N-acetylprocainamide induced changes in refractoriness and monophasic action potentials of the dog heart in situ.

The major metabolite of procainamide (PA) is N-acetylprocainamide (NAPA). The effect of this drug on the electrophysiology of the right ventricle was studied in the dog heart in situ by means of programmed electrical stimulation and monophasic action potential (MAP) recordings. A decrease in the beat interval caused a progressive decrease in refractoriness and MAP duration. Intravenous injection of NAPA 50 mg.kg-1 body weight increased refractoriness to a greater extent than MAP duration at shorter, while the opposite was observed at longer beat intervals. These observations are compatible with delayed recovery of excitability. On the basis of these findings, it is concluded that NAPA belongs to antiarrhythmic drugs of type I according to Vaughan Williams' classification, and NAPA may be an alternative drug for treatment of cardiac arrhythmias.