A supraventricular tachycardia with two atrial activation sequences: What is the mechanism?

Case presentation A 62-year-old woman presented with a history of paroxysmal regular palpitations. She did not have any structural heart disease. Treatment with antiarrhythmic drugs was stopped 5 half-lives before the electrophysiological study. Basic intervals were normal. On right ventricular (RV) apical pacing 500 ms in the basal state, the ventriculoatrial (VA) conduction pattern was distal to proximal in the coronary sinus (CS) and the earliest atrial signal (A) was in the His bundle electrogram (Figure 1A). The VA block cycle length was 410 ms. During isoprenaline infusion, the VA block cycle length was 300 ms, with the earliest A at the His and a CS atrial activation pattern that was proximal to distal (Figure 1B). During infusion of isoprenaline, a sustained narrow-QRS tachycardia with 1:1 VA relationship started spontaneously. The initiation was with a spontaneous atrial premature depolarization (Figure 2), and termination was with a ventricular signal (V). The tachycardia cycle length (VV) and HH interval were constant at 320 ms. However, the CS activation sequence was distal to proximal at times and proximal to distal at other times. The VA interval changed from 130 to 156 ms as the CS activation pattern switched from a distal-to-proximal pattern to a proximal-to-distal one (Figure 3). What is the tachycardia mechanism?