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Fourteen patients with atrial fibrillation or flutter and a ventricular rate of greater than or equal to 120 beats/min occurring after cardiac surgery entered a double-blind placebo-controlled conditional crossover trial of intravenous propafenone. Patients randomly received either propafenone (2 mg/kg body weight) or placebo during a 10 minute intravenous infusion. If 20 minutes after the initiation of this infusion there was no conversion to sinus rhythm, the patient received a second intravenous infusion over 10 minutes (either propafenone or placebo, whichever was not given first). The electrocardiogram was recorded continuously throughout the study. Fourteen patients received propafenone and 10 received placebo. No patient's rhythm converted to sinus rhythm after placebo. In six patients (43%) (p less than 0.001), the arrhythmia converted to sinus rhythm between 5 and 10 minutes after the end of the propafenone infusion. After propafenone, the ventricular response to atrial fibrillation or flutter decreased significantly from 141.6 +/- 15.2 to 116.0 +/- 15.5 beats/min. Ventricular rate did not change after placebo. The mean propafenone plasma concentration was 3.46 +/- 2.17 mg/liter. The only side effect of propafenone noted was a decrease in systolic blood pressure of 9 +/- 9 mm Hg. Propafenone was useful for management of atrial fibrillation after cardiac surgery both for control of rapid ventricular response and for conversion to sinus rhythm.