Typical atrioventricular nodal reentrant tachycardia with 2:1 conduction block: What is the mechanism?

A 66yearold woman with a history of frequent premature ventricular contractions (PVCs), and wide QRS complex tachycardia, underwent electrophysiological testing. During the electrophysiological study, multipolar catheters were positioned in the coronary sinus (CS), His bundle (HB) region, and right ventricular apex. She underwent successful catheter ablation of PVCs at the mitral annulus. Following this, an induction of wide QRS complex tachycardia was attempted. Programmed atrial stimulation demonstrated no evidence of dual AV nodal physiology. Any pacing maneuvers from the right side could not induce any tachycardias. Programmed ventricular stimulation from the left ventricle induced a nonsustained supraventricular tachycardia (SVT) with 2:1 atrioventricular (AV) conduction block (Figure 1). An SVT with 2:1 AV conduction block was induced again, but the Hisatrial (HA) interval was shorter than that during the first SVT (Figure 2). This SVT immediately converted to a wide QRS complex tachycardia with 1:1 AV conduction (Figure 2). During the tachycardia, there was a slight oscillation in the tachycardia cycle length (Figures 1 and 2), and the preceding His to His (HH) intervals equaled the following intervals between the atrial activations (AA intervals). Rapid ventricular pacing was performed, and the tachycardia was terminated without any retrograde atrial capture. What is the mechanism?