A man in his 20s with complex congenital heart disease (CHD) that included D-transposition of the great arteries, single ventricle physiology (upstairs-downstairs configuration of the ventricles with a large ventricular septal defect with outflow extension), and Fontan circulation was admitted to a hospital with lightheadedness and syncope that occurred while he was walking. He had a heart rate of 183 beats per minute (bpm) in the emergency department and was diagnosed as having atrial fibrillation (AF) (Figure 1A). He was given diltiazem, 15 mg intravenously, and converted to a regular complex tachycardia at 120 bpm. He was admitted overnight and discharged with a prescription for diltiazem, 120 mg daily, and apixaban, 5 mg twice per day. A few days later, he returned with persistent tachycardia and palpitations, and had a transesophageal echocardiogram, which showed no intracardiac thrombus. The patient underwent electrical cardioversion, with restoration of normal sinus rhythm.