Ventriculoatrial block during a narrow-QRS tachycardia: what is the tachycardia mechanism?--IV.

A 28-year-old man underwent an electrophysiologic procedure becau.se of frequent episodes of paroxysmal supraventricular tachycardia. The baseline sinus cycle length. alrial-His (AH) interval, and His-ventricular {HV) intei-vals were 800, 90, and 40 msec, respectively. A narrow-QRS tachycardia having a cycle length of 250 to 280 msec was reproducibly induced by ventricular programmed stitnulation with a single extrastimulus (Figs. ] and 2). There was an inverse relationship between the coupling interval of the ventricular extrastimuius that induced the taebyciirdia and the interval between the extrastimulus and the first tachycardia complex. The HV interval during taehycatdia was 40 msec. A ventricukir premature depolaiization introduced during the tachycardia simultaneous with the His-bundle depolarization did not reset or tenninate the tachycardia. The tachycardia was consistently temiinated by ventricular pacing at a cycle length of 230 msec. What is the mechanism of this tachycardia?