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Wide QRS Tachycardia with Atrioventricular Dissociation and an HV Interval of 60 msec

A 63-year-oId man had an episode of unexplained syncope. He had triple-vessel coronary artery disease, a history of an anterior and inferior myocardial infarction, and had undergone a coronary artery bypass operation 3 years earlier. Cardiac catheterizalion demonstraied a left ventricular ejection fraction of 0.17, and patency of all three saphenous vein grafts. An elecuophysiologic test was perfonned. The baseline sinus cycle length was 760 msec, the QRS duration was 110 msec, and the atriat-His and His-ventricuJar (HV) intervals were 120 and 55 msec, respectively. Sinus node function was normal, and there was no carotid hypetsensitivity, infranodal block during atrial pacing, or evidence ol dual AV nodal physiology. The AV block cycle length was 370 msec. Tachycardia was not inducible by atrial pacing. A wide QRS complex tachycardia having a cycle length of 210 msec was induced by ptogramined ventricular stimulation (Fig. 1). The tachycardia had a left bundle branch block configuration (Fig. 2). When sustained, the tachycardia resulted in severe hypotension and required electrical catdioversion. Sometimes the tachycardia was nonsustained (Fig. 3). What is the tachyciirdia mechanism?