The dynamic nature of ventriculoatrial conduction.

An endless loop tachycardia starts when the atrial sensory amplifier of a dual chamber pacemaker identifies an early atrial signal originating from a ventricular or atrial premature depolarization or from myopotential noise. The tachycardia will continue as long as ventriculoatrial conduction is sustained. By selecting the appropriate atrial sensitivity setting, postventricular atrial refractory period, or upper rate limit, it is possible to eliminate sustained endless loop tachycardia. Electrophysiological data obtained at the time of dual chamber pacemaker implantation can assist the physician when selecting these settings. This report summarizes our intraoperative data on ventriculoatrial conduction obtained from 432 consecutive patients. One hundred sixty-two patients had evidence of ventriculoatrial conduction including 14% of patients with antegrade complete heart block and 32% with 2:1 AVB. The majority of patients with preserved antegrade conduction had sustained retrograde conduction. During incremental ventricular pacing, ventriculoatrial conduction prolonged in the majority of patients, and with faster ventricular pacing rates, ventriculoatrial block developed. Ventriculoatrial block developed in half of the patients at a ventricular pacing rate exceeding 120 bpm. Analysis of these data suggests that by selecting an upper rate limit of 140 bpm, a postventricular atrial refractory period of 300 msec, and an atrioventricular interval of 125 msec, approximately 90% of patients will not have sustained endless loop tachycardia.