Pharmacodynamics of intravenous procainamide as used during acute electropharmacologic testing

Abstract

No previous studies have determined the pharmacodynamics of intravenous procainamide when administered in a dose of 15 mg/kg and at a rate of 50 mg/min, as is common practice during etectropharmacologic testing. In this study, 30 patients received procainamide in this fashion; the right ventricular effective refractory period and the QRS duration at a ventricular pacing rate of 120/minute were then determined every minute for 20 minutes. Ten patients received no maintenance infusion of procainamide (group A), 10 received a 4 μg/min maintenance infusion (group B) and 10 received an 8 mg/min maintenance infusion (group C). Ten additional patients received no procainamide and served as control subjects (group D). The plasma procainamide concentration was measured at 1, 5, 10,15 and 20 minutes after the loading dose was administered. A stable plasma procainamide concentration was not present in group A, B, or C until 15 minutes after infusion of the loading dose. The effective refractory period and QRS duration increased compared with baseline at 1 minute, decreased between 1 and 10 minutes and then remained essentially unchanged between 10 and 20 minutes in all 3 treatment groups. Concentration-effect relation was linear in each treatment group. The plasma procainamide concentrations in group C were significantly greater than in group A; however, the effects on refractoriness and QRS duration were similar in both groups. These findings indicate that with a procainamide dosing method commonly used during electropharmacologic testing, the plasma procainamide concentration decreases significantly during the first 15 minutes after the loading dose is administered; the effects of procainamide on ventricular refractoriness and conduction parallel the changes in the plasma procainamide concentration; and an 8 mg/min maintenance infusion of procainamide results in higher plasma procainamide concentrations without an associated increase in ventricular refractoriness or slowing of conduction.