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 ms, respectively. After intravenous encainide antegrade conduction over the AP was blocked in 4/9 patients and the antegrade effective refractory period (ERP) was prolonged in another 4. Oral encainide blocked AP conduction in 4 cases and prolonged ERP considerably in the others.(ABSTRACT TRUNCATED AT 250 WORDS)