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Differential Diagnosis of Supraventricular Tachycardia with Concentric Atrial Activation

A 72-year-old man was referred for evaluation of paroxysmal tachycardia. In the electrophysiology laboratory, the 12-lead ECG showed a narrow QRS tachycardia with a ventricular rate of approximately 180 beats/ min (Fig. 1). The endocardiac bipolar recordings showed a concentric atrial activation sequence during the tachycardia, which could be terminated by a single ventricular premature depolarization (Fig. 2). What is the mechanism of the tachycardia?