Spontaneous atrial premature depolarizations during paroxysmal reentrant tachycardia.

The spontaneous onset and termination of many episodes of paroxysmal tachycardia, each initiated by an atrial premature depolarization, were studied in one patient. Surface electrocardiograms alone were inadequate to define the mechanisms underlying the frequent irregularity of atrial and ventricular cycle lengths during the tachycardia and the nature of spontaneous termination of the tachycardia. Unipolar atrial electrograms demonstrated that the irregularity during the tachycardia was due to premature atrial depolarizations that reset the reentrant cycle sustaining the tachycardia, and each spontaneous termination was due to an even more premature atrial depolarization interrupting the reentrant pathway. The genesis of the atrial premature depolarizations resetting and terminating the tachycardia, and their relationship to those initiating the tachycardia, are discussed.