Reversal of proarrhythmic effects of flecainide acetate and encainide hydrochloride by propranolol.

The use of membrane-active antiarrhythmic agents may be complicated by aggravation of existing arrhythmias or development of new drug-induced arrhythmias. Four patients, referred because of out-of-hospital cardiac arrest or symptomatic sustained ventricular tachycardia, were receiving class IC antiarrhythmic agents in an attempt to prevent inducibility of sustained ventricular tachycardia. New or worsening spontaneous arrhythmias developed while they were on flecainide acetate (n = 3) or encainide hydrochloride (n = 1) therapy. Spontaneous runs of rapid nonsustained and sustained ventricular tachycardia developed in two. Increased frequency of premature ventricular contractions and repetitive forms of ventricular ectopic activity developed in one, despite the fact that inducibility of sustained ventricular tachycardia had been prevented. Salvos and nonsustained ventricular tachycardia developed in the fourth patient. Propranolol had failed to prevent inducibility of sustained ventricular tachycardia during previous programmed stimulation studies in three of the four patients, but it reproducibly suppressed drug-induced arrhythmias that appeared only after administration of the IC agents in each patient. Suppression of the proarrhythmic effects by beta-adrenergic blockade suggests a possible interaction of these drugs with autonomic function in the genesis of the observed proarrhythmic effects. Direct pharmacologic control of proarrhythmic drug effects has not previously been reported.