Alternating Narrow and Wide Complex Tachycardia:

A 38-year-old man presented with a 20 year history of repeated episodes of paroxysmal palpitations, terminating spontaneously or with verapamil. Clinical examination and 12-lead electrocardiography (ECG) during sinus rhythm were normal, and echocardiography showed a structurally normal heart. The patient was referred for electrophysiological study and radiofrequency ablation (RFA). During electrophysiological study, tachycardia was induced; the ECG (leads I and V1) is shown in top panel of Figure 1. What is the best explanation for this rhythm?

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