Possible contribution of encainide metabolites to the long-term antiarrhythmic efficacy of encainide.

To establish long-term efficacy and the relation between drug plasma concentration and antiarrhythmic response, 12 patients with encainide-responsive frequent complex ventricular ectopic activity underwent 1 year of therapy with encainide. Twenty-four hour ambulatory electrocardiograms were obtained at baseline and every 2 months. Drug withdrawal with concomitant plasma sampling and electrocardiographic monitoring was performed at 6 and 12 months. Average group premature ventricular contraction (PVC) suppression during the year was 97 to 99%, with nearly total suppression of pairs and salvos. The most common adverse effects were transient visual disturbances and dizziness or lightheadedness. During a dose interval (6 to 12 hours) the concentration of encainide metabolites exceeded that of encainide by several-fold. The median time of arrhythmia return after drug withdrawal was 12 to 14 hours. At the time of arrhythmia return encainide was generally no longer detectable but the average concentration of O-demethylencainide and 3 methoxy-O-demethylencainide was 72 +/- 49 and 172 +/- 74 ng/ml, respectively. It is concluded that encainide therapy is extremely effective for continuous long-term suppression of complex ventricular arrhythmias and its metabolites contribute significantly to its antiarrhythmic action during chronic oral therapy.