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We studied transient entrainment and interruption of atrioventricular (AV) bypass pathway-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 of slow conduction.The data from this study suggest that (1) if one can demonstrate constant fusion beats during transient entrainment of a tachyarrhythmia except for the last transiently entrained beat; or (2) if during transient entrainment of a tachyarrhythmia at two or more different pacing rates, one can demonstrate constant fusion at each of the different pacing rates, but different degrees of fusion at the different rates; or (3) if interruption of a tachyarrhythmia by overdrive pacing is associated with localized conduction block to a site followed by activation of that site by the next pacing impulse from a different direction and with a shorter conduction time, then the underlying mechanism of the arrhythmia can be best explained by reentry.

circulation

Transient entrainment and interruption of the atrioventricular bypass pathway type of paroxysmal atrial tachycardia. A model for understanding and identifying reentrant arrhythmias.