A regular, wide-QRS complex tachycardia: what is the tachycardia mechanism?

A 45-year-old woman underwent an electrophysiology procedure because of a 20-year history of recunent episodes of paroxysmal tachycardia. The baseline sinus cycle length, atrial-His. and His-ventricular intervals were 700, 60, and 45 msec, respectively, and the QRS duration was 80 msec. Ventricular pacing at a cycle length of 300 msec induced a tachycardia that had a cycle length of 320 msec and a QRS duration of 120 msec, with a right bundle branch block configuration (Fig. 1). The tachycardia was terminated by ventricular pacing at cycle lengths shorter than 290 msec. Atrial pacing at a cycle length of 320 msec was performed in the setting of sinus rhythm (Fig. 2). What is the mechanism of this tachycardia?