Atrial pacing during a narrow QRS complex tachycardia: what is the mechanism?

A 25-year-old woman with a 5-year history of paroxysmal supraventricular tachycardia underwent an electrophysiologic procedure. The baseline sinus cycle length, atrial–His-bundle (AH) interval, and His-bundle–ventricular (HV) interval were 750, 60, and 50 msec, respectively. Paroxysmal supraventricular tachycardia with a cycle length of 320 msec was induced by programmed atrial stimulation with a single extrastimulus (Fig. 1). The response to atrial pacing at a cycle length of 260 msec during the tachycardia is shown in Figure 2. The tachycardia was terminated by atrial pacing at a cycle length of 240 msec. Upon termination of the tachycardia, atrial pacing at a cycle length of 260 msec was performed during sinus rhythm (Fig. 3). Ventricular overdrive pacing always terminated the tachycardia. During ventricular pacing, the atrial activation sequence was the same as during tachycardia. What was the effect of atrial pacing on the tachycardia, and what is the mechanism of the tachycardia?