Effects of oral verapamil in patients with atrioventricular reentrant tachycardia incorporating an accessory pathway.

In 14 patients with atrioventricular reentrant tachycardia incorporating an accessory pathway, electrophysiologic studies were performed before and serially at 0.5--1-hour intervals for 6 hours after the fourth dose of 80 mg of oral verapamil given every 6 hours. Verapamil increased both the longest atrial paced cycle length producing type 1 atrioventricular block and the effective refractory period of the atrioventricular conduction system at all measurements. Before verapamil, sustained tachycardia could be induced in all 14 patients. After verapamil, six patients had induction of echo beats alone at all measurements, and in eight patients nonsustained or sustained tachycardia could be induced, particularly after the fourth hour. Follow-up study with oral verapamil at the same dosage in 13 patients for 7 +/- 5 months (+/- SD) revealed that the six patients with induction of echo beats alone have been free of symptomatic arrhythmia, while six of the remaining eight patients had occasional attacks of sustained tachycardia. Thus, oral verapamil increases atrioventricular nodal refractoriness, with an effect lasting up to 6 hours. Electrophysiologic study performed 5-6 hours after verapamil can be used to predict clinical responses in patients with atrioventricular reentrant tachycardia.