Pacemakers.docx

Pacemakers.docx

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Pacemakers

Pacemakers12/04

1-Whatisapacemaker?

2-Whatdoes“intrinsic”mean?

3-Howexactlydopacemakersworkontheheart?

4-Whatarethepartsofapacemaker?

5-Aretheredifferentkindsofpacemakers?

6-Whatistheadvantageoftwowiresoverone?

7-Howarepacemakersinserted?

8-Whatdotheymeanbytransvenous,transcutaneous,andtransthoracic?

9-Howdoesthegeneratorboxwork?

10-Howlongdotheimplantedbatterieslast?

11-Howmuchelectricitydoesthepacemakerusetoactuallypacetheheart?

12-InEnglish,please?

13-Whatis“capturethreshold?

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14-WhydopacedbeatsgeneratedbyaventricularwirelooklikePVCs?

15-Whatdoes“asynchronous”mean,andwhatdoes“demand”mean?

16-Whatdothoseletters:

VVI,DDD,etc.standfor?

17-Whatis“failuretocapture?

”

18-Whatis“failuretosense”?

19-Howcananimplantedpacemakerbereprogrammed?

20-Whatisthemagnetthing?

21-Whataresomereasonsforplacingapermanentpacemaker?

22-WhatisanAICD?

23-CanAICD’salsofunctionaspacemakers?

24-WhatproblemsdoAICD’shave?

25-HowdoyoustopanAICDfromshockingthepatientincorrectly?

26-Canyoushockapatientwithapacemaker?

27-Whatisexternalcardiacpacing?

28-WhowasZoll,anyhow?

29-WhatelsedoIneedtoknowaboutrunningtheexternalpacemaker?

30-HowdoIknowifcapturehasbeenachieved?

31-What’sthetrickypart?

32-Doesexternalpacinghurt?

33-Howlongcanapersonstayontheexternalpacemaker?

34-HowdoestheZollgointodemandmode?

35-AnyotherZolltricks?

36-CanyoudoCPRwiththeexternalpacerinplace?

37-HoweffectiveistheZoll?

Pacemakers

1.Whatisapacemaker?

Apacemakerisanelectronicdevicethatprovidesanelectricalsignaltomaketheheartbeatwhenit’sown,built-inpacemakersfail.Theanatomical,built-inpacemakersprovidewhat’scalledthe“intrinsic”rhythm,andtheycanbedisruptedbyvariousconditions–ischemiaforexample,orbyanMI.

2.Whatdoes“intrinsic”mean?

Intrinsicmeans“builtin”.Inthissituation,itmeans:

“comingfromthepatient’sownbuilt-in,naturalpacemakers”:

theSAorAVnodes;orsometimesfromlowerdownintheventricles.

3.Howexactlydopacemakersworkontheheart?

Thepacemakeressentiallydoestwothings:

itsensesthepatient’sownrhythmusinga“sensingcircuit”,anditsendsoutelectricalsignalsusingan“outputcircuit”.Ifthepatient’sintrinsicrhythmbecomestoosloworgoesawaycompletely,theelectronicpacemakersensesthat,andstartssendingoutsignalsalongthewiresleadingfromthecontrolboxtotheheartmuscle.Thesignals,ifthey’re“capturing”properly,providearegularelectricalstimulus,makingtheheartcontractataratefastenoughtomaintainthepatient’sbloodpressure.

Here’sthe“box”,implanted.

Howmanypacingwires?

4.Whatarethepartsofapacemaker?

Thepacemakerboxitselfiscalledthe“pulsegenerator”–thegeneratorisconnectedtoeitheroneortwowires,whichcarrytheelectricalsignalstotheheartmuscle.Permanentpacinggeneratorsareimplantedinthechestundertheskin–nowadaysthey’reverysmall–andthewiresleadingtotheheartarethreadedthroughthesubclavianvein.

5.Aretheredifferentkindsofpacemakers?

Pacemakerscanbeeithertemporaryorpermanent.ThetemporarypacemakersthatweseeintheMICUaremadeupofacontrolboxandonesingleoutputwireleadingtotheinnerwalloftheRV（thuscalledaventricularwire,or“V-wire”）,andprovidesimpleratecontrolbypacingtheventricles.Permanentpacemakerscomeinseveralflavors,butthemaindifferencebetweenthemisthatsomehaveonlyonewireleadingtotheRV,andsomehavetwo–onetotherightatrium（RA）,andanotheronetotheRV.ApacingsystemthatpacesboththeRAandtheRViscalledan“atrioventricular”pacer,andpacesbothrightheartchambersinsequence.Thesignalaffectstheleft-sidechambersandstimulatesthemtocontractaswell.ThesignalfromthewiregeneratesavisualsignalontheEKGthatlookslike,andiscalleda“spike”.

Here’sanexampleofatemporary,external,single-wirepacingbox.Thewirehastwopoleconnections,soonepairofconnectors:

singlewirebox.

Here’saniceexampleofsingle-wirepacing,withspikescomingatarateinthe70’s.Thereasonwecantellthatthere’sonlyonewiregoingissimplythatthere’sonlyonespike.Two-wiresystemsgeneratetwospikes.

ThesespikesgobothupanddowninfrontoftheQRS’s,whichyouseesometimes–a“normalvariant”.

Howmanyspikesthistime?

Everybodyseethetwospikes?

Thearrowsaren’tperfect–butclearlytherearetwopacingfunctionsgoingonhere:

thefirstspikeisgeneratingatrialkick,andthesecondiskickingtheventricles:

twospikes,twowires.

Twosetsofconnectors:

twowires.Sowhatkindofboxisthis?

6.Whatistheadvantageoftwowiresoverone?

Ifyoupacetheventriclealone,thepatientdoesn’tgetthe“atrialkick”–thepushoftheatriaintotheventricles.Thiscanactuallyadd20-25%tothecardiacoutput,andimprovethebloodpressureaccordingly.AniceexampleofthiswasapatientwesawrecentlywhohadatemporaryV-wireinplace.Whenshewaspacedatarateof70,sheonlygotabloodpressureofabout85,systolic.Butwhenthepacerratewasturneddown,herintrinsicratetookoverataboutarateof60–sinusrhythm,whichmeantthatshestartedgettingheratrialkickagain–andherbloodpressurerose,evenwiththeslowerrate,toabout105.Alsoaniceexampleofhowthefastestratewillcapture:

firstthewire,thentheintrinsicone.

6.Howarepacemakersinserted?

Atemporarypacingwireisthreadedthroughanintroducerplacedinacentrallinesite,usuallytherightinternaljugularorsubclavian.（TherightIJisthestraightestshotdownintotheRV,whichiswhereyouwantyourtemporarywiretogo.）Nowadayswerarelyusetransvenouswireinsertionatthebedside–weapplythetranscutaneouspacer,or“Zoll”instead.Onceinagreatwhileatransthoracicwireplacementisattempted,usuallyattheendofacodewhennothingelseisworking…

7.Whatdotheymeanbytransvenous,transcutaneous,andtransthoracic?

Transvenousmeansthatthepacingwireisthreadeddownthejugularveinthroughanintroducer（thesameasaPAlineintroducer）.Theintroducerisputinfirst,likeanycentralneckIVline,andthewireispassedthroughit,likeaPAlineis,untilitmakescontactwiththeinnerwalloftheRV.Thenthewireisattachedtoageneratorbox,andtheheartispacedusingthewire.Wehardlyeverdothisatthebedsideanymore,sincethecomingoftheZollexternalpacer.

TranscutaneouspacingmeansusingexternalpacingpadsconnectedtoadeviceliketheZollmachine,oroneofthedefibrillatorsthathasexternalpacingability.

Transthoracicpacingmeansusingwiresinsertedeitherduringcardiacsurgery–smallwiresthatsitontheouterwalloftheheart–“epicardial”wires,thatleadoutofthechest,toacontrolbox-ordoingamaneuverthatinvolvespushingapacingwireintotheRVupthroughthechestwall,subxiphoid,duringacode.I’veonlyeverseenthistriedonceanditdidn’twork.Accordingtoawebsourcethatwelookedat,theprocedureisn’tverypopular,cancreateawholeslewofnastycomplications,andusuallyisn’tanyuseanyhow.

8.Howdoesthegeneratorboxwork?

Thegeneratorboxconsistsofasmallcomputerizedchipcontrollerthat’srunbyabattery.Theboxsensesandpacesthroughthesamesetofwiresthatleadtotheendocardium.

9.Howlongdoesthebatterylast?

AsIunderstandit,implantablepacersuselithiumbatteriesthatlastanywherefrom6to10years.Thebatteryinatemporarypacingboxisaregularhardware-storetype9-voltbattery.

10.Howmuchelectricitydoesthepacemakerusetoactuallypacetheheart?

Theoutputofthepacemakerismeasuredintwoways:

“signalamplitude”,and“pulsewidth”.

11.InEnglish,please?

Signalamplitudemeans“howmuchjuicetheboxputsoutthroughthewirewitheverypulse”.Thisismeasuredinmilliamperes,andiscalledMA–there’satwistydialtocontroltheMAonthefrontofthetemporarypacerbox.Pulsewidthmeans“howlongeachpulselasts”.Theelectricalpulsehastobestrongenough,andlastlongenough,tocapturethemyocardium.

12.Whatis“capturethreshold”?

Capturethresholdistheminimumamountofelectricitythattheboxhastoemittopacetheheart–asabove,it’smeasuredinmilliamps,andthetwistyknobisturnedupuntiltheheartispaced100%-thenturneddownagainuntiltheminimumisdetermined.

13.WhydopacedbeatsgeneratedbyaventricularwirelooklikePVCs?

ThereasonthatPVCslook“wideandbizarre”isbecausetheyoriginatedownatthebottomoftheheart,attheoppositeendfromwheretheyusuallycome-thepathofdepolarizationisbackwardsasaresult.ThinkofthenormalleadIIsignal,goingfromnorthwesttosoutheast:

OregontoFlorida.Apacedsignalisgoingbackwards,upwards（“retrograde”）-thereverseofthenormalQRSwaveform.Sincethev-wiregeneratesarhythmbyemittingelectricityfromawirewhosetipisembeddedinthewalloftheRV,thedeflectionfollowsthesamepathasaPVC–soitlookslikeone.

Here’sthetwo-wirepacingstripagain:

onewireisintherightatrium,andtheother’sintherightventricle.Withtwowires,theventricularpathisstillbackwards,justaswithone–makessense,right?