V-A-A-V Activation Sequence at the Onset of a Long RP Tachycardia: What is the Mechanism?

A 52-year-old woman with a history of multiple episodes of paroxysmal supraventricular tachycardia underwent electrophysiologic studies and a catheter ablation procedure. The 12-lead electrocardiogram during tachycardia showed a long RP tachycardia with negative P waves in leads II, III, and aVF. At baseline, dual atrioventricular (AV) and ventriculoatrial (VA) nodal conduction was elicited by atrial and ventricular premature stimulation. During intravenous administration of isoproterenol, the earliest site of atrial activation during ventricular pacing was observed near the His bundle, and a previously recorded narrow QRS tachycardia was reproducibly induced by premature or rapid ventricular apical pacing, with an initial V-A-A-V activation sequence (Fig. 1). During tachycardia, the HA and AH intervals measured 71 and 342 milliseconds, respectively, and the earliest atrial activation was recorded at the ostium of the coronary sinus (CS). The earliest first “A” of the initial “V-A-A-V” activation sequence was recorded in the His bundle region, reflecting conduction over a fast pathway (FP), as was observed during ventricular pacing, while the second “A” was recorded near the CS ostium, as was observed during the tachycardia. The interval between the first and second “A” was often shorter than the subsequent tachycardia cycle length. Premature ventricular stimuli delivered during tachycardia while the His bundle was refractory did not reset the atrial cycle. From these observations, what is the mechanism of tachycardia?