Forty-one patients undergoing coronary artery bypass grafting were randomly assigned to receive prophylactic timolol or placebo, given in a double-blind fashion. /3-Adrenoceptor-blocking therapy was stopped at least one half-life before surgery. Three to 7 hr after surgery (304 + 56 min), 0.5 mg of timolol or placebo was given intravenously twice daily in a double-blind manner. When oral medications were resumed postoperatively, 10 mg of timolol twice daily or placebo was continued orally. Continuous electrocardiograms were recorded for 24 hr before and for 7 days after surgery with a standard cassette recorder. No patient received digoxin. Both groups were comparable for frequency of preoperative supraventricular arrhythmias, left ventricular ejection fraction, duration of cardiopulmonary bypass, aortic cross-clamp time, number of bypass grafts, and total duration of monitoring. Analysis of arrhythmias was done by hand counts, and supraventricular arrhythmias were divided into supraventricular tachycardia and atrial fibrillation and/or flutter. Timolol decreased the frequency of supraventricular tachycardia (581 episodes placebo vs 84 timolol; p < .05) and of atrial fibrillation and/ or flutter (291 episodes placebo vs five timolol; p < .05). Timolol decreased the number of patients with severe (heart rate > 200 beats/min, duration > 50 beats) episodes of supraventricular tachycardia (four placebo vs 0 timolol; p < .05) and also decreased the number of episodes of severe (heart rate > 200 beats/min, duration > 5 min) atrial fibrillation and/or flutter (16 placebo vs one timolol; p < .005). There were no differences in the durations of supraventricular arrhythmias. All 1 1 episodes of supraventricular tachycardia and 11 episodes of atrial fibrillation and/or flutter with heart rates over 200 beats/min occurred in the placebo group (p < .005). Four patients on placebo and one patient on timolol required treatment for symptomatic supraventricular arrhythmias. Two patients in the timolol group were nauseated and one patient developed wheezing while on placebo. The results of this study demonstrate that prophylactic use of timolol after coronary bypass surgery is effective and safe therapy for decreasing the frequency and severity of supraventricular arrhythmias. Circulation 70, No. 3, 479-484, 1984. SUPRAVENTRICULAR arrhythmias are common after coronary artery bypass surgery, occurring in 20% to 54% of patients. -110 These arrhythmias may be associated with excessively rapid ventricular rates that jeopardize hemodynamic stability; in such cases urgent treatment is required to slow the heart rate or restore sinus rhythm. On occasion, when antiarrhythmic drug therapy is ineffective, it is even necessary to

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