Andersons pediatric cardiology 565
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UpperRateBehavior
Upperratebehaviorreferstohowadual-chamberpacemakerwillbehavewhen
theatrialrateexceedstheprogrammedmaximumtrackingrate.Thesumofthe
AVintervalplusthePVARPyieldswhatiscalledthetotalatrialrefractory
periodanddefinesthehighestratethepacemakerwilltrackanatrialevent
before2:1AVblocksetsin.Itisimportanttotrytoprogramapacemakersoas
nottohaveapatientgofromaheartrateof180beats/minimmediatelyinto2:1
AVblockandat90beats/min.Thepacemakershouldbeprogrammedsoasto
achievetheuppertrackingratefirst,priortotheTARP,andhavethepatient
graduallyslowtheheartinaWenckebachfashionratherthan2:1AVblock
(Fig.22.38).
FIG.22.38 Tracingofupperratebehavior.Thepacemakerrhythmisatrial
sensed-ventricularpaced(AS-VP)(arrows).Thebluntarrowdemonstrates
anatrialbeatthatfallsintothepostventricularatrialrefractoryperiod
(refractory)modeandasaresultaventricularpacedbeatdoesnotfollow.
Troubleshooting
Knowledgeofthedevice'scapabilitiesandhowitrelatestothepatient'sclinical
statusiskeytosuccessfultroubleshooting.Asystematicapproachwillfacilitate
success.Themostcommonpatientcomplaintsarepalpitations,tachycardia,or
recurrenceofthepatient'spreimplantsymptoms.Oneshouldperforman
assessmentoftheoriginalindication,pacemakerdependence,activity
immediatelyprecedingtheclinicalevent,anddetailedsymptomsaswellas
durationencounteredduringtheevent.Oftensymptomsofpacingmalfunction
aresubtle,suchasfatigue,weakness,confusion,neckpulsations,activity
intolerance,shortnessofbreath,palpitations;thesemay,infact,be
asymptomatic,especiallyinchildren.
Fromadeviceperspective,themostcommonfindingsarefailuretosense,
failuretocapture,outputfailure,orachangetomagnetrate.Thefirststepduring
interrogationofthedeviceistoidentifyallbasiccomponentsincluding
manufacturer,devicemodel,leadmodels,andprogrammedsettings.Utilize
othersourcesthroughtheprocess,suchaschestx-rayforcomparisonof
postimplantfindingsandcurrentdevicefindingsonanteroposteriorandlateral
views.PerformanECGwithevaluationofrhythm,pacing,andintervaltiming
interpretation.RoutineHolterevaluationcanbehelpfulindetectingsubtle
interactionsbetweenthepacedrhythm,andtheintrinsicrhythmthatcango
unrecognized.Forexample,inapatientwithvaryingdegreesofAVconduction
who,duringinterrogation,has1:1intrinsicAVconduction,undersensingmay
goundetected.
Giventhevulnerabilityoftheleadsystem,athoroughevaluationmustbe
performed.Toidentifythepatient'sintrinsicrhythmandabilitytotoleratetesting
maneuvers,itishelpfultofirststartwithsensing.Tofunctionproperly,the
devicemustappropriatelysensetheintrinsicrhythm.Mostdevicesperform
automaticthreshold,sensing,andimpedancechecksandprovideagraphof
thesedataforevaluation.
Thesensingsettingistheamplitudeinmillivolts(mV)thatservesto
recognizethepresenceoftheintrinsicbeat.Inordertoidentifythesensing
threshold,thepacingrateisgraduallydecreasedtopermitintrinsicbeatsto
occur.Whenthisisdoneoneshouldnotethecharacteristicsandpresenceofthe
underlyingrhythm.Attimessensingisnotpossible,ifthereisnounderlying
rhythmortheintrinsicrateislessthan30beats/min.
Thresholdtesting(Fig.22.39)identifiestheminimumelectricalstimulus
requiredtoconsistentlycapturethemyocardiumandtriggeratrialorventricular
depolarization.Thethresholdisthemeasurementofamplitudeinvolts(V)and
thepulsewidthismeasuredinmilliseconds.Toperformatrialthresholdtesting,
thepacingratemustbefasterthanthepatient'sintrinsicatrialrate.Ventricular
testingcanbeperformedbyavarietyofmethods.Ifthereisasingle-chamber
ventricularpacemaker,thetechniqueissimilartoatrialtesting.Ifthereis
completeAVblockwithadual-chamberpacemaker,thereisnoneedtochange
thepacingrate.WhenAVconductionispresent,testingoftheventricular
capturethresholdcanbeassessedbyprogrammingtheAVintervaltoatiming
shorterthanintrinsicconduction.Whencapturethresholdsaretested,theoutput
isincrementallydecreaseduntillossofcaptureoccurs.Thecessationofthetest
resultsinreversionbacktoimmediatepretestpacedorsensedvalues.Onceyou
haveidentifiedtheatrialand/orventricularcapturethresholds,theoutputshould
beprogrammedtotwotimesthemeasurethresholdorthreetimesthepulse
width.Insomecasesitmaybehelpfultodrawastrengthdurationcurveto
optimizesettingsandoptimizecapturewhileavoidingexcessivebattery
depletion.Anotherprogrammingfeaturepresentinmostdevicemodelsisthe
abilitytosetthethresholdtoadaptivemode.Thiswillenablethedeviceto
performautomaticthresholdtestingandadjusttheoutputdependingonthe
detectedthresholdatthattime,preventingthedevelopmentofexitblock.The
developmentofexitblockduetothelossofmyocardialcapturecanbe
influencedbyclinicalstatus,hemodynamicinstabilitysuchasviruses,
medications,severeelectrolyteabnormalities,metabolicdisorders,pHbalance,
andnewmyocardialfibrosis.
FIG.22.39 Ventricularthresholdtest.Inthisexample,theamplitudeis
keptstableat2.0Vandthepulsewidthisprogressivelydecreased.
Captureisdemonstratedbyaventricularpacingartifactfollowed
immediatelybyawideQRScomplexwithaTwaveintheopposite
directionofdepolarization.At0.12ms,eachpacemakerartifactisfollowed
byacapturedbeat.At0.09ms,onlythefirstbeatiscaptured.Twofurther
pacingartifactsarenotfollowedbyventriculardepolarization,andlater
thereisaventricularescapebeat.Thethresholdisdescribedas0.12msat
2.0V.VP,ventricularpace.
ImplantConsiderations
Manychallengesareinvolvedinconsideringdevicetherapyforsmallpatients
andthosewithcongenitalheartdisease.Decisionsmustbeindividualizedtothe
patient'scomplexanatomy,size,residualshunts,myocardialfunction,fibrosis,
long-termvenousaccess,riskforthrombusformation,endocarditis,andlongtermarrhythmiarisk.Thetypeofdevicesystemimplantedisguidedby
individualpatientcircumstances.Selectionregardingasingle-versusdualchamberversusbiventricularsystem,unipolarleadsorbipolarleads,andMRI
