Contrasting effects of verapamil and procainamide on intraventricular conduction and reentry within the His-Purkinje system in man.
The effects of intravenous verapamil (0.15 mg/kg) and procainamide (12-14 mg/kg) on intraventricular conduction and reentry within the His-Purkinje system were studied in eighteen patients using His bundle electrograms and ventricular extrastimulus method. Verapamil's effects were studied in eight patients and procainamide's effects in ten patients. Plasma verapamil concentrations ranged from 56 to 192 ng/ml (mean +/- SD: 139.0 +/- 46.0); plasma procainamide concentrations ranged from 11.3 to 19.0 mg/liter (mean +/- SD: 14.9 +/- 2.5). Verapamil caused no change in latency, intramyocardial conduction (duration of QRS complex), and His-Purkinje (V2H2 interval) conduction of even the earliest premature impulses introduced before the completion of repolarization of the His-Purkinje system and the ventricular myocardium, i.e. when some fibers were presumably at the level of membrane potential at which conduction becomes, wholly or in part, dependent on slow inward calcium current. Verapamil did not abolish or modify the zone of reentry, and did not significantly change the determinants of reentry in any of the eight patients. In contrast, procainamide significantly prolonged the latency, the duration of QRS complex and the V2H2 interval in each of the ten patients, abolished reentry in seven patients, and decreased the width of reentry zone in three patients. The contrasting effects of verapamil and procainamide on intraventricular conduction and reentry within the His-Purkinje system suggest that slow conduction of early premature impulses is due to an incompletely reactivated rapid inward sodium current and not to a fully activated slow inward calcium current. Further, our observations suggest that the system responsible for slow conduction cannot be recognized from the magnitude of conduction delay.