Encainide for treatment of atrioventricular reciprocating tachycardia in the Wolff-Parkinson-White syndrome

Oral encainide, varying from 75 to 300 mg/day (mean 174 mg/day), was administered to 52 patients with drug-resistant atrioventricular reciprocating tachycardia (AVRT) associated with the Wolff-Parkinson-White syndrome. Electrophysiologic studies were performed before and during drug treatment. Encainide resulted in anterograde accessory pathway block in 15 of 37 (41%) and retrograde accessory pathway block in 11 of 46 (24%) patients. In patients With residual accessory pathway conduction, encainide significantly prolonged the shortest pacing cycle length maintaining anterograde (261 ± 26 to 404 ± 85 ms) and retrograde (279 ± 46 to 436 ± 87 ms) accessory pathway conduction, as well as the anterograde accessory pathway effective refractory period (271 ± 32 to 329 ± 73 ms). AVRT could not be induced during encainide therapy in 20 of 49 patients (41%). In the remaining patients, AVRT cycle length increased (319 ± 44 to 426 ± 90 ms, p <0.001) due to prolongation of HV and ventriculoatrial intervals. During follow-up (mean 38.5 months), 30 patients continued to take the drug and 7 patients with favorable drug response subsequently elected to undergo surgical accessory pathway ablation (71% overall favorable response). Encainide was ineffective in 11 patients, was discontinued because of drug intolerance in 2 patients and exacerbated ventricular tachycardia in 2 patients. Lack of AVRT inducibility at encainide electrophysiologic study did not always predict recurrence-free follow-up. Encainide is an effective and well-tolerated drug to prevent recurrence of AVRT in patients with Wolff-Parkinson-White syndrome.