Encainide for treatment of supraventricular tachycardias associated with the Wolff-Parkinson-White syndrome.

Thirty-three patients with supraventricular tachycardia associated with the Wolff-Parkinson-White syndrome were treated with encainide for 26 months (mean). Encainide at a mean dosage of 187 mg/day abolished or markedly decreased episodes of palpitations in 24 of 33 (73%), and no patient had syncope or required cardioversion while receiving the drug. Encainide was well tolerated and was discontinued in only 2 patients because of side effects (6%). Only 1 patient (3%) had a proarrhythmic effect while taking encainide (ventricular tachycardia). Fourteen of 16 patients (88%) with atrial fibrillation continue receiving encainide. Episodes of palpitations have been abolished or markedly decreased and no patient has had syncope or required cardioversion. All 14 of these patients had either anterograde block in the accessory pathway during atrial fibrillation or greater than or equal to 75 ms increase in the shortest R to R interval formed by 2 preexcited QRS complexes. Encainide prolonged refractory periods of the atrial (p = 0.064) and ventricular (p = 0.061) muscle. It prolonged the cycle length at which 1:1 conduction of the accessory pathway in both the anterograde and retrograde directions occurred (both, p less than 0.001). Induction of atrioventricular-reciprocating tachycardia (AVRT) was prevented in 36% of patients at repeat electrophysiologic study. The AVRT cycle length increased 112 ms (mean, p less than 0.001) in those patients in whom AVRT was still inducible. The loss of delta waves recorded with the 12-lead scalar electrocardiogram during encainide therapy was a significant predictor of anterograde accessory pathway block (p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)