Encainide and flecainide: Are they interchangeable?

Abstract

Encainide and flecainide are two recently released class IC antiarrhythmic agents that have been reported to be highly effective for suppressing all forms of ventricular arrhythmias. Both drugs are potent blockers of the sodium channel and produce marked slowing of conduction within the His-Purkinje system and myocardium. To establish if the response to one agent predicts the response to the other, the antiarrhythmic effect of the two drugs was compared in a group of 18 patients with malignant ventricular arrhythmia. Fourteen patients had sustained ventricular tachycardia and four patients had ventricular fibrillation. The average ejection fraction of this group was 37%. All patients had a high density of spontaneous ventricular arrhythmia and were evaluated noninvasively. Both drugs significantly reduced all forms of spontaneous arrhythmia. By Holter monitoring criteria, the effect of the two drugs was concordant in 61% ( p = NS). By exercise criteria, the concordance rate was 50% ( p = NS). Overall, 12 patients (67%) responded to encainide and eight (44%) patients responded to flecainide ( p = NS). Only 10 of 18 (56%) patients had the same response to both agents ( p = NS). There was no significant difference in ejection fraction or in average daily dose of drug between responders and nonresponders. Thus despite their similar electrophysiologic profile, the clinical antiarrhythmic effect of these two agents was different. For the noninvasive management of patients the response to one agent should not be used to predict the response to the other.