Clinical efficacy and electrophysiologic effects of intravenous and oral encainide in patients with accessory atrioventricular pathways and supraventricular arrhythmias

Abstract

The electrophysiologic effects and clinical efficacy of intravenous (i.v.) and oral encainide were studied in 13 patients with accessory atrioventricular (AV) pathways (7 overt, 1 intermittent and 5 concealed) and drug-resistant Supraventricular arrhythmias (5 paroxysmal atrial fibrillation, 1 atrial tachycardia and 7 with orthodromic circus movement tachycardia). Previously, therapy had failed with a mean of 3 conventional antiarrhythmic agents. In 5 patients, amiodarone administration had also been unsuccessful. All patients underwent programmed electrical stimulation of the heart before and after 1.5 mg/kg of i.v. encainide. Seven patients were restudied during oral encainide therapy (mean 155.8 ± 54.2 mg/ day) 3 days to 6 weeks (average 21 days) later. Anterograde conduction over the accessory AV pathway blocked in 4 of 7 patients after i.v. encainide. Oral encainide blocked anterograde conduction over the accessory pathway or prolonged the refractory period of the accessory pathway in 3 of 4 patients. This change in anterograde conduction was independent of the predrug value for the anterograde refractory period of the accessory AV pathway. Intravenous and oral encainide had minimal effects on retrograde conduction over the accessory AV pathway. The clinical effect of oral encainide was studied in 12 patients. Four patients responded to oral encainide and have been free of arrhythmia or side effects for 2 to 20 months (average 10.5). Encainide failed to prevent the clinical arrhythmia in 2 patients. In 4 patients with atrial arrhythmias, circus movement tachycardia developed during oral encainide therapy. In 1 patient the frequency of circus movement tachycardia increased with oral encainide treatment. Five of the previous 7 patients also suffered from central nervous system side effects, and in 1 patient serious central nervous system effects led to withdrawal of encainide. In conclusion, encainide has a marked effect on anterograde conduction and a minimal effect on retrograde conduction over the accessory AV pathway. Oral encainide was effective in controlling Supraventricular arrhythmia in 4 of 12 of this selected group of patients with accessory AV pathways. Failure to control the initial arrhythmia, facilitation of circus movement tachycardia and central nervous system side effects led to discontinuation of oral encainide in the other patients.