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A Wide Complex Tachycardia: What is the Mechanism?

A 33-year-old female was referred for an electrophysiologic study because of recurrent episodes of palpitations. Catheters were placed in the high right atrium (HRA), the His bundle (HIS), and the right ventricular apex (RVA). Baseline cycle length in sinus rhythm and atrial-His (AH) and His-ventricular (HV) intervals were 790, 90, and 45 ms, respectively. Spontaneous onset of a wide complex tachycardia with a left bundle branch block morphology and variable cycle length was observed on several occasions (Fig. 1). Atrial pacing at a rate of 600 ms with an extrastimulus with a coupling interval of 390 ms was conducted, as shown in Figure 2. What is the mechanism of this wide complex tachycardia?