING

The breathing rate is faster in children, whoare often mouth-breathers. This must be takeninto account when using a face mask since itmay make it more difficult to maintain a freeairway.

13.1.4 HYPOVENTILATION

Children are more susceptible and sensitiveto hypoventilation. Obstruction of the airwaysis quick to result in bradycardia and respiratoryacidosis.

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13.1.5 COMPRESSIBLE VOLUME

This should be minimized when children areanesthetized, which makes the choice ofbreathing system and ventilator crucial.

13.1.6 CIRCULATION

Children's heart rate is faster, their bloodpressure lower and their cardiac output highin relation to body weight. Even small bloodlosses may be significant in children and aretherefore compensated. The aim should be toavoid hypovolemia and maintainnormovolemic patients.

13.1.7 METABOLISM

Children have a higher metabolic rate thanadults, which means that they break downdrugs and pharmaceuticals more rapidly. Theconcentration of anesthetic gases thusgenerally needs to be raised.

13.1.8 BODY TEMPERATURE

Children's body temperature, especially thatof neonates and infants, is less stable thanthat of adults. It is therefore important tomonitor body temperature more closely. Oftena hot air warmer is used to maintain normalbody temperature.

13.2 ANESTHESIA FORNEUROSURGERY

The focus here is on lowering intracranialpressure, which means special demands onanesthesia, including:

maintaining free airways at all times;

adequate oxygenation and CO2 elimination.

Previously, hyperventilation was used tolower intracranial pressure, although thetendency now is to strive fornormoventilation;

suitable intravenous infusions using agentssuch as cortisone and hypertonic fluids toreduce edema in the brain and thus lowerpressure;

maintaining stable blood pressure withoutfluctuations;

careful patient positioning, sometimes inthe sitting position, and the use of specialdrugs and accessories to enhanceperipheral circulation;

using suitable anesthetic agents and drugssuch as muscle relaxants to prevent thepatient from coughing or straining.

13.3 ANESTHESIA FORCARDIOVASCULAR SURGERY

Thoracic surgery imposes special demandson the anesthetist, including:

effective ventilation/respiration must, asduring all surgery, be maintained;

it must be possible to ventilate the lungsindividually during lung surgery;

equipment to counter induced hypothermiamust be at hand;

special cardiovascular drugs must beavailable in case of emergency;

pleural drains are almost always needed;

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it is even more important than usual for thepatient to have an adequate cough reflexshortly after recovery so as to avoidpostoperative complications.

13.4 ANESTHESIA DURINGLAPAROSCOPY

This type of surgery using a laparoscope isincreasing in popularity and more and moreinterventions are now performedlaparoscopically.

The technique requires the patient to beslightly tipped and the abdomen to be filledwith CO2. Although it requires special skills in

the operating surgeon, it enables relativelycomplicated surgery to be performed withoutlarge skin incisions. The high level of visualdetail contributes to enhancing accuracy andpatient safety.

It may also give rise to a number ofphysiological effects including:

major changes in compliance

CO2 retention and embolism

risk of bradycardia and arrhythmia

other cardiovascular effects caused by therise in intraabdominal pressure

pneumothorax.

13.5 ANESTHESIA DURINGARTHROSCOPY

The patients here are often relatively healthyoutpatients, necessitating rapid recovery fromanesthesia if regional anesthesia, often themethod of choice in such cases, is not used.

This in turn means that inhalation anesthesiausing agents eliminated via the lungs and withlow solubility is generally preferred whengeneral anesthesia is given. The idea is toavoid long stays in hospital. Short-acting IVagents such as propofol are also popular.

13.6 ANESTHESIA DURINGTRANSPLANTATION

The special demands imposed here dependon the organ to be transplanted and include:

blood flow and perfusion have to beadequate and constant before, throughoutand after surgery;

total muscular relaxation is often used;

special drugs must be at hand.

13.7 ANESTHESIA IN BURNS UNITS

Burns patients are often anesthetized duringpainful bandaging and may face specialcomplications, including:

frequent hypovolemia

serum potassium levels may increase duringanesthesia if depolarizing muscle relaxantsare used, causing large amounts ofpotassium to be released from cells indamaged tissues, with an ensuing risk ofcardiac arrest

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a high risk of respiratory distress

a high risk of circulatory instability

frequent septicemia.

13.8 ANESTHESIA FOR EAR, NOSEAND THROAT SURGERY

ENT anesthesia techniques may differ fromthose used in other specialized fields. Themost important aspects include the sharedairway, the difficult airway, emergence andextubation, special airway equipment, apneaand rapid case turnover. Preoperativeassessment is crucial, particularly predictionof difficult intubation. Patients may be of allages and include those with:

stridor

intubation difficulties

sleep apnea

concomitant diseases.

Special points to remember include:

The use of local techniques and surfaceanalgesia;

The special nature of ENT emergencies(including croup, epiglottitis and foreignbodies);

Frequent use of laryngoscopy andbronchoscopy;

The need for familiarity with special tubes,gags and equipment for microlaryngoscopy,bronchoscopy´and laser surgery (e.g.Venturi devices, ventilating bronchoscopeand fiber optic bronchoscopy);

The use of HFO, ventilation with highfrequency oscillation;

Middle ear surgery may involve specialhypotensive techniques;

Major head and neck surgery sometimesentails special positioning;

Emergency airway management may beneeded, including tracheostomy;

ENT anesthesia is often pediatricanesthesia;

ENT anesthesia often involves assessingand managing the difficult airway, includingfiber optic intubation;

ENT anesthesia may also involve managingmajor facial injuries.

13.9 ANESTHESIA FOROUTPATIENTS

General anesthesia is frequently offered toparticularly sensitive patients who suffer fromsevere anxiety or are afraid of pain. In addition,it is becoming increasingly popular to performsurgery on outpatients who are sent home thesame day as surgery is performed. Thisimposes special requirements including:

use of fast-acting drugs

relatively healthy patients

rapid recovery

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effective analgesia.

13.10 ANESTHESIA IN MRENVIRONMENTS

Many patients, and especially children, findMRI examinations difficult to cope with, sincethey have to lie still for long periods inside themachine. As the use of MRI equipmentbecomes more widespread, the need toanesthetize certain patients is increasing.

This involves the same kind of requirement aslisted above for outpatients, but because theMR environment is a highly specialized one,it also imposes specific requirements on theequipment used there, which must be capableof withstanding the powerful magnetic fieldsused. Another requirement is that suchequipment should not affect the MRapparatus.

13.11 ANESTHESIA IN THE ICU

Anesthesia in the ICU focuses mainly onsedation and analgesia. It is often necessaryto sedate patients or anesthetize them whilecertain procedures are performed, or even forlong periods while they undergo treatment.These patients are often extremely ill andrequire specialized anesthesiological care.Generally speaking, an ICU ventilator is usedand intravenous anesthetic agentsadministered via a central line.

The needs of the ICU are imposing growingrequirements on the equipment used there. Ithas been suggested that special anesthesiamachines could be developed with access tosophisticated modes of mechanical ventilationand the highest levels of reliability for use inthe ICU.

When patients who are agitated or anxiousrequire sedation ahead of difficult procedures,ICU personnel need to consider various meansof sedation and pain relief so as to meet thepatients' needs.

The following are some of the indications forsedation in the ICU:

facilitation of mechanical ventilation/airwaymanagement

pain relief

fear and/or anxiety

sleeping problems

amnesia during neuromuscular blockade

control of agitation

The drugs most commonly used includebenzodiazepines, propofol and opioids (foranalgesia).

13.12 ANESTHESIA AND THEOLDER PATIENT

The elderly population is growing as peopleare living longer. In Europe, Japan and theUSA, people over 65 represent 15% or moreof the population and many of them seeksurgical care. The increased risk involved inhospital care of older patients is related bothto their age and to the fact that more and moreof them are seriously ill when hospitalized.

Although the ageing process is highlyindividual, it is often characterized bydegeneration of organ systems and tissuesand loss of functional reserves, as well as adecreasing ability to cope with anesthesia andsurgery. The systems most affected include

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the respiratory, cardiovascular, renal, andcentral nervous systems. It is also importantfor the anesthetist to remember that patients'skin, dental and nutritional status should betaken into account.

13.13 ANESTHESIA ANDMATERNITY CARE

Obstetric anesthesia and analgesia constitutethe only area of anesthetic practice where twopatients are cared for simultaneously.Pregnancy is a physiological rather than apathological state and patient expectationsare therefore high - the mother expects fullinvolvement in her choice of care.

The majority of the workload is the provisionof analgesia in labor and anesthesia fordelivery, although multidisciplinary care forsick mothers is increasingly important.Knowledge of techniques such as epiduraland spinal anesthesia is essential, as well asfamiliarity with and planning for obstetricemergencies, should they arise.

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13.14 ANESTHESIA IN EMERGENCYCONDITIONS

Whenever there is a risk that the patient'sstomach may not be empty (emergencysurgery or other unforeseen situations), theanesthetist will generally choose rapidsequence induction (described in more detailin Chapter 6).

Anesthetists and other healthcareprofessionals may also be called to the scenesof accidents and emergencies to take care ofpatients on site. This involves familiarity withroutines that apply in the pre-hospitalenvironment, pre-hospital analgesia andpractical methods for rapid patientassessment. Special drugs, equipment andprocedures may be used and multidisciplinarytrauma teams are often formed. Thesespecialists are often experts on advancedairway management, intravenous and otherforms of cannulation, thoracotomy andlaparotomy.

One of the main complications among traumapatients is shock, or acute circulatory failure,with hypovolemia as the most common cause.

Treatment routines in case of trauma will ofcourse differ in different countries andhospitals, as will access to the mostsophisticated equipment for saving lives.

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13.15 ANESTHESIA IN THEDEVELOPING WORLD

In 1960, the per capita gross domestic product(GDP) of the 20 richest countries was 18 timesthat of the 20 poorest. By 1995, the gapbetween the richest and poorest nations hadmore than doubled to 37 times. Only a smallminority of the world's population have accessto a computer, and there is also a strikingdisparity between rich and poor nations.According to the InternationalTelecommunications Union, there were 61.1personal computers per 100 people in NorthAmerica as compared with 6 in the EastAsia/Pacific or Latin America/Caribbeanregions in 2001. Access to qualified medicalcare is likewise unevenly distributed at theglobal level. Most of the sophisticatedequipment and high standards of care that wetake for granted in the western world areunavailable to people in developing countries.

There is a great need to collaborate withdeveloping nations on the education ofanesthetists and to work with governmentsand other agencies. In many cases,collaboration involves assistance in the formof advice and even of trained and experiencedspecialists who can provide help whereneeded. There is also a tremendous need fortraining and personnel, as well as material andequipment. Fortunately, some such efforts arenow being made as attitudes begin to changeand progress is made.

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14 GLOSSARY AND ABBREVIATIONS

Medical professionals often use terms andexpressions that may be unfamiliar tooutsiders. They also sometimes assignspecialised meanings to otherwise familiarwords and this may require explanation. As isthe case with all specialist terminology, its usemay be more justifiable in some cases than inothers. As a rule, however, excessive use ofjargon is a barrier to understanding, ratherthan an aid.

Health professionals in general also have anunfortunate tendency to use acronyms andabbreviations that occasionally make theirlanguage obscure or difficult to interpret.Some of these terms and abbreviations areexplained below, although there are of courselocal variations that are beyond the scope ofthis short glossary.

Acidosis - condition characterized by a lowpH (acidity) caused, in the case of respiratoryacidosis, by accumulation of CO2 due to

inadequate respiration, as opposed tometabolic acidosis, where the excess acid isdue to metabolic processes.

Alveoli (plural form of alveolus) - the smallestspace units in the lungs formed by the terminalair-filled sacs at the end of the bronchioles,where gas exchange takes place.

Analgesia - originally a state of painlessness,although it now mainly means pain relief.Analgesics are thus drugs used to relieve pain.

Amnesia - lack of memory.

Anesthesia - a state of being "withoutfeeling", insensibility to most external stimuli,including pain.

Angina pectoris - chest pain originating fromthe heart, sometimes referred to (in English)simply as angina. It is generally a sign ofmyocardial ischemia (inadequate blood supply)and is a relatively common symptom,particularly after stress or effort, in patientswith known heart disease.

Anticoagulants - pharmaceuticals designedto prevent, suppress or delay the clottingprocess in blood.

ASA - American Society of Anesthesiologists.In medical charts and case histories, the termmay also be used to refer to acetyl salicylicacid, a drug generally known as aspirin.

Aspiration - accidental sucking in of foodparticles or fluids into the lungs.

Awareness - a condition in which the patientis not fully anesthetized and may therefore beaware of some of the things that go on in theOR without being able to communicate thisfact.

BGPC - blood/gas partition coefficient, ameasure of the solubility of an anestheticagent in blood.

BMI - Body Mass Index, a key index forrelating body weight to height and assessingobesity. BMI is calculated by dividing aperson's weight in kilograms (kg) by theirheight in meters (m) squared.

Bradycardia - a slower heartbeat than isnormal, i.e. generally < 50 in adults.

Bronchospasm - spasmodic contraction ofthe bronchi, as in asthma.

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Capnometry/Capnography - monitoring ofpatients' inhaled and exhaled carbon dioxidelevels.

Carbon dioxide retention - inadequateelimination of carbon dioxide, CO2.

Cardiac output (CO) - the effective volumeof blood pumped out by the heart per unit oftime (l/min).

Catecholamines - amines derived from aspecial amino acid (tyrosine), examples ofwhich include adrenaline, noradrenaline anddopamine. ´They act as hormones orneurotransmitters, raising blood pressure andheart rate, among other things.

CCO - continuous cardiac output. Definitionas for "Cardiac output" above, but monitoredcontinuously.

Coagulation and hemostasis status - a panelof tests that provide information aboutcoagulation status and bleeding tendencies.

Compliance - a measure of the elasticity ofthe lungs and thoracic wall, expressed as thevolume change per unit change in pressure.

Compressible volume - the part of theinspiratory minute volume needed to compressthe gas in the apparatus and tubing andtherefore not reaching the patient.

COPD - chronic obstructive pulmonarydisease, comprising any disorder thatpersistently obstructs bronchial airflow. COPDmainly involves two related diseases - chronicbronchitis and emphysema. Both causechronic obstruction of air flowing through theairways and in and out of the lungs. Theobstruction is generally permanent andbecomes worse over time.

Coronary steal - the detrimental redistributionof coronary blood flow whereby blood isdiverted from underperfused areas towardbetter perfused areas.

Cricoid cartilage - ringshaped cartilageforming the lower and back part of the larynxat the level of C6.

CSF - cerebrospinal fluid, which surroundsthe brain and spinal cord.

Dead space - for each breath taken, there isa part that is not involved in gas exchange.There are three types of dead space -anatomic (the trachea and bronchi, which haveno alveoli), apparatus or external (theendotracheal tube and other pieces of tubingwith bidirectional gas flow), and physiologic(the apical areas of the lung with ventilationbut no perfusion).

Diathermy - the use of electrocautery forcutting and coagulation or cauterization, asfor sealing a blood vessel, resulting in localtissue destruction. Diathermy may be eithermonopolar or bipolar.

Edema - accumulation of fluid in serouscavities (e.g. lungs) or connective tissue.

ECG - electrocardiography uses electrodesplaced on the body to monitor electricalactivity in the heart. The ECG curve has acharacteristic appearance that also providesinformation about the condition of the heartmuscle.

EEG - electroencephalography useselectrodes placed on the head to monitoractivity in the brain cortex, which is thengraphically displayed in the form of waves.

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Elective - a procedure that is planned andscheduled, as opposed to emergencyprocedures.

Emphysema - a pathological increase in thesize of the alveoli, involving their distension orrupture and leading to progressive loss ofpulmonary elasticity. It is a commoncomplication among smokers.

Endocrine disease - disease involvingdisorders of the endocrine glands andhormones of the body, for example, andincluding such conditions as diabetes andthyroid disease, as well as many less commondisorders.

etCO2 - end-tidal concentration of carbon

dioxide.

ETT - endotracheal tube used for endotrachealintubation.

Extubation - removal of an endotracheal tube.

FA - alveolar concentration (or fraction) of a

gas.

FI - inspired concentration of a gas.

FA/FI ratio - the change in this ratio over time

describes the wash-in curve of a gas such asan anesthetic agent (and thus its solubility inblood).

FGF - fresh gas flow.

FOB - fiber optic bronchoscope. a flexibleinstrument used to view the trachea andbronchi.

Hemoglobin - the molecule in red blood cellsthat transports oxygen.

Hypercapnia - a condition characterized bya higher than normal level of carbon dioxidein the blood.

Hyperpyrexia - see hyperthermia.

Hypertension - elevated arterial bloodpressure.

Hyperthermia - a condition, sometimesknown as pyrexia, in which body temperatureis elevated.

Hyperventilation - ventilation that exceedsnormal physiological requirements.

Hypnosis - although now use in a slightlydifferent sense, the term originally refers to asleeplike condition.

Hypnotic agent - a drug that puts patients tosleep.

Hypotension - low arterial blood pressure.

Hypothermia - a condition in which bodytemperature is lowered.

Hypoventilation - inadequate or reducedventilation that does not meet physiologicalrequirements.

Hypovolemia - decreased volume ofcirculating fluid in the body.

Hypoxia/hypoxemia - condition in which O2

levels in the blood are reduced.

Icterus - jaundice.

Intrathecally - drugs given intrathecally areadministered into the cerebrospinal fluidsurrounding the spinal cord and brain.

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Intubation - insertion of a tube, generallyendotracheal.

Laparoscopy - examination of intra-abdominalorgans using a laparoscope. The term is nowoften used to denote surgical interventionduring the above procedure.

Laryngoscope - a flexible, lighted tube usedto look at the inside of the larynx. It is insertedthrough the mouth into the upper airway.

Laryngospasm - spasm of the larynx causingclosure of the vocal chords and severe airwayobstruction. A complication ofintubation/extubation.

Larynx - upper part of the respiratory tract(above the trachea) containing the vocal cords.

Liver status - a group of laboratory valuesthat reflect liver function, the most importantof which are the ASAT, ALAT, LD, ALP andGT (liver enzymes) and bilirubin tests.

LMA - laryngeal mask airway, a device usedfor maintaining a patent airway withoutendotracheal intubation. It consists of a tubeconnected to an oval inflatable cuff that sealsthe larynx.

MAC - Minimum Alveolar Concentration, anindex of the anesthetic effect and potency ofan inhalation agent in relation to alveolarconcentration. 1.0 MAC is the concentrationrequired for lack of reflex response to skinincision in 50% of patients. 1.3-1.4 MAC isthe concentration usually required for surgicalanesthesia. MAC values are affected bypatient age (lower for old people and higherfor infants and children) and the use of otherinhalation agents (when MAC values areadditive).

Malignant hyperthermia - a genetic disorder(sometimes referred to as malignanthyperpyrexia) triggered by exposure to volatileagents that causes a life-threatening conditionof rapidly increasing body temperature,hyperventilation and tachycardia. Early signsinclude high muscle tonus verging on rigidityand an increase in etCO2. Untreated, the

condition can lead to cardiac arrest.

Metabolism - the range of biochemicalprocesses occurring within any living organismand involving the build-up and breakdown ofsubstances. The term is commonly used torefer specifically to the breakdown of food andits transformation into energy.

Metabolites - breakdown products (often ofpharmaceuticals) resulting from metabolismby the body.

Mixed venous blood - the blood in thepulmonary artery that has returned to the rightside of the heart from all parts of the body.

Myocardial infarction - an infarct in part ofthe heart muscle, often known as a heartattack.

Myocardium - the heart muscle.

OGPC - oil/gas partition coefficient, a measureof the lipid solubility of an anesthetic agent.

Opioid - chemical substance with amorphine-like action in the body, mainly usedused for pain relief.

Peroral - administered through the mouth asopposed to other administration routes,generally in relation to medication.

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Plethysmograph - curve measuring theamount of blood flowing through an organ andthe organ's blood content.

Pneumothorax - a collection of air or gas inthe pleural space of the lung, causing the lungto collapse. It may be caused by an openchest wound allowing air to enter, by a ruptureof the surface of the lung due to lung diseasesuch as emphysema, or by a severe bout ofcoughing. It may also occur spontaneously.

Pop-off valve - valve used in breathingsystems for releasing excess gas at a certainpressure.

Regurgitation - passive backward flow, e.g.of stomach contents from the ventricle.

RSI - rapid sequence induction, a variation ofthe standard induction technique for patientsunder anesthesia. It is performed whenimmediate definitive airway managementthrough intubation is required, especially inemergencies when there is a risk of aspiration.

Serum electrolytes - laboratory valuesindicating levels of important electrolytes inthe blood, The main electrolytes tested aresodium and potassium. High or low levels ofthe latter (hyper- and hypokalemia), inparticular, may have a negative impact oncardiac rhythm.

Serum creatinine - laboratory value thatprovides a quantitative estimate of kidneyfunction.

Shivering - a condition resulting from lowbody temperature following anesthesia inwhich metabolism increases, causing higher

oxygen consumption. It is therefore treatedwith pure oxygen and sometimes small dosesof pethidine given intravenously, as well as bywarming the patient and preserving body heat.

Solubility - the ability to dissolve, generallymeasured by the amount (in ml) of a substancethat will dissolve in another quantifiedsubstance.

SpO2 - the percentage of oxygen-carrying

hemoglobin molecules, or oxygen saturation,in peripheral blood as measured non-invasivelyby pulse oximetry.

Stridor - a harsh high-pitched crowing soundduring inhalation or exhalation associatedprimarily with airway obstruction.

Stroke volume - the volume (in ml) of bloodpumped out by the heart with each beat.

SvO2 - oxygen saturation in mixed venous

blood (as opposed to arterial blood - SaO2, or

peripheral blood - SpO2), a spot value

measured invasively as the percentage ofhemoglobin occupied by oxygen.

Tachycardia - an accelerated heartbeat,generally > 100 in adults.

TIVA - total intravenous anesthesia, a form ofgeneral anesthesia involving the intravenousadministration of hypnotic agent, analgesicdrugs and muscle relaxants and excludingsimultaneous administration of any inhaleddrugs.

TOF - train of four, a method used formeasuring magnitude and type ofneuromuscular blockade. It is based on the

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ratio of the amplitude of the fourth evokedmechanical response to the first one, whenfour electrical currents are applied for 2seconds to a peripheral motor nerve.

Trachea - the part of the airway referred to asthe windpipe between the larynx and thebronchi.

Trendelenburg position - position on theoperating table in which the patient is supinewith head tilted down in relation to the rest ofthe body.

Vagal reflexes - reflexes relating to the vagusnerve and causing bradycardia, reducedstroke volume and a fall in blood pressure.

Vasoactive agent - a drug causingconstriction or dilation of blood vessels.

Vasoconstriction - narrowing of the diameterof blood vessels resulting from contraction oftheir muscular walls. This is the opposite ofvasodilation.

Vasodilation - widening of the diameter ofblood vessels resulting from relaxation of theirmuscular walls. Vasodilation is the oppositeof vasoconstriction.

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15 REFERENCES

American Society of Anesthesiologists (ASA): ASA Physical Status Classification System.

Anaesthesia UK (FRCA): Resources.

Cashman, Davies: Lee's Synopsis of Anaesthesia.

Euliano, Gravenstein: Essential Anesthesia. From Science to Practice.

Food and Drug Administration (FDA): Anesthesia Apparatus Checkout Recommendations.

Halldin, Lindahl: Anestesi.

Jacobsson, Murray: Medical Devices: Use and Safety.

MAQUET Critical Care AB: Anesthesia. An Introductory Guide.

Moyers: A nomenclature for methods of inhalation anesthesia.

The Virtual Anaesthesia Textbook: The Anesthesia Gas Machine.

Tung: New Developments in Anesthesia Ventilators.

Tung: New Anesthesia Techniques.

www.medterms.com

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