Reversal of electrophysiologic effects of flecainide on the accessory pathway by isoproterenol in the Wolff-Parkinson-White syndrome.

To determine the reversibility of the effects of flecainide on accessory pathways, electrophysiologic studies were performed in the drug-free control state, after flecainide loading and with isoproterenol infusion during flecainide treatment in 12 patients with symptomatic preexcitation syndrome. After the baseline drug-free evaluation, oral flecainide was given in dosages of 50 to 200 mg twice daily (mean daily dose 282 +/- 75) for at least 4 days before the repeat electrophysiologic study. Isoproterenol infusion was given in dosages of 1 to 4 micrograms/min to increase the heart rate at rest by 50%. Anterograde block in the accessory pathway was observed in 3 patients with flecainide therapy, whereas in the other patients the anterograde refractory period increased from 243 +/- 20 to 315 +/- 23 ms (p less than 0.05). The shortest preexcited RR interval during atrial fibrillation lengthened from 234 +/- 27 ms before flecainide to 313 +/- 38 ms (p less than 0.05). Retrograde block occurred in 2 patients after flecainide, whereas the retrograde refractory period of the accessory pathway increased from 247 +/- 26 to 337 +/- 45 ms in the other patients. Orthodromic atrioventricular reciprocating tachycardia, inducible in 10 patients before therapy, became noninducible in 3 patients. Its rate was significantly slowed in the other 7 patients (from 346 +/- 50 to 471 +/- 81 ms). In 2 patients the tachycardia was nonsustained during flecainide treatment. Atrial fibrillation, inducible in all patients at baseline, was rendered nonsustained and more difficult to induce in 7 patients with flecainide. When isoproterenol was infused during flecainide treatment, complete anterograde (3 patients) or retrograde block (2 patients) persisted in the accessory pathway.(ABSTRACT TRUNCATED AT 250 WORDS)