Comparison of the electrophysiologic effects of intravenous and oral verapamil in patients with paroxysmal supraventricular tachycardia

The electrophysiologic effects of intravenous verapamil (a bolus dose of 0.15 mg/kg body weight followed by infusion of 0.005 mg/kg per min) were compared with those of oral verapamil (80 mg every 6 hours for 48 hours) in eight patients who had paroxysmal Supraventricular tachycardia. The mechanism of tachycardia was atrioventricular (A-V) nodal reentry in four patients and A-V reentry utilizing an accessory pathway for retrograde conduction in the remaining four. The electrophysiologic effects of oral and intravenous verapamil were similar. Both preparations significantly prolonged anterograde effective and functional refractory periods of the A-V node (p < 0.001). Both significantly increased the shortest pacing cycle length maintaining 1:1 anterograde conduction over the A-V node (p < 0.001). Retrograde conduction over the A-V node was greatly prolonged with verapamil in one patient but was unaffected in the others. There was no significant effect on sinoatrial conduction time, sinus nodal recovery time or atrial or ventricular refractoriness. Both preparations prevented induction of tachycardia in six patients none of whom had recurrence of sustained tachycardia while receiving long-term oral therapy (5 to 10 months). Neither preparation had a significant effect in two patients and this predicted failure of long-term oral therapy in one of these patients.The results of acute drug testing with intravenous verapamil can be extrapolated to predict the electrophysiologic results and response to long-term therapy with oral verapamil.