Transient Entrainment andInterruption ofthe Atrioventricular BypassPathwayType ofParoxysmal Atrial Tachycardia AModelforUnderstanding andIdentifying Reentrant Arrhythmias

SUMMARY We studied transient entrainment andinterruption ofatrioventricular (AV)bypass pathway-type paroxysmal atrial tachycardia in15patients during overdrive pacing fromselected atrial sites. Overdrive atrial pacing atlessthana critically rapidrateforinterruption transiently entrained the tachycardia. Transient entrainment was duetorepeated early entrance ofthewave front fromthepacing impulse into thereentry loopinbothantidromic andorthodromic directions. Theantidromic wave front of eachpacing impulse was repeatedly blocked asitcollided withtheorthodromic wave front oftheprevious beat, ineffect extinguishing thetachycardia. However, theearlyentrance oftheorthodromic wave front of eachpacing impulse repeatedly reset thetachycardia. Theresult was that during transient entrainment, the tachycardia rateincreased tothepacing rate.Interruption ofthetachycardia occurred whenoverdrive pacing produced block within thereentry loopofboththeantidromic andorthodromic wave fronts ofthe same pacing impulse, theblock occurring either atseparate sites within thereentry loop oratthesame site. Atrial fusion beats were demonstrated during transient entrainment inninepatients andresulted from intraatrial collision oftheantidromic wave front fromthepacing impulse withtheorthodromic wave front oftheprevious beat. Thepresenceoffusion beats depended critically on therelationship ofthepacing site to thereentry loopandtheduration ofconduction aroundthereentry loop, particularly through theareaof slowconduction. Thedatafromthis study suggest that(1)ifone can demonstrate constant fusion beats during transient entrainment ofa tachyarrhythmia except forthelasttransiently entrained beat; or(2)ifduring transient entrainment ofa tachyarrhythmia attwoor more different pacing rates, one can demonstrate constant fusion ateachofthedifferent pacing rates, butdifferent degrees offusion atthedifferent rates; or (3)if interruption ofatachyarrhythmia byoverdrive pacing isassociated withlocalized conduction block toasite followed byactivation ofthatsite bythenextpacing impulse fromadifferent direction andwithashorter conduction time, thentheunderlying mechanism ofthearrhythmia can bebestexplained byreentry. PAROXYSMALatrial tachycardia that involves antegrade conduction fromtheatria through theatrioventricular (AV)node-His-Purkinje