Effect of procainamide on reentry within the His-Purkinje system in man

The effects of a single intravenous infusion of 500 mg of procainamide on macro-reentry within the His-Purkinje system were assessed, in 10 patients using bundle of His electrograms and the ventricular extrastimulus method. Procainamide did not abolish reentry in any of the 10 patients and produced repetitive reentry (two or more consecutive reentrant beats) in 8 of 10 patients. On the average, procainamide caused reentry to be initiated at longer S₁-S₂ intervals, widening the zone of reentry. In all but one patient reentry was manifest at significantly longer S₂-H₂ intervals than control values. The H-V intervals of reentrant beats (H₂-V₃ intervals) after procainamide were longer than control values at comparable S₂-H₂ intervals. Procainamide shortened or abolished the retrograde gap zones thereby causing reentry to be present more frequently within the zone of reentry. No significant change was seen in effective refractory period of ventricular muscle. On the assumption that many clinically encountered ventricular arrhythmias that respond to procainamide result from micro-reentry, the observed differences between the effects of procainamide on macro- and micro-reentry may be explained on the basis of the relation between drug effect and size of the reentrant circuit. Procainamide may produce a greater increase in refractoriness in a micro-reentrant circuit than in macro-reentry, thereby producing bilateral block and abolishing reentry in the former; or if the degree of electrophysiologic changes produced by procainamide were the same in both types of circuits, then for a given change, the pathway in a micro-reentrant circuit may be too small to sustain reentry.