Mechanisms of spontaneous alternation between reciprocating tachycardia and atrial flutter-fibrillation in the Wolff-Parkinson-White syndrome.

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Abstract
In a group of 36 consecutive patients with the Wolff-Parkinson-White (WPW) syndrome undergoing electrophysiological studies because of paroxysms of reciprocating tachycardia (RT) and/or atrial flutter-fibrillation (AF), 7 patients (19%) had repeated episodes of spontaneous alternation between RT and AF. Electrophysiological studies demonstrated left-sided anomalous pathways (AP) in all 7 patients. Atrial vulnerability, as evidenced by the occurrence of repetitive atrial responses or a paroxysm of AF following a single atrial premature stimulus, was noted in all. Invariably, spontaneous conversion of RT to AF (7 patients) was triggered by an atrial premature depolarization which resulted in atrial asynchrony during the atrial vulnerable phase. Spontaneous conversion of AF to RT (3 of the 7 patients) required the presence or the development of antegrade unidirectional block in the AP prior to the cessation of AF. The demonstration of atrial vulnerability in association with the phenomenon of spontaneous alternation between RT and AF provides further information pertaining to the understanding of the mechanisms of tachyarrhythmias in the WPW syndrome. The occurrence of this electrophysiological phenomenon may be more common than is generally appreciated, and optimal medical treatment should be directed toward controlling both RT and AF in this group of Wolff-Parkinson-White patients.