Encainide in paroxysmal supraventricular tachycardia: Long term clinical effects

The clinical effects and safety of oral encainide were evaluated in 29 patients with paroxysmal supraventricular tachycardia. During a control electrophysiologic study, inducibility and mechanism of tachycardia were determined: 14 patients had atrioventricular (AV) node reentrant tachycardia (AVNRT); 15 had AV reentrant tachycardia (AVRT) involving either a patent (9 patients) or a latent (6 patients) accessory pathway. The mean tachycardia rate was 186 ± 25 beats/min. All patients underwent a repeat electrophysiologic study on treatment with encainide (75-150 mg daily). Tachycardia was reinduced in 8 patients (28%): 7 with AVRT (47%) and only 1 with AVNRT (7%). The tachycardia rate in these patients decreased from 194 ± 21 to 152 ± 26 beats/min (p <0.001). The mean plasma concentrations of encainide and its metabolites O-demethyl-encainide and 3-methoxy-O-demethyl-encainide were 108 ± 219, 167 ± 109, and 120 ± 86 ng/ml, respectively. Three poor metabolizers (10.3%) were found in this series. All patients but one were discharged on encainide at the same dose tested and followed-up for 32 ± 18 months; in 3 patients the dose was subsequently reduced, so the overall mean dose decreased to 114 ± 39 mg daily. Only 3 patients (11%) had less frequent, short-lasting and better tolerated recurrences of tachycardia. The other 25 patients (89%) were totally or almost totally protected by encainide (7 patients with only one short-lasting recurrence). There were only 4 patients complaining of minor side effects: 1 transient hand tremor and 3 mildly blurred vision. In 2 of the latter the symptom disappeared after dose reduction; the last patient refused a dose reduction and was dropped out. We conclude that encainide is extremely effective in preventing paroxysmal supraventricular tachycardia during a prolonged follow-up. When the drug is used at low dosages the incidence of side effects is negligible.