Induction of supraventricular tachycardia by ventricular pacing: what is the mechanism?

A 29-year-old woman underwent an electrophysiologic procedure because of a 10-year history of recurrent episodes of paroxysmal supraventricular tachycardia. The baseline cycle length, atrial-His (AH), and His-ventricular (HV) intervals were 550, 50, and 45 msec, respectively. The AV block cycle length was 270 msec. With programmed atrial stimulation, there was no evidence of dual AV nodal pathways or inducible tachycardia. During ventricular pacing, there was occasional VA conduction at cycle lengths of 500 to 400 msec, and VA dissociation at paced cycle lengths below 400 msec (Fig. 1). On several occasions upon cessation of ventricular pacing, a supraventricular tachycardia with a cycle length of 300 msec was initiated (Fig. 2). What is the mechanism of the tachycardia, and how was it initiated?