Electrophysiologic and clinical effects of intravenous and oral encainide in patients with Wolff-Parkinson-White syndrome and paroxysmal atrial fibrillation

The electrophysiologic effects of encainide were studied in 10 patients with Wolff-Parkinson-White syndrome after intravenous (1 mg kg^-1 in 60 minutes) and oral administration of two dose regimens (75 and 150 mg daily). Under control conditions atrial fibrillation (AF) with a rapid ventricular response was induced in all patients and atrioventricular reciprocating tachycardia (AVRT) in 9 patients. After intravenous encainide AF was no longer induced in 3/9 patients; in 3 of the remaining the accessory pathway (AP) was totally blocked and in the others the shortest RR interval increased from 213 ± 6 to 297 ± 91 ms and the mean RR interval from 293 ± 39 to 362 ± 79 ms. The lower dose of oral encainide prolonged the shortest RR interval from 206 ± 24 to 273 ± 64 ms and the mean RR interval from 280 ± 48 to 368 ± 52 ms in 6 patients; in 2 cases no preexcited beats were recorded and in 1 AF was not inducible. After the higher dose of oral encainide AF was still inducible in 7/8 cases; in 3 the AP was blocked and in the others the shortest and mean RR intervals increased from 202 ± 30 to 280 ± 24 ms and from 276 ± 59 to 436 ± 80, respectively. After intravenous encainide antegrade conduction over the AP was blocked in 4/9 patients and the antergrade effective refractory period (ERP) was prolonged in another 4. Oral encainide blocked AP conduction in 4 cases and prolonged ERP considerably in the others. Induction of AVRT was prevented in 1/8 patients after intravenous and in 5/9 patients after oral encainide; in the 4 patients in whom AVRT remained inducible cycle length increased from 306 ± 31 to 354 ± 49 ms after intravenous encainide and to 392 ± 46 ms after oral administration. All patients were discharged on encainide (mean maintenance dose, 127 mg daily) and followed for 21 ± 7 months; no recurrence of AF was observed; two patients complained of transient mild side effects. These data show that in patients with Wolff-Parkinson-White syndrome encainide prolongs refractoriness and slows conduction over the AP; it prevents induction of AVRT and markedly slows ventricular response during AF, thus protecting against life-threatening arrhythmias.