kh 33 ventilator c.pdf

8/14/2019 Kh 33 Ventilator C.pdf

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lb HAPTER

entil tors

II 33 1 M ECHA ICS O F RESPIRATIO

Respiration is the processof supp lying oxygen to an d removing carbo n di oxide from the tis sues .These gasses are carried in th e blood, oxygen from the lun gs to the tiss ues and carbo n d ioxide fromth e tissues to the lun gs. The ga s exchan ges in th e lung s a re called ex ternal r esp iration a nd tho sein the t iss ues a reca lled intern al respiratio n . The re is a very d elicateb alan ce between the absorptionand exc reti on o f oxyg en a n car bon d ioxid e in the lun gs an d t issues, a nd thi s b alance is

maintained by the res pira tory or b reathing a ctivity ..The o rgans of respiratio n ar e shown in Fig. 33 .1. They are typically divi de d in to th e follow ing

parts:

ond u ing Sect io n This inclu d es th e na sal caviti es , pharynx, larynx , trache a, bronchi and br onch io les. Th ese org ans a re th ick -walled and do n ot pa rticipa te in th e gas excha nge t o ca pillaries.

Respiratory Sec tion his includes respiratory bro nchioles , alveolar d ucts an d alveo lar sacs. Thesecontain thin walls an d pe rmit gas exchange to blood capillaries .

Both sections function through the muscles of respiration consisting of dia phra gm an d in tercostal/ chest m uscles , nb s , and ste rnum. The chest is formed b y twelve pairs of ribs jo ined tog ether

by m uscl es a nd c on n ective tiss ue . t fonns a closed cavity sea led off f rom th e o u tside ai r excep tfor a fle xibl e non-collaps ible tub e, the trach ea , whic h leads up t o th e larynx, nose and mo ut h .Just bel ow t he lev el of the colla r bo nes, the tr achea d ivides int o rig h t an d left d ivisions o r th ebronchi ;

Bronchi branches into about 20non-s ymmetrical branches and then leads to br o nchioles , ea chhavinga small diameterof about mm . The bron chioles become progressivelysmall er un til fina ythey lead into th e a lveoliw here th e exchange of gasses between th e bl ood an d lungs takes pl ace .At th e alv eoli , only two thin laye rs of ce lls sepa rate th e ai r from the blood an d g asses can diffusefree lybetw een th em . As it tr ave rses th e alveo lar cap illa ries , the ha emoglobin of the blood takes upoxyg en an d carbo n dioxi de passes ou t of th e b lood int o the al veolar spa ces . Each alveolus isextremely tiny 0.2 mm di ameter ), but because o f the very larg e num ber of th e a lveo li presen t


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838 H andbook of iomedical Inst rum nt ation

Trachea

ronchu s

Bron chi ol e f [ l

-

Fig . 3 3.1 a O rgans of res pira tion b ia gr m o f a lveo lar g as exc ha n ge

300 milli on) , th ey co m prise abo u t 70 m J o f th e s urface a rea . This gives rise to a total lun g ca pacityo f3. 6 to 9.4 li tres in th ea d u ll m ale a nd 2.5 to 6.9 in th e n or m al fem ak

Th e lun gs consist o f tw o co ne-s ha pe d spo ng y o rgans th at con t ai n the a lveoli air sa cs) th at tr apai r for ga s excha nge with th e blood . Th e lu ng s are covered by a smooth glistening mem b ranecalled th e pleura , which tums back at th e roo t of each lung and covers th e inne r surfa ce o f th e chestwall . No rmally , there ar e tw o lay ers of p leura: a mois t mem bran e , th e visce ral pleura which coversth e lu ng surface and the p ari eta l p le ur a wh ich lin es t he thoracic cavity. Th e flui d-lined s pace

betwe en tw o m embrane s acco un t s for eas y s lippa ge b et w een th e lun g a nd ch es t w alls durin gbr eat h in g . Th e lun gs ar e n ormally str etched o r ex p an de d ag ains t th e resis tan ce of elasti c fib resinsp ir a tion a nd ex p iration . Th e tw o sets of mu scles invo lved a re th e diaphr agm a thin she e t ofmuscle which separates the th o rax fr om the abdomin al cavity ), th a t moves up and down an d th einte r-costal muscl es surro un di n g th e thora cicc avity), that m ov e th e rib cage in an d o ut-Inspirationresults from contraction of th e diap hragm and th e inter-costal muscl es , where as expirati on resultsfrom th e ir re laxa tion. Th ere is no acti ve pa rticipation of the lu n gs in th e m ov em ents . The ra te a ndd epth of br eathin g are contr o lled fr om th e brain in th e m edull a re gion . Th e co ntrol imp ulses reachres p irat ory m uscl es via th e s pina l co rd , co ntr o l th e co nt r actio n of the di aphragm an d raise th e ribsto incre ase th orax cav ity . A lso, th e ch an ges in th e m et abolis m d ue to d ifferen t ty pe s of c hemica l


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reacti ons in th e o rgans of the bo d y regu late th e process of respi ration. Th e sensors kn own aschem o-receptors also regu late th e breathin g p rocess . Th e chem o-receptors are directl y influencedby the concentration of 0 2 N 2, 2, carboni c a cid, temperature and the flow rate of blood .

br eathing s tops fo r mo r ethan five minu tes, deat h or p erm anen t da mage w almost certainly

occu r. This ma y happen in man y co nditions such as as phyxia , carbo n mo no xid e poisonin g,drowning and electri c s hock, and artifici al respiration is then essential.

33 .2 ARTIFI CIAL VENTILATI O

For redu ced br ea thi n g o r respi ra tory failur e insufficie n cy), mech an ical devic es o r resp iratorsare used in hospit als. Th ese d evices pr o vid e a rtificia l ventil a tion, su pp ly enoug h o xygen andeliminate th e rig ht am ount of carbo n dioxide , m aint ain th e d esi red arterial partial pres sure ofcarbon dioxide PaCOV and d esired arteri al oxy gen tension PaOV

Mechani cal ai ds for m anua l artificia l ventil ation consist of a m ask , breathing valve ands l -

filling ba g Fig . 33.2 ). The mask , w hich is of so ft rubber or pl astic, is held fi rmly over th e patient smouth an d nose so that it fits tigh tly . The breathing valve serv es to guid e the a ir so thatfresh air orair enriched wi th oxyge n is su pplied to th e patie nt an d ex pired ai r is co nduc ted away. Th e bag issq ueezed with one hand and functions a s a pum p It is self -expandin g an d fill s au tomatically

with fresh ai r or oxyg en when the patient breath es o ut.

5eIf-fiIl ing bag

-

Air or o xygen

Breathing v tve _

L . . i i l - - - I ' f Y Y Y Y > Y Y Y ' f ' r Y Y V Y ' f Y Y Y Y Y \ . . - - - -

Mask

Patient

xpi red air 1 k ~ - v . . AA J , . - Av . . A A J , . - Av . . A A J , . - A -

Inspi ration Expi ration

P atient

Fig. 33 .2 Ma sk breathing va lve and se lf i tt ing ba or artificia l oen ti la ti on


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840 Handbook o Biomedical Instrumentation

33 .3 VENTIL TOR SWhen artificial ven tilation n eeds to be ma intained for a lo ng time a ve ntilator isuse d . Ve ntilatorsare al so used d u ring a naesthesia and are designed to ma tch huma n br eathing waveformpattern . These are so phisticated eq uip ment with a large n umber of con trols which assist inma intainingp roper an d regula ted breathingactivity . For short -term or emergency use resuscitatorsare employed. These d epend upon mechanical cycle o peration and are generally ligh t-weight an dportable.

The ma in fun cti on of a respirator i s to v entilate th e lun gs in a mann er as cl ose to na turalresp irat ion as possible . Since n atural inspirati on is a result of nega tiv e p ressu re in th e p leur alcavity ge nerated by the m oveme nt of the d ia phr agm ve ntilators we re initi ally d esigned to createth e sa me effect. Th ese ve n tilators ar e ca lled negative pressureventila tors thi s d esign th e flow o fa ir to the l ungs is facilitated by ge nerating a negative-p ressure a round th e patient s thoraciccage .The neg ative-pr ess ure moves the th oracic wa lls outwa rd expandin g th e intra -th ora cic vo lu me

and dr o pping th e p ress ure ins ide th e lu ngs resul ting in a p res sure gr adien t b etw een theatm osphere an d th e lun gs w hich c auses the flow o f atmosp heric air int o th e lun gs. The ins piratoryand exp iratory ph ases of the respiration are controlled by cycling the pressure inside the bod ycha mber. H owever bec ause of several engineering problems impeding the implementation of theconcept and th e d ifficulty of accessing the patien t for car e and monitori ng negative press ureve ntilators ha ve not become really po pular.

Patientcircuit

Inspiratory f lowdelivery y t m

Exhal ation contro l tsystem

Con troller

Fig 33 .3 Fu nctiona l d iagram o a positiv pressu re ven t il ator

Pos itive pressure ve nti lato rs ge ne rate th e in spiratory flow by applying a p ositive pr essur greater than th e a tmospheric pr essure to the airw ays. Fig . 33.3 shows th e p rinciple ofa positivepressure ven tilator. Durin g the inspir ation th e ins p iratory fl ow d elivery sys tem cr eates a positivepressure in th e patient ci rcuit an d th e exhalation control sys tem cl oses the ou tlet t o the a tmosp he re.Durin g the e xpiratory p hase th e in sp iratory fl ow de livery sys tem sto ps th e positive pre ss ur e a tth e exhalation sys tem and opens th e valves to a llow th e exhaled a ir to the atmosphere. Po sitivep ressu re ve ntilator s hav e be en found to be qu ite successfu l in treatin g patien ts w ith a w ide rangeof p ulmonary disor de rs .


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Positive pressureventilators operate either in mandatory or spontaneous mode. In spontaneousbreath delivery, the ventilator responds to the patient s effort to breath independently. Therefore,the patient can control the volume an d the rate of respiration. Spontaneous breath delivery is usedfor those patients who are on their wa y to full recovery bu t are no t completely ready to breathe from

the atmosphere without mechanical assistance. In contrast, when delivering mandatory breaths,the ventilator controls all parameters ofthe breath such as tidal volume, inspiratory flow waveform,respiration rate and o xygen content of the breath . Mandatory breaths are normally delivered to thepatients who are incapable of breathing on their own.

In general, most ventilators n clinical use employ positive pressure during inspiration to inflatethe lung s with mixture of gasses air, oxygen) . Expiration is usually passive , though under certainconditions, pressure may have to be applied during the expiratory phase in order to improvearterial oxyg en pressure .

33.4TYP S OF V NTIL TORS

Anaesthesia Ventilators : These are generally small an d simple equipments used to give regularassisted breathing during an operation .

Intensive CareVentilators: Int ensive care ventilators are more complicated, give accurate controlover a wider range ofparameters and often incorporate patient triggering facility: i.e.the ventilatordelivers air to the patient when the patient tri es to inhale.

33 .5 V NTIL TOR T RMS

Lung Compliance: The complian ce of the pa ti ent s lungs is the ratio of volume del ivered to thepressure rise itur rig the inspiratory phase n the lungs. This includes the compliance of theairways. Compliance is usually expressed as Iitres / ern H 2 0

Lung compliance is the abil ity of the alveoli and lung tissue to expand on inspiration. Thelungs are passive , but they should stretch easily to ensure the sufficient intake of the air.

A ventilator and other p arts of the breathing circuit also have compliance and some of thedelivered volume is used to compress gas or e xpand gas in these parts.

The compliance of a pa ti en t s lungs is th e rat io of pressure drop across the airway to theresulting flow rate through it. t is also expressed as ern H 2 0 l i t r e s pressure drop / flow rate).

irw y Resistance: Airway resistance relates t o the ease with which air flows through the tubularrespiratory structures. Higher resistances occur in smaller tubes such as the bronchioles an dalveoli that have no t emptied properly.

Mean irw y Pressure MAP : An integral taken over one complete cycle expresses the meanairway pressur e Fig. 33.4).

Inspiratory Pause Time: When the pressure in the patient circuit and alveoli is equal , there is aperiod of no flow. This period is called inspiratory pause time Fig. 33.5).

Inspiratory Flow : Inspiratory flow is represented as a positive flow above the zero line Fig.33.6).

Expiratory Flow: Expiratory flow is a negative flow below the zero line Fig. 33.6).


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Ventilat ors 843

Tidal Volume : Tidal volume is the depth of br eathing o r the volume of gas in spired or e xpiredduring each r espiratory cy cle. can be calculated by multiplying the flow rat e (l s ec) setting bythe set ins piratory time (seconds ). Calibrated tidal vo lume se ttings range from 0 .010 litre t o 4.8

litr es. f

the fl ow is set at 0.6 1/ san d

insp ira t

ory time is se t at 1 sec , th e tidal volume i s = 0.6litre s .Minute Volume: This refers to volumeof gas exchanged per minute during quietbreathin g . Minut evo lume i s obtained b y multiplying the tidal volumeb y the breathing rate .

.Respiration Rate: This is the number ofbreaths p er seco nd. represents t otal respiratory r ateofth epatient ln the assist-eontrol mode an d SIMV (Sync hro nized lntermittentMandato ry Ventilation)mode , the ventilator m easures the previous four breaths and shows the average total rate , which isprescribed r ate plus the additionalbreaths taken b y th e patie nt.

n v n t o na Mechanical Ventilation (CMV,\-TIris pro vides the force which determin es the tidalvolume (VT) at a respiratory frequen cy f to achieve th e desired minute ventilation (VE )

VE=V Tx fIntermittent Mandatory Ventilation IMV \- Th is a llows the insertion of a variable time dela ybetween ea ch br eath.

_ Inspiratory Expiratory Phase Time Ratio l : ERatio \-TIris signifies the ratio of inspirato ry intervalto expiratory interval of a mandatory breath .This ratio is normally limited to 1 :1, i.e. the inspiratorytime should not e xceed 50 of the total v entilator cy cle time as set by the breath / minut e control.lnverse I :E rati o is pr evented.

Synchronized Intermittent Mandatory Ventilation SIMV \- represents a combinationof machineventilation and spontaneous breathing . SIMV enables th e patient to breathe spontaneously in

regular prescribed cycles, with the mechanical mandatory ventilation strokes providing aminimum ven@atlonduringtheremainingcycles . .

Synchronized Int ermittent Mandatory Ventil ation d elivers a prescribed tidal volume andrespi ratory rate . The p atient ma y then breath e s pontaneously in between the deliv ered breaths(Fig.33.7).

sens itivity level

= = I - =_ _=_ = = c o --,-,-, >-'- --c

SIMV+i ?ill

l O ~ ~ ~ S V intern l k = l ynchronizingwindo w yn hronizingwindow> Fig. 33 .7 Synchronized intermittent mandatory ventilation. is activated simul

taneously only when the p ti nt s breath is detected during the l stquarter of the s t tot l breath cycl e

Sigh Volum e: On e s igh br eath is 150 of the set tidal vol ume.

Patient Circuit : Th is includes a set o f tools co llecting th e p ati en t airwa y to th e outlet of aventilator.


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844 Handboo k of Biomedical In strumentation

Oxygen Percentage F 2 : In all ventilatory modes , oxygen is delivered during the inspirator yphase an d the percentage (F 2) is adjustable from 21 to 91 .

Peak irway Pressure: It is the highest level of pressure reached ov er several breaths .

Spontaneous Ventilation: This is a ventil ation mode in which the p atient initiates and breathesfrom the ventilator at will .

Bias F low: In bias flow , mixed gas from the mixer isdirected through the patientcircuit in-betweenmechanical breaths . Bias flow stabilizes baseline pressure fo r spontaneously breathing patientsan d decreases the respon se time of the demand valve.

Sensi tivity: It is used to de t ect spon taneous effort b y the patient . In order to trigger mandatoryventilation with the se t Respiration rate .

/ Mandatory Minutes Volume Ventilation MMV): This operating mod e appliesmandatory ventilati ononly if sp ontaneous breathing is not yet sufficient an d has fallen bel ow a pre-selected minimumventilation . Unlike SIMV , the mandat ory s trokes are no t appli ed regularl y but onl y in c ases o finsufficient ventilation .

Control/e d a ndatory Ventilati on: This term refers to mandatory ventilation of patients wh o arenot able to initiateor r espire on their ow n .

AssistedS pontaneous Breathing AS B : I t refers to the pressuresupport ofinsufficient spontaneousbreathing.

.Yosit ive End Expiratory Pressure PEEP : PEEP isa th erapist-selected pr essure level for -the patientairway at the en d of e xpiration in either m and atory or spont aneous br eathing. PEEP i s used toincrease the end-exp iratory lung volum e EELV or pr olo ng expiration with a potentiall y s imilareffect on the EELV (F ig.33.8 ).

Inspi rati onsensiti vity

- . - .. . --,

Pre ssuresuppo rt level

..... . . .../ .. _ .--- - - - - - f

: f-- T < - - - ~ .. __ ... .. __ .. . __ .a. , ,

, ,,

,, ,, , Ins pirati on i _ Expiration :, ,

> Fig.33.8 Concept of PEEP

v Co n tin uous P ositive irway P ressure CPAP : CP AP is aspontane ousv entila tion mod e in whichth e ventil ator m aintains a co nstant p ositive pr es su r e, near o r below P EEP Level, in th e pa tie nt sa irway w hile th e patient b reathes at will .

ssist /Control Ventilation: During this p rocess, a positi ve pr essu r e br eath is deli vered with ea chp atient s s pontaneous insp iratory effor t to reach th e trigger l evel setti ng . In vo lum e controlled


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Ventilators 84 5

assist control , tidal volum e is determined by flow an d inspir atory time se ttings . the patient d oesnot trigger th e v en tilation, it auto maticall y delivers breaths accor di n g to th e se t ra te .

Relief Valve : t determines the maximum p ressure th at can be r each ed in the p atie ntcirc uit d urin g s pontaneo us m echanical a nd m an ual ve ntila tion . t is adju stable fro m 0-100 em /H 2 and func t ions in all m od es.

3 3 .6 C LASSIFICA TI O N OF VENTILATO RS

Ventil ators can be classifi ed in te rms of various methods . Discussed be low are the general criteriafo r sy stematic lis ting and d escription of th ese classifi cations .

.6 . 1 Bas ed on the Method of Initiating the In spiratory Ph a se

Controller: A ventilato r which operates ind ependent of t he pat ient s i nsp iratory effort . Th einspiration is initiated b y a mechanism which is controlled with respect to time , pressure ora nother similar factor. Controlled ve ntilation is required for pati ents who are un a ble to brea th onth eir o wn . isto r A v en tilat o r w hich aug m ents th e in spi r at ion of th e p ati ent b y o pera ting in respo nse toth e pa tient s ins piratory e ffort. A p res su r e sensorde tects t he s light negative pr ess ur e th at o ccurseach time the patient a ttemp ts to in h a le and trig gers the process of infla ting the lu n gs . Th us t heven tilator h elps the patient to inspire when needed . A sens itivity ad justment prov ided on theequipment helps to select the amoun t of effort require on th e patient s pa rt to trigger th e inspirationpro cess. Th e assist m ode is re qui red for th ose pat ien ts w ho are ab le to br e at h e but ar e un ab le to

in h ale a s ufficie nt amo un t o f ai r or for wh om b rea thi n g r equir es a grea t dea l o f effo rt. sistor lContro ller : A ve n tila tor whi ch com bi ne s bo th the contro lle r and a ss is tor fun ctions. Inthese de vices , if the patie nt fails to br ea the within a pre-determin ed time, a time r aut om atica llytri ggers th e inspi rati on proces s to inflat e the lu n gs . Theref ore , th e breathing i s co ntrolled by thepatien t as long as it is p ossible , but in case the pati en t sh ould fail t o d o so , the machine is able totake over th e functi on . Suchd evices ar e most frequ ently u se d in criti cal care units .

l 2 Based on Power Tr ansm issionDir ect Power Transmission: A v entil ator wh ich delivers t he gas di rectly fro m the so urce of

co mpressed g as to th e patient Fig. 33 .9 a .In direct P ower Tr ansmission: A ventilator wh ich ha s se pa rate patient and power sys tems Fig.33 .9 b . The pr ess ur e in the p owers ys tem dete rmines the fl ow rat e.

.6 .3 Ba sed on Pr es su r e Pattern

Positive Atmo sphere: A v entilator whic h pr od u ces a p ositive pr ess ur e in th e pati ent s lung sduri ng inspir ation , w ith an end exp iratory p ressur e that is equal t o the a tm o spheric p res sure . nthis m od e, the m ean airway press ur e is alwa ys highe r than th e at m osphe ric p ressure an d thepatient n ormally br eat h es s po nt a neously w ith th is mo d e o f o peration Fig. 33. 10 a .


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846 Handbook of Biomedical Instrumentation

T L = = = = H I E xhu~ . J \ IPa tient


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Ventil tors 84i

Pos it i oe Pos itic e: A ventilato r which produces a positive p ressure in th e pa tien t s lungs durin ginspiration. with an end expiratory pressure that is greater than the atmospheric p ressure Fig. 33 Oc ). In order to obtain an en d expiratory pressure that is greater than th e atmosphericpressure, it is necessary to s tart the inspiratory pha se before th e airway pressure reaches th e

a tmosph eric p res s ure .

33 .6 .4 Ba sed on t he Typ e of Safet y l i mit

o l l l ~ Limited: A venti lator in which pre-detennined volume cannot be exceeded duringinspiration. Volume limit normally refers to tidal volume .

P ressure Limit ed: A ventilator design ed in su c h a way that predetermined p ressu re cannot beexceed e d durin g inspiration .

Tim e imit ed: A ve ntila tor in which pred et er m ined pha se tim e ca nn o t be ex ceeded t limit s theexpiratory phase tim e if th e patient does not initiate the inspira tory ph ase and is co mmo n t oventilators used for assis ted ventilation,

33 .6.5 Ba ed o n C ycling Control

Cyclin g co ntr o l o f a v entil ator i s the d evice w hich d e termines the ch an ge from th e inspi rato ryphase to th e expi ratory phase an d i ce versa . Th e cycl ing of a v entilator m ay be based u pondifferent factors such as pressure , volume , time and the inspiratory effort made by the patient, Thecommon types of cycling controls ar e described below .

33 6 6 1 Cycli ng fr om In sp iration to Exp ira tion

I lIIff Cycltll : A v entilator which s tarts the expiratory ph ase after a pr es t tidal vo lume h as beendelivered int o the pat i ent ci rcuit. Thi s d evice norm a lly has a pr essur e ov e r-ride va lve so that iwhile th e machine is in th e process of administering th e set volume , th e press ur e exceeds a predetermined maximal value , th e ventilator Mil cycle whether or not the appropriate volume hasbeen administered .

Pr essur e Cyclr d: A ve ntilator which begins the expiratory phase after a preset pressure has beena ttain W .

Time Cy clrd: A ventila tor whi ch initia tes the ex piratory ph ase a fter a pr eset t ime period f or th einspira tory phase has passed .

33 6 6 Cycling from Expiration to pirstion

Pre ssure C yclrd : A ven tilator which begins the inspira tory phase afte r a pre-set en d expiratorypr essure ha s bee n attained .

Time Cy cled: A ve ntilator wh ich initia tes the ins piratory p hase aft er a pr ese t tim e period f or th eexpiratory phase has passed .

at i ent Ins piratory Ef for t c l d A ventilator which starts the inspiratory phase in response tothe inspiratoryeffort .


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848 Handbook o f i omed ical I nstrum en tation

6 7 Based on the Source of Power

Pn um ti c A ventilat or p owered b y compr essed gas .

Elec tric A venti lator p owered b y an electrical de vice s uch as an e lectric m otor , or similar ga dge t.

lila > .7 PRESSURE-VOLUME-FLOW DIAGRAMS

In orde r to un derst and th e perf or m ance of a ven tilator , it is necessary to be familiar wi th thep ress ure-time , flow-time an d vo lume-tim e d iagrams.Th e ve ntilated system consists of th e pati en tcircu it, the airwa y and th e alveoli , each h aving its ow n compliance . After th e s tart of th e ins piratory ph ase , a cer t ain gas vo lum e is de liver ed int o the sy s tem, r esultin g prim arily in an mhease ofp res su re in the p ati en t circuit a nd sub se quently, in a flow throu gh th e airwa y. During theinsp iratory ph ase, th e airway p res sure an d alveolar p ressure in crease g radually with th e airway

pr essur e always b eing hi gher th an th e a lveolar pr essure .The equal p ress ur e of the p atient c ircuit and al veoli d etermines th e end o f th e inspira tory fl owand b eginnin g o f th e ex pira tory fl ow , du e to th e fact that th e pr es su r e in th e p ati en t sys tem isallowed todecrease . Th e expirat ory flowisde termined b y the d ifferenc e between a lveolar pr es su r eand p ressure i n th e pati en t circu it. t ma y t hu s be no ted tha t:

- an airw ay p ress ur e high er than th e a lveolar pressur e ch aracterizes an inspiratory flow;and

- an airw ay pre ssur e lower t han th e a lveol ar pre ss ur e characteri zes an expiratory fl ow . t ma y be obse rv ed th at i t is necessary to prov ide for a tim e d elay pause time between th e

cycling of th e v entilato r and th e ch ange from ins pir at or y flow to ex piratory fl ow in th e a irway .Dur ing this pause tim e, th e flow become s ze ro w hen th e a lveolar pr es sur e equals th e a irwaypr es sur e a nd constant vo lume i s m aintained in th e lungs . Ventilator s p roducing a pau se timedurin g in spir a tion o r expiration h ave ce rtain advantages over ventilat ors w ith ou t such a p au sean d are the refore p referred ove r the latter . Figu re 33 .11 shows p ress ur e, flow a nd vo lume patt er nin a v enti lated sys tem with and wi thout p au se.

lil a> 8 MODERN VENTilATORS

Th e current and future trend s in critical ca re ventilat or y m an agem en t d em an d pre cise flow ,pr essure an d o xygen c on t rol for ap plication t o both adult and pa ediatri c patien t s. ad dition,pati en t monitoring an d rap id , u nder stan d ablea la rms a re e xtr em ely imp ortant fo r tim ely ca re ofthe p atien t.Thi s h as beco me pos sible by m aking use o f com p uter techn ology in t h e ven tilators toachie ve a w ide range of fun ction s a nd co ntrols.

Modern ve ntila tor machin es consis toftw o sepa rate bu t int er -eonnected s ystems: the p ne um aticflow sys tem and an el ectronic co ntrol sy st em . Figur e 33 .12 shows a bl ock di ag ra m of a typicalventilator.

Th e p neumatic fl ow sys tem enab les th e flow of g as through the ventilat o r. Oxyg en an d m edicalgrad e air ent er the ve ntil ator a t 3.5ba r 50 psi p res sur e thr ou gh built- in 0 .1 mi cron filt ers . Th e

norm al opera ting rang e is 2 10 6 bar or 28 to 86 ps i. Theseg asses en te r th e a ir / oxyge n mix er w here


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Ventilators 849

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ausetime

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W Fig . 33.11 ressur e lo w and volume diagram

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th ey co mbine at the r equired p ercen tag e an d reduc ed i n pres sure to 350 cm H 20 . The gass es th enent er a large reservo ir tank whi ch hol d s ab ou t 8litr es o fmixed ga sses wh en compr essed to 350 cmH 20 . An electro nically c ontrolled flow valve prop ortions th e gas flow f ro m th e reservoir tank toth p n nt br eathin g ci rcui t. In so me ve ntilators a n a ir co mp r esso r is u sed in pl ace o f a

com pressed ai r tank . Th e p rimary ob jective of th e d evice is to ens ure pr op er level of o xygen in th ein sp iratory a .r and de liver a tid al vo lu me according t o th e clini ca l requ irements .As th e gasses leave the ventil ator they p ass by a n o xygen a nalyser a safety a mbient air inlet

valv e an d a b ack-up m echa nical over pr essure valve . The amb ient valve provid es the pati en t theabili ty to br eat h e ro om a ir wh en th e ventila to r fail s or th e pr ess ur e in the p ati en t circuit dropsbelow 1 em of H 20 . In th e pati ent br eathing circ ui t is a bi-di rectional flow sens or to m easur e th egas flows. Th e exhaled gasses ex it t hrou gh a n electronically con trolled exha lation valv e located atth e ve ntilator . W ith th e introduction of mi croprocessors for co ntro l of met ering d evices electrom echanical va lves h ave ga ined p opulari ty. Th e mi cropr oces so r controls each va lve to d eliver th ede si r ed inspiratory ai r and o xygen flow s for m an d atory and s po n taneous v entilation. A high


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850 andbook o f iomedical Instrumentation

irway pressur e ; { P EEPcontrol ler

L ~ ...: l pressu re support

: avel sensitivity Microprocessor : Sp t an eous

_ __ _ __ _ ____ _ _____ b reath se tting

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PEEP

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:> Fig . 33.12 lo ck diagram a micro pro cesso r contro lle d ventila to r

pressure valve i s used to p rovide sa fety in case th e pressure in the patient circuit e xceeds 110 emof o .

Th e e lec tro nic c ont ro l sys te m may use o ne or m ore mi croprocessors and so ftwa re to p erformm onitoring a nd co ntrol functions in a ve ntilator . The se p ar amete rs includ e se tting of therespirati on rat e flow w avefo rm tida l volume oxygenconcentratio n of the delivered b reath pe akflow an d PEEP . The PE EP selected in the mandat ory mo d e is o nly used for control of exhalationflow . The mi corproces so r utiliz es the abo ve param eters to compute the des ired inspiratory flowtrajectory.The sys tem consists of moni tors for pressure flow an d oxygen fraction . The sensors ar e

conn ec ted to electron ic p roces sin g circuits w hich m a kes th em ava ilable for digi tal r eadout s. Thesigna ls are a lso co mpar ed w ith p re- se t alar m lev els s o th a t i f th ey fall outside a pr e-det erminednorm al range a larms ar e so un de d .

The pressure sensors ar e normally of se miconducto r s train gau ge type placed in a bridgeconfiguration. F or measurement of fracti on of o xygen in th e inspired a ir a fuel cell type ox ygensenso r is used . This senso r ge nerates a current proportional t o p l As this senso r is tempe raturese ns itive com pe ns ation for it s o pe rat ing t emp erat ur e is incl ud ed in the circuit. Us ually atherm isto r is u sed to car ry o u t thi s fun ction . The flow se ns or u su ally consis ts of a varia bleorifice a nd by m easuring the p ress u re d ro p ac ros s the va riable orifice the p a tien t flow s ca n becalculated .

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entil to rs 851

Ventila tors are lif e sav ing equipment and th erefore need reg ular m aint enan ce a nd calib ra tionw hich should b e ca rried out as p er th e inst ru ctions of th e manu facturers .

33 .9 I FR EQUEN Y VEN TIL TO

A new technique fo r ventilating pa tients at frequencies m uch high er th an the resp ira tion rate hasrece ntl y been in t rodu ced . This me tho d has been s ho w n to improve CO 2 was h out and prov idead e quat e oxyg enat ion w ithou t t h e requir em en t for h igh i n spir atory pr ess ur es (Chan andGreenough, 1993 ). Th e key p rin ciple in this techn iqu e is to provid e tid a l vo lumes equa l t o o rsma ller tha n th e d ead s pace , a t ver y h igh rates .

conven tional po sitive p ressure ve ntilation, CO 2 elimination is direc tly contro lled by theamo u nt o f applied min u te v entil at ion . Ho w ever, it is known tha t mean airway p res sure is thepa ra meter t hatbes t correlates with improvem en t in oxygena tion . Gas t ransport during conventio na l ven ti lation is a ttributed to tw o b asic mechani sms : (i) co nvec ti on or flow of gas th r oughthe co nducting airways , a nd (ii) m olecular diff usion of gasses in to th e a lveoli and p ulmonarycap illaries . The tidal v o lume (VT) appli ed to the patient at the Y piece can be divided into thevolume used t o ve ntila te the dead space (Vo) an d th e alv eolar volume (VTa1v) Onl y the alv eol arvo lum e takes part in th e g as exchan ge p rocess. Th erefore,

V Talv V T V

Th e porti on of t he tida l vo lume u sed to ven tilate th e de ad space d oes not take p ar t in ca pillarygas exchange and i s therefore was ted .Toove rcome th e pr ob lem of w as ted venti la tion in con ventiona lve nti lation , the ins pi r ato ry p ress ur e is in creased in ord er to increase the tota l tid al vo lume. Unfortu nat ely, ho w ever , thi s a lso increas es th e m ech anical s tress on th e lun g a nd ha s been assoc iated

w ith various tr au m as. H igh frequency ve ntilation h as bee n shown to p rovide ad equatealveo larve ntilation an d oxyge na tion witho ut th e requireme n t for h igh inspi ratory p ressures (H am ilt onet al., 1983).

H igh frequ ency ( HF)v entilators a re n ow com mercia lly availab le, th e mos t po pu la r b eing th eBabylo g 8 from M s Drage r, Germa ny. Th e ve ntilator generates hig h freq uenc y ra te from 5 to20 H z (300 to 1200 puls e minu te ). Alth ough se veral m ethods ar e availabl e to generate th e highfrequency pressur e wa ves, the Bab ylog 8000 makes use of an osc illating diaphragm mechani sm .Th is mechanism is computer-controlled an d ca n pr ecisely d etermi ne th e sha p e o f th e pr essureswings and J:E ratio.

An alt ernative method of achieving H F vent ilation is based on th e jet prin ciple i n w hich a sma lld iamet er tu be is passe d do w n a tr ach eal ca nnula and is e ithe r t erminated a t its d istal e nd orexten ded into th e tr achea itself . Short pu lses of highe r p ress ur e oxyge n a re intr od u ced in t o thea irw ay th ro ug h the cann u la at freq uenci es w ell above the no rma l respi ration r ate.This technique

. has th e disadva nt age of for cing vo lume in to th e patie nt and th en leav ing th e p ati ent to exh a le. pass ive ly, wh ich m ay lead to so me tra pp ed vo lume inside th e lun g in creasing t he m ean lu n g

p ress ure . Thi s prob lem is ove rcome b y e nsur ing th at the p res sur e du rin g th e ex halati on ph ase isn egative w ith resp ect to th e se t PEEP.

Boynton et a (1984) found th at co mbin ed hig h frequenc y ve ntilation and conve ntionalve ntilation facilita ted gas exchange in certain criticall y ill neona tes. Blanco e t a (1987) rep orted


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852 andb ook iomedical In strumentation

th at th e use o f hi gh freq u ency v entilation in co mbination with conventional ventilati on produceda s ignifican t im proveme nt in gas exc h ange at a lower airway p ress ur e .

The commerc ial HF ve n tilators o pera te at fr eq u encies of bet w ee n 5 to 20 Hz . Ins pirationto- expiration rati os ca n u sually be varied fr om 1 : 1 t o 1 : 4. Th e wav e sha pe c an be s in uso ida l or

rect angular . Like conventional v entila to rs, HF ventilators ar e usually microproc essor-eontrolledand prov ide an integrated sys tem featurin g high frequency , tri ggered and conventionalv entilation.

33 .10 H UMI IFIER S N EBU LIZ ERS AN D ASPI R T O RS

Ap ar t from ve ntilation, humidifi cation of th e bre athin g ga s p lays a leadin g rol e in th e int ensivecar e of pati ent s. The main task of a humidifie r is t o replace humi di ty in th e upp er a ir pas sageswhichh as been l ost by intubation . The h umidity sh ould e as cl ose to 100 as pos sible, or speakingin t erms o f wa ter, the absolute co ntent per litre breathin g g as should b e m ore than 30 m g, rega rdlessof e nviro nm e ntal conditions.The ref ore , in o rder t o pr event d amage to th e pati ent s lung s, th e airo r oxyge n ap plied du ri n g res pir atory th erapy mu st be hum idified. Thus, a ll ve nti lators incl udear r angements to hu m idify th e a ir, either by h eat vapo urization strea m) or by bubbling an a irstream throu gh a jar of wate r.

Wh en w ater o r som e typ e of m edication susp ended in th e inspir ed a ir as an a erosol is to beadminister ed to th e pati ent , a d evice c alled a n ebulizer is used. this d evice, th e water or medication is p icked up by a hi gh velocity jet of ai r o xygen and ma d e to impact again st on e o r morebaffl es to br eak th e su bstance int o controUed-sized dr oplets whic h ar e th en appli ed to th e p atientvia a respirator. M ore e ffective a nd efficient nebulizers are base d on th e use o f high in tensityultra sound energy whi ch v ibrates th e su b stan ce w ater or medi cati on ) to produ ce a high vo lume

of mi nu te pa rticles. Ultra sonic nebulizer s

d ono t

d epen dupon

breathin g ga s for o peration andthu s therap eutic agents can be co nveniently administered during ventilation pro cedur e.Aspir ators a re oft en inclu de d as part of a ventilator to remov e m ucu s an d oth er fluids fr om th e

airwa ys. Alt ern ative ly, a se parate suction de vice m ay be utili zed to a chieve th e sa me purpo se.

kh 33 ventilator c.pdf

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