Comparative vagolytic effects of procainamide and N-acetylprocainamide in the dog

Procainamide exerts vagolytic effects which are deleterious in clinical therapy for supraventricular arrhythmias. The purpose of the present study was to determine if N-acetylprocainamide (NAPA), an active metabolite of procainamide which has been proposed as an effective and less toxic alternative, would exert an equivalent degree of vagal blockade. In anesthetized dogs, the right cervical vagus nerve was electrically stimulated at supramaximal voltage using frequencies from 0.5 to 20 Hz to slow the sinus rate. The ability of NAPA and procainamide to block this response was tested with infusion of equimolar doses (1.0 and 0.87 mg/kg/min i.v., respectively) continuously over a period of 40-78 min. Both drugs exerted statistically significant vagolytic effects at the higher frequencies of stimulation. Although the vagolytic effect appeared to be more pronounced with procainamide, this could not be demonstrated by statistical analysis of the data.