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V‐A‐A‐V activation sequence followed by an induction of long RP tachycardia: What is the mechanism?

A 76‐year‐old woman with a history of multiple episodes of a long RP tachycardia with biphasic P waves in inferior leads underwent electrophysiological studies and a catheter ablation procedure. No dual anterograde atrioventricular (AV) nodal conduction was elicited by atrial extrastimulation. No constant ventriculoatrial (VA) conduction was observed at baseline. Therefore, we tried to perform ventricular extrastimulation (S2) after an S1‐S1 simultaneous overdrive pacing train from the atrium and ventricle (VPS‐S) to promote VA conductivities and reproducibly induced narrow QRS tachycardia documented previously. On induction of the tachycardia, the initial atrioventricular activation sequence was not uniform and was ventricular‐atrial‐atrial‐ventricular (V‐A‐A‐V) at 12 types of S1‐S2 coupling intervals ranging between 220 and 420ms (Figure 1A) or ventricular‐atrial‐ventricular (V‐A‐V) at 2 types of coupling intervals (Figure 1B). During the tachycardia, the earliest atrial activation was in the His bundle region. Premature ventricular extrastimuli on His bundle refractoriness did not reset atrial cycles. An injection of adenosine triphosphate (ATP) of a dose of 2mg terminated the tachycardia with an end of ventricular deflection. What is the mechanism of the tachycardia, and the V‐A‐A‐V and V‐A‐V activation sequence on ventricular induction of the tachycardia?