Andersons pediatric cardiology 564
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variationsofasinglecompletepacingcycle.Ineachmodeapacingoutputis
deliveredattheendoftheescapeintervalthatwillcorrespondwiththe
programmedlowerratelimit.Ifthereisasensedevent,thepacingoutputis
inhibitedandtheescapeintervalisreset.Theescapeintervalisthelongesttime
betweenanytwoatrialorventricularevents.Tosenseandpacethesame
chamber,refractoryperiodsmustbeconsidered.Directlyfollowingapaced
event,willbeablankingperiodofabout50to100ms.Duringtheblanking
period,noeventissensed.Thisfeatureisdesignedtopreventoversensingofthe
afterpotentials.Followingtheblankingperiodistherefractoryperiod.The
generatorcansensetheevent,butthiseventwillnotresetthetimingcycle.
Blankingandrefractoryperiodshelptodecreasetheriskofoversensing
afterpotentials,suchasnear-fieldT-waveoversensing,far-fieldatrialor
ventricularactivity,myopotentials,electromagneticinterference,leadnoisefrom
apossibleinsulationdefect,orheaderconnectionproblems.Oversensing
happenswhensignalsotherthanthelocalcardiacactivationattheleadelectrode
siteiscountedasacardiacsignal.
FIG.22.36 (A)Tracingshowingadualchamberpacemakerwith
appropriateatrialtracking(AS-VP).(B)Tracingdemonstratingadualchamberpacemakerwithatrioventricularsequentialpacing(AP-VP).
Modeswitchisdesignedtopreventundesirablerapidventricularpacing
causedbytrackingatrialarrhythmiasduringDDDoperation,butitcanaddto
thecomplexityofinterpretation.Ifatrialarrhythmiasaredetected,theresultmay
bereversiontoanontrackingmode(DDIorVVI).Undersensingoccurswhen
thedevicefailstosenseanintrinsiceventandthereforedeliversapacedbeat.
ProgrammableFeatures
Knowingthatdefaultsettingsaredesignedfortheadultpopulation,adetailed
understandingofallthepossibleprogrammablefeaturesisparamountwhen
programmingthedeviceforapediatricpatient.Programmingthelowerratelimit
determinesthelowestacceptableratebeforeapacingstimuluswilloccur.Ifrate
modulationisprogrammed,therateisdeterminedbysensoractivity.Ifthereis
anattempttominimizeventricularpacing,thehysteresisratecanbe
programmed.
Hysteresisallowsforalongerescapeintervalfromthelastventricularsensed
eventtothefirstventricularpacedevent.Programmingahysteresisratepermits
alowerintrinsicheartratebeforepacingstimulation.Forexample,ifthedevice
isprogrammedwithalowerratelimitof60beats/min(technicallythedeviceis
programmedatppmnotbeatsperminute)andahysteresisrateof50beats/min,
thedevicewillnotpaceaslongastheintrinsicrateisabove50beats/min.
However,iftheratedropsbelow50beats/min,thedevicewillpaceatthe
programmedlowerratelimitof60beats/min.Anotheroptiontodiscourage
pacingisbyprogrammingthesleepmode.Becausecardiacdemandduringsleep
islessthanthechronotropicneedwhileawake,somedeviceshaveasleepmode
option.Sleepmodehastheabilitytodetermineaprogrammedwindowoftime
thatwilldecreasethelowerratelimittotheprogrammedrateduringthosehours.
Forexample,ifthelowerratelimitissetfor60beats/min,onecanprogram
nighttimehourstopaceat50beats/min.
TheAVinterval(AVI)isaprogrammedintervalthatdeterminesthemaximum
timeafteranatrialeventinwhichaventriculareventcanhappenbefore
deliveringaventricularpacedstimulus.ProgrammingofthepacemakerAVI
optimizeshemodynamicsbypermittingcoordinatedatrialandventricular
contractionsthatmimictheintrinsicPRinterval.Thechallengeistoachievea
shortenoughAVItopromotesAVsynchronyandalongenoughAVItoallow
timeforintrinsicactivationtooccur.
Thepostventricularatrialrefractoryperiod(PVARP)isaprogrammable
parameterthatoccursafterasensedorpacedventriculareventinwhichatrial
sensedeventsdonotresultinacorrespondingAVintervalfortheprogrammed
timeperiod.PVARPsettingscanbehelpfulinpreventingsensingofretrogradeP
wavesafteraventricularpacedorsensedevent.Withoutthisprogrammed
feature,theatrialchannelmaysensearetrogradeatrialsignalasanintrinsic
eventandstimulateaventricularpacedeventthatalsoconductsretrograde.This
recurringcycleisknownaspacemaker-mediatedtachycardia(PMT).IfPMTis
presentoninterrogationofthepatient'sdevice,youwillcommonlyfindthe
pacemakerpacingatorbelowtheMTR.TopreventPMT,thePVARPisoften
programmedtobelongerthantheretrogradeconductiontime.Extendingthe
PVARPtoolongcanleadtoanabsenceofatrialtrackingatrateslowerthanthe
upperratelimit(Fig.22.37).
FIG.22.37 Pacemaker-mediatedtachycardiaisanendless-loop
tachycardiathatissustainedbythepacemakerandrequiresretrograde
ventriculoatrialactivation.Extensionofthepostventricularatrialrefractory
periodplacestheatrialbeatinrefractorymodeandsuccessfullyterminates
thetachycardia.
ThePVARPperiodpreventssensingretrogradePwavesand/ortrackingfarfieldventricularrepolarizationontheatrialchannel.
