Right ventricular entrainment of a narrow QRS complex tachycardia: what is the mechanism?

A 65-year-old woman underwent an electrophysiologic procedure because of a 20-year history of paroxysmal supraventricular tachycardia. The baseline sinus cycle length was 800 msec, with an atrial-His interval (AH) of 100 msec and a His-ventricular interval (HV) of 40 msec. There was evidence of dual AV nodal pathways, but no inducible tachycardia. During infusion of isoproterenol, the sinus cycle length shortened to 460 to 500 msec, with an AH of 80 msec and an HV of 40 msec. Atrial pacing at a cycle length of 360 msec induced a supraventricular tachycardia that had a cycle length of 460 to 500 msec (Fig. 1). The response to atrial pacing at a cycle length of 410 msec during tachycardia is shown in Figure 2. A single ventricular extrastimulus introduced during tachycardia when the His bundle was refractory had no effect on the tachycardia. Double ventricular extrastimuli reliably restored sinus tachycardia with an AH interval of 80 msec (Fig. 3). What is the mechanism of this tachycardia?