The short-term effects of incremental doses of
procainamide (7.5, 15, 22.5, and 30 mg/kg) on right ventricular effective refractory period, intraventricular conduction, and induction of
ventricular tachycardia were determined in 31 patients who had a history of sustained, unimorphic
ventricular tachycardia. QRS duration during incremental ventricular pacing was used as an index of rate-dependent changes in intraventricular conduction. The mean plasma
procainamide concentrations corresponding to the incremental doses were 5.5 +/- 1.2 (+/- SD), 9.0 +/- 1.6, 12.6 +/- 2.2, and 16.3 +/- 3.2 mg/liter. Each incremental dose of
procainamide up to a dose of 30 mg/kg resulted in a significant increment in right ventricular effective refractory period and each dose up to 22.5 mg/kg potentiated a rate-dependent prolongation of QRS duration. After the 7.5 mg/kg dose of
procainamide, induction of
ventricular tachycardia was suppressed in eight of 31 patients. After higher doses of
procainamide, induction of
ventricular tachycardia was suppressed in two additional patients. In three of 10 patients in whom the induction of
ventricular tachycardia was suppressed by 7.5, 15, or 22.5 mg/kg of
procainamide, sustained unimorphic
ventricular tachycardia was again inducible after a higher dose of
procainamide. In three of 31 patients, only
nonsustained ventricular tachycardia was inducible after a 7.5 to 22.5 mg/kg dose of
procainamide; however, in two of these three patients, sustained
ventricular tachycardia was again inducible after administration of a higher dose of
procainamide. In conclusion, during electropharmacologic testing with
procainamide, it is worthwhile to test a dose of 7.5 mg/kg, because this dose is often effective in patients who respond to this
drug. However, the results of this study indicate that
procainamide may be effective in suppressing the induction of sustained
ventricular tachycardia at a relatively low plasma concentration, but not at a higher plasma concentration. Therefore, during long-term
therapy with
procainamide it may be important to avoid plasma
procainamide concentrations not only lower, but also higher than the concentration that results in the suppression of induction of
tachycardia.