A Model for Understanding and Identifying Reentrant Arrhythmias

SUMMARY We studied transient entrainment and interruption of atrioventricular (AV) bypass path-way-type paroxysmal atrial tachycardia in 15 patients during overdrive pacing from selected atrial sites. Overdrive atrial pacing at less than a critically rapid rate for interruption transiently entrained the tachycardia. Transient entrainment was due to repeated early entrance of the wave front from the pacing impulse into the reentry loop in both antidromic and orthodromic directions. The antidromic wave front of each pacing impulse was repeatedly blocked as it collided with the orthodromic wave front of the previous beat, in effect extinguishing the tachycardia. However, the early entrance of the orthodromic wave front of each pacing impulse repeatedly reset the tachycardia. The result was that during transient entrainment, the tachycardia rate increased to the pacing rate. Interruption of the tachycardia occurred when overdrive pacing produced block within the reentry loop of both the antidromic and orthodromic wave fronts of the same pacing impulse, the block occurring either at separate sites within the reentry loop or at the same site. Atrial fusion beats were demonstrated during transient entrainment in nine patients and resulted from intraatrial collision of the antidromic wave front from the pacing impulse with the orthodromic wave front of the previous beat. The presence offusion beats depended critically on the relationship of the pacing site to the reentry loop and the duration of conduction around the reentry loop, particularly through the area o