We compared the electrophysiological effects of intravenous propafenone and flecainide on accessory pathway conduction by a randomized crossover study in 16 patients with Wolff-Parkinson-White syndrome. The antegrade refractory period of the pathway increased from 256 +/- 18 msec at baseline to 288 +/- 13 msec on propafenone (P less than 0.05) and to 296 +/- 27 msec on flecainide (P = 0.075). The minimum preexcited RR interval during atrial fibrillation or incremental atrial pacing was prolonged from 225 +/- 37 msec to 262 +/- 22 msec by propafenone (P less than 0.05) and to 301 +/- 31 msec by flecainide (P less than 0.005). The prolongation was significantly greater with flecainide than propafenone (P less than 0.05). Both drugs increased tachycardia cycle length (TCL) from 310 +/- 35 msec to 354 +/- 37 msec (propafenone P less than 0.005) and to 352 +/- 37 msec (flecainide P less than 0.01). Both propafenone and flecainide blocked antegrade conduction in the pathway in five patients. Both drugs rendered atrial fibrillation noninducible in seven patients and orthodromic tachycardia noninducible in five patients.
Conclusions: (1) Flecainide causes a greater prolongation of minimum preexcited RR interval than propafenone; (2) There is no significant difference between propafenone and flecainide on the inducibility of arrhythmias, TCL, or incidence of antegrade conduction block.