What is the tachycardia mechanism? - III.

A 66-year-old woman with a 24-year history of paroxysmal supraventricular tachycardia underwent an electrophysiologic procedure. The baseline sinus cycle length, atrial-His (AH), and Hisventricular (HV) intervals were 8(X). 70, and 40 msec, respectively. Programmed stimulation with a single atrial extrastimulus did not demonstrate dual AV nodal physiology and did not result in the induction of tachycardia. Supraventricular tachycardia with a cycle length of 350 msec was induced by rapid atrial pacing (Fig. 1). A single ventricular extrastimulus introduced during tachycardia coincident with His-bundle refractoriness did not preexcite the atrium. Double ventricular extrastimuli introduced during the tachycardia resulted in advancement of the atrial electrograms. At times, the tachycardia was terminated (Fig. 2); at other times. it was not (Fig. 3). Termination of the tachycardia always was associated with shortening of the AH interval after the double ventricular extrastimuli. What is the mechanism of this tachycardia?