Unusual responses of a short RP tachycardia to His-refractory ventricular premature depolarizations: What is the mechanism?

A 68-year-old man underwent a diagnostic electrophysiology study because of a symptomatic short RP supraventricular tachycardia (SVT). Transthoracic echocardiography was normal. Atrio-His and His-ventricular intervals measured 70 and 55 ms, respectively. The patient had retrograde ventriculoatrial activation that was earliest only at the anteroseptal right atrium and decremental. Programmed atrial extrastimulation elicited dual atrioventricular (AV) nodal physiology. Rapid atrial pacing repeatedly induced his SVT with an episode of termination shown in Figure 1A.