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?
”
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?