Treatment of atrioventricular node reentrant tachycardia with encainide: Reversal of drug effect with isoproterenol

To determine the efficacy of encainide in the treatment of atrioventricular (AV) node reentrant tachycardia, Holter electrocardiographic (ECG) monitoring, exercise treadmill testing and programmed electrical stimulation were performed in 16 patients while they were taking no medication and after steady state levels were reached during treatment with encaini+de (75 to 200 mg/day; mean 117 ± 47). In addition, to study the possible reversal of drug effects by sympathetic stimulation, AV node conduction and tachycardia induction were reassessed during isoproterenol infusion (1 to 3 μg/min), a dose calculated to increase the rest heart rate by 25 ± 10%. Sustained AV node reentrant tachycardia could be initiated in all 16 patients in the control state, in 2 patients after encainide and in 10 patients during isoproterenol infusion. The shortest mean atrial paced cycle length sustaining 1:1 AV conduction was 358 ± 57 ms during the control study, which increased to 409 ± 59 ms with encainide (p < 0.01 versus control) and decreased to 313 ± 31 ms during isoproterenol infusion (p < 0.01 versus control and encainide). he shortest mean ventricular paced cycle length with 1:1 ventriculoatrial conduction was 337 ± 56 ms in the control study, 551 + 124 ms with encainide infusion (p < 0.01 versus control) and 354 ± 72 ms during isoproterenol infusion in the encainide-loaded state (p < 0.01 versus both control and encainide). During a mean follow-up period of 19 ± 10 months, significant clinical recurrences occurred in 4 of the 10 patients in whom tachycardia could still be initiated with encainide (with or without isoproterenol). In contrast, no significant episodes occurred in the six patients in whom tachycardia could not be induced with encainide despite isoproterenol infusion. Suppression of atrial or ventricular ectopic activity with encainide during Holter ECG monitoring did not appear to correlate with clinical recurrence. It is concluded that: 1) encainide is an effective agent for control of AV node reentrant tachycardia, primarily by virtue of its depressant effect on retrograde pathways; and 2) failure to induce tachycardia in the medicated state despite isoproterenol infusion predicts freedom from tachycardia recurrence.