Phase 3 and phase 4 block of the accessory pathway in Wolff-Parkinson-White syndrome--a case report.

A 12-year-old girl presented with recurrent episodes of supraventricular tachycardia. A 12 lead electrocardiogram showed normal sinus rhythm with a normal PR interval and no evidence of preexcitation. A 24 h Holter monitor showed intermittent preexcitation. Both phase 3 and phase 4 block in the accessory pathway were demonstrated with introduction of atrial extrastimuli during sinus rhythm at electrophysiology study. However, Holter recording failed to reveal a consistent relationship between coupling intervals and accessory pathway conduction. The discrepancy between the Holter monitor and the electrophysiologic study can be explained by variability of factors such as autonomic tone during the Holter recording. This case illustrates a potential mechanism for intermittent preexcitation in the Wolff-Parkinson-White syndrome.