Perioperative arrhythmias after Fontan repair. Arrhythmias are well-recognized sequelae of the Fontan repair. A prospective analysis of perioperative arrhythmias after Fontan repair was performed. Thirty-three patients had 24-hour ambulatory monitoring on the day before surgery and for 24 hours immediately after surgery; the 27 surviving patients had an additional 24-hour study on the seventh postoperative day. The most common preoperative rhythm was normal sinus, which was present in 79%. Hemodynamically significant preoperative arrhythmias were present in only two patients-atrial flutter (n = 1) and complete heart block (n = 1). Other asymptomatic preoperative arrhythmias included ectopic atrial rhythm (n = 2), supraventricular tachycardia (n = 1), accelerated junctional rhythm (n = 1), and frequent ventricular premature beats (n = 1). On the first postoperative day, the most common rhythm was accelerated junctional rhythm, which was present in 48%. Hemodynamically significant arrhythmias were present in nine patients (27%) -rapid accelerated junctional rhythm (rate, greater than 190 beats/min) (n = 3), complete heart block (n = 2), atrial flutter (n = 2), supraventricular tachycardia (n = 1), and ventricular tachycardia (n = 1). Three of six deaths occurring between postoperative days 2 and 7 were in patients with rapid accelerated junctional rhythm and associated low cardiac output. On the seventh postoperative day, the most common rhythm was normal sinus, which was present in 41%. Nine of the 27 surviving patients had complete resolution of their arrhythmias between postoperative days 2 and 7. On postoperative day 7, hemodynamically significant arrhythmias were present in four patients-atrial flutter (n = 2) and complete heart block (n = 2). 