LOCAL ANAESTHETICS

INTRODUCTIONANAESTHESIA :

GENERAL ANAESTHESIA

LOCAL ANAESTHESIA

DIFFERENCE BETWEEN GENERAL ANAESTHESIA & LOCAL ANAESTHESIA

FEATURES Gen.Anaesthsia Local AnaesthsiaSite of action CNS Peripheral nerves

Area of body involved Whole body Restricted area

Consciousness Lost Unaltered

Care of vital functions Essential Usually not needed

Poor health patients Risky Safer

Use in non cooperative patients

Possible Not possible

Major surgery Preferred Cannot be preferred

Minor surgery Not preferred preferred

LOCAL ANAESTHETICS

DEFINITION: are drugs which, when applied directly to peripheral nervous tissue, block the nerve conduction and abolish all sensations in the part supplied by the nerve without loss of consciousness.

HISTORY

Erythroxylon coca

500’s B.C Coca leaves first used by Peruvians for psychotropic properties

Cocaine –first discovered in 19 th century-local anasthetic action

Andean natives –chewed extract of leaves-stimulatory &euphoric action

1850’s Cocaine isolated, hypodermic needle developed

Albert niemann-isolated -18601884 Sigmund Freud studies the effects of cocaine it was used as ocular anasthetic1884 Carl Koller introduces cocaine into medical practice1884 Local anesthesia- used in dentistry by Halsted and Hall1905 Procaine synthesized by Einhorn

FEATURES OF LOCAL ANAESTHETICS

Should have quick onset of action

Should not be irritating to skin & mucous membranes

Duration of action must be long enough to allow desired surgery to be completed

Should be effective on both injection & local application

Should have low Systemic toxicity

Should not cause any permanent damage on any tissue.

Should be relatively free from producing allergic reaction.

Should be stable in solution and readily undergo biotransformation.

No LA in use today satisfy all of these criteria , however all anesthetics do meet a majority of them.

Contd…

Chemistry

All local anesthetics are weak bases,  they have amphiphilic property

Consist of hydrophilic secondary or tertiary amine on one side Lipophilic aromatic residue on other sideTwo are joined by an alkyl chain through an ester or amide linkage

Based on linkage they can be classified as

Esters: cocaine, procaine, tetracaine, and chloroprocaine.

hydrolyzed in plasma by pseudo-cholinesterase.

Amides: lidocaine, mepivicaine, prilocaine, bupivacaine, and

etidocaine.

metabolized in the liver to inactive agents.

True allergic reactions are rare (especially with lidocaine)

ADVANTAGE OF AMIDE LAs OVER ESTER LAs

Produce more intense and longer lasting anaesthesia .

Bind to α1 acid glycoprotein in plasma.

Not hydrolysed by plasma esterases.

Rarely causes hypersensitivity reaction.

CLASSIFICATION

1. INJECTABLE ANAESTHETIC:

LOW POTENCY, SHORT DURATIONprocainechloroprocaine

INTERMEDIATE POTENCY AND DURATIONLidocaineprilocaine

HIGH POTENCY, LONG DURATIONtetracainebupivacaineropivacainedibucaine

2. SURFACE ANAESTHETIC:SOLUBLE INSOLUBLEcocaine benzocainelidocaine butylaminobenzoatetetracaine oxethazainebenoxinate

MECHANISM OF ACTION OF LAs

MECHANISM OF ACTION OF LAs

LA blocks the nerve conduction by reducing entry of Na+ through the voltage gated channels

Due to this, they block the initiation & propagation of nerve impulse.

At higher doses it also blocks

1. Voltage gated Ca2+ channels

2. K+ channels

VASOCONSTRICTOR

Vasoconstrictor used – adrenaline (1:50,000 to 1:200,000).

prolongs duration of action of LAs by decreasing rate of removal from local site into circulation

Enhances intensity of nerve block

Reduces systemic toxicity of LAs

Provides more bloodless field for surgery

Order of sensory function block

1. pain 2. cold 3. warmth 4. touch 5. deep pressure 6. motor

If applied to the tongue, bitter taste is lost first followed by sweet & sour and salty taste last of all Recovery in reverse order

SYSTEMIC ACTIONS

CNS: All LAs are capable of producing a sequence of stimulation followed by depression

CVS: LAs are cardiac depressants, but no significant effects are observed at conventional doses. But at high doses they decrease automaticity, excitability, contractility, conductivity.

BLOOD VESSELS: LAs tend to produce fall in BP. This is primarily due to sympathetic blockade, but higher doses cause direct relaxation of arteriolar smooth muscle

SYSTEMIC TOXICITIES OF LOCAL ANESTHETICSCentral Nervous System Toxicities:

- Excitement: Tremors, shivering, and convulsions characterize the CNS excitement.

- Depression: respiratory depression at higher doses

CVS derangement— High plasma titers may depress the cardiovascular system directly.

Blood pressure may fall because of arteriolar dilation, myocardial depression, and/or cardiac conduction disruption.

Treatment includes patient positioning, IV fluids, and vasopressors.

Haemotological: large dose of prilocaine->10mg/Kg-Acumalation of it’s metabolite- o toludine-oxidises haemoglobin to methehaemoglobin Patient –cyanosed , blood –choclate colouerd –decompensate in patients with preexisting cardiac /res. Disease

Treatment: I.V methylene blue /ascorbic acid-a reducing agent – convert methhemoglobin to Hb

Hypersensitivity: Some patients are hypersensitive (allergic) to some local anesthetics.

There are two basic types of local anesthetics (the amide type and the ester type). A patient who is allergic to one type may or may not be allergic to the other type.

PHARMACOKINETICS Absorption

Local anesthetics are absorbed when ingested. Some local anesthetics may be absorbed in toxic amounts after topical use.

Absorption after an injection depends on drug

solubility in lipid and in water, tissue vascularity and local anesthetic and vasoconstrictor effects on local circulation.

Distribution: amides-wide distribution –I.V-lipophilics taken up by highly perfused organs-then moderately perfusedEster type- short plasma half life

Metabolism and excretion Esters are hydrolyzed by plasma and liver esterases.

Longer-acting esters are often metabolized more slowly.. Patients with altered pseudo-cholinesterase activity may be highly sensitive to these drugs.

Amides are metabolized in the liver by cyp450.-N-dealkylation then hydrolysis except prilocaine- hydrolysis first-o toludine-can cause methhamoglobinemia

Patients with severe hepatic damage or advanced

congestive heart failure may be unusually sensitive to these drugs.

Some amides are partially excreted unchanged in the urine.

Acidification can enhance excretionPK properties of amide LAs :

PRECAUTIONS AND INTERACTIONS

Aspirate lightly to avoid intravascular injection.

Inject the LA slowly &take care not to exceed the maximum safe dose, especially in children.

Propranolol may reduce metabolism of lidocaine and other amide LAs by reducing hepatic blood flow.

Vasoconstrictor (Adr) containing LA should be avoided for patients with ischemic heart disease, cardiac arrhythmia, uncontrolled hypertension those receiving β-blockers or tricyclic antidepressants

Techniques of Local Anaesthesia

Surface Anaesthesia

Application of a local anesthetic to nose, mouth, throat, tracheobronchial tree, esophagus.

Onset & duration depends on the site, the drug, its concentration and form.

Absorption of soluble LAs from mucous membrane is rapid.

Except for eutectic lidocaine\prilocaine , no other LA is capable of anaesthetizing intact skin.

used to relieve itching, burning, and surface pain (for example, as seen in minor sunburns).

Infiltration Anaesthesia

Injection of LA directly into tissue under the skin.

used primarily for surgical procedures.

LAs most frequently used are lidocaine (1%), bupivacaine (0.25%), etidocaine(0.5-1%), ropivacaine(0.5-1%), mepivacaine(1-3%) and prilocaine(1-4%).

mix with adrenaline (1:20000) to prolong the action.

Conduction block:

Injected around nerve trunks so that area distal to injection is anaesthetised and paralyzed.

- Choice of LA and concentration is mainly determined by the required duration of action.

- Lidocaine for intermediate duration of action

- longer lasting anesthesia bupivacaine may be selected.

Field block: - produced by injecting the LA subcutaneously in the

surrounding area of nerve so that all nerves coming to particular field are blocked.

- herniorrhaphy, appendicectomy, dental procedures, scalp stitching, operations on forearms and legs etc.

- - Larger area can be anaesthetized with lesser drug

compared to infiltration.

Nerve Block: local anesthetic is injected around a nerve that leads to

the operative site.

Usually more concentrated forms of local anesthetic solutions are used

eg: radial nerve block, ulner nerve block so on.Nerve block lasts than field block or infiltration anaesthesia.

Lidocaine (1.5%), mepivacaine(1.5%), bupivacaine (0.25-0.35%) can be used.

Epidural Anaesthesia.

spinal dural space is filled with semi liquid fat through which nerve root travel.

Injected in this space- acts primarily on nerve roots and small amounts permeates through intravertebral foramina to produce multiple paravertebral blocks.

used to produce analgesia or anaesthesia in surgical and obstretric.

Divided into 3 categories depending on site of action:1. Thoracic:2. Lumbar:3. Caudal:

Spinal Anaesthesia.

Injected into the subarachnoid space between L2-3 or L3-4 of the spinal cord .

Suitable LA like lidocaine (3-5%), bupivacaine (0.5-0.8%), tetracaine(0.3-0.5%).

Primary site of action is cauda equina rather than spinal cord.

Used to anaesthetize lower abdomen and hind limbs.

Use of hyperbaric(in7.5-10% glucose) or hypobaric (in distilled water) solution of LA .

proper positioning of the patient is also limiting the block to the desired level.

Advantages over general anaesthesia are: Safer Produces good analgesia and muscle relaxation without

loss of consciousness. Cardiac, pulmonary, renal disease and diabetic pose

less problem.

complication of spinal anaesthesia: Respiratory paralysis Hypotension Headache Cauda equina syndrome Septic meningitis

Contraindications: Hypotension & hypovolemia Infant & childrens- control of level is difficult Vertebral abnormalities - kyphosis

Intravenous regional anaesthesia

Also referred as Bier’s block & used for upper limb and orthopedic procedures.

Regional analgesia produced within 2-5min and last till 5-10min.

Only ¼ of the injected drug enters systemic circulation when tourniquet is removed.

Bradycardia can occur and bupivacaine should not be used because of higher cardio toxicity.

LAs IN A NUT SHELL