Andersons pediatric cardiology 555
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FIG.22.25 Catheterelectrodespositionedinthehighrightatrium,right
ventricularapex,siteoftheHisbundle,andcoronarysinus(poleslabeled).
Thefluoroscopicviewsarefrontal(A),leftanterior(B),andrightanterior
(C)orientations.
IntracardiacRecordingsandProgrammed
Stimulation
Baselinemeasurements:Usingthecathetersplacedasdescribed,intracardiac
conductiontimescanbemeasuredinsinusrhythm.ThePAintervalallowsan
assessmentoftheintraatrialconductiontime;theAHintervalmeasuresAV
nodalconduction;andtheHVmeasurestheHis-Purkinjeinterval.Intervalsare
measuredinmillisecondsandcyclelengthmeasurementsareusedratherthan
heartrates.Thecyclelengthis60,000dividedbytheheartrate.
Programmedstimulation:Thisisusedtomeasuretheelectrophysiologic
propertiesoftheatria,ventricles,andAVnodeaswellasaccessorypathways
whenpresent.Thesemeasuresareusedtoinitiatetachycardia.Typicallythis
includesthefollowing:
▪Incrementalpacing:Atrialorventricularpacing
startsatalongcyclelength(slowheartrate),andthe
cyclelengthisgraduallyshorteneduntilconductionis
blockedortachycardiaisinitiated.Thisisdoneto
assessthefunctionoftheanterogradeaccessory
pathwayandisreportedastheshortestatrialpacing
cyclelengththatconductswithpreexcitation.
▪Extrastimulustesting:Traditionallythisinvolves
atrialorventricularpacingusingatrainofeightbeats
atafixedcyclelength(600,500,and400ms)andis
followedbyaprematureextrastimulus.The
extrastimuliareaddedatgraduallyshortercycle
lengthsuntilblockorlackofcaptureoccurs.This
allowsforthedeterminationofatrial,AVnodal,
accessorypathway,andventricularrefractoryperiods.
Whentheprematureextrastimulusfailstoexcitethe
tissueofinterest,thisistheeffectiverefractory
period.Forexample,atrainof8ventricularbeatsata
cyclelengthof500isfollowedbyaprematurebeat;
whenthisisdeliveredat480ms,conductionchanges
frommidline(concentric),overtheAVnode,to
eccentricconductionoveranaccessorypathwaythat
conductsonlyintheretrogradedirection(concealed).
Theaccessorypathwaycontinuestoconduct
retrogradeuntiltheprematurebeatisaddedat360
ms.Thus,inthiscasetheretrogradeAVnode,the
effectiverefractoryperiodis480msandthe
retrogradeaccessorypathway'seffectiverefractory
periodis360ms.ThistechniquecanexposedualAV
nodalpathwaysandcaninitiatetachycardia.Once
refractorinessisreachedinonelimbofatachycardia
circuit,theextrastimuluscan“jump”toanotherlimb
ofthecircuitandthencirclebackoverthenow
recoveredlimb.Single,double,ortripleextrastimuli
canbeaddedafterthedrivetraintoattempttoinitiate
tachycardia.Extrastimuliintroducedintotachycardias
canhelptocharacterizethetachycardiaorcan
terminateit.
▪Burstpacing:Thisistheadditionofaperiodof
atrialpacingatcyclelengthsthatareoftenveryshort,
includinganattempttoinitiateatrialflutterorAF.
Thisisusuallynotperformedintheventricleasit
risksinitiatingahemodynamicallyunstableand
possiblyclinicallyunimportantventricular
arrhythmia.
