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An Unusual Cause of Irregularly Irregular Supraventricular Tachycardia

A 49-year-old man with no known structural heart disease presented with palpitations. He described his symptoms as a nearly incessant sensation of fullness or pounding in the head and neck rather than precordial thumping. He was diagnosed with paroxysmal atrial fibrillation (AF) based on ambulatory electrocardiography (ECG), and started on aspirin and sotalol. His symptoms persisted, and he was referred for consideration of catheter ablation of AF. ECG monitoring at presentation to our center suggested an alternative diagnosis (Fig. 1). What is the mechanism of the tachycardia?