Efficacy and electrophysiologic effects of encainide for atrioventricular nodal reentrant tachycardia

To prospectively determine the clinical efficacy and electro-physiologic effects of encainide in atrioventricular nodal reentrant tachycardia (AVNRT), 49 patients refractory to 2.7 ± 1.5 previous antiarrhythmic drug trials underwent electrophysiologic study before and 47 did so after administration of oral encainide (75 to 240 mg/day). Encainide prolonged the minimum atrial pacing cycle length maintaining 1:1 atrioventricular (AV) nodal conduction from 334 ± 55 to 391 ± 55 ms (p = 0.0001). Encainide induced ventriculoatrial (VA) block in 12 patients (25%) and slowed the minimum ventricular pacing cycle length maintaining 1:1 VA conduction from 315 ± 46 to 485 ± 89 ms (p = 0.0001) in the remaining 35 patients. After encainide, AVNRT was not inducible in 32 of 47 patients (68%) primarily because of the effects on retrograde AV nodal conduction. In the remaining 15 (32%) patients, AVNRT remained inducible; however, the tachycardia cycle length slowed from 397 ± 86 to 492 ± 90 ms (p = 0.0001). There was no significant difference in the baseline minimum ventricular pacing cycle length maintaining 1:1 VA conduction in patients whose inducible tachycardia was or was not suppressed. Forty-seven patients were treated for 18.9 ± 12.9 months (range 1 to 50) with oral encainide. Encainide was completely effective in eliminating recurrences of supraventricular tachycardia in 26 of 47 patients (55%) and partially effective in an additional 42%. Recurrences of arrhythmia occurred in 15 of 32 patients (47%) whose inducible tachycardia was suppressed by encainide and 7 of 15 patients (47%) whose inducible tachycardia was not suppressed by encainide (p = not significant). It is concluded that encainide is effective in the treatment of patients with AVNRT. Encainide's primary antiarrhythmic effect is depression of conduction in the retrograde limb of the reentrant circuit. Results of serial electrophysiologic testing with encainide in AVNRT are not predictive of long-term clinical response.