An unusual incessant long RP tachycardia—What is the mechanism?

A 40-year-old man underwent a diagnostic electrophysiology study because of a slow incessant long RP supraventricular tachycardia (SVT). Echocardiography revealed an ejection fraction of 35%. Upon presentation to the electrophysiology laboratory, he was in SVT. His refractory ventricular premature depolarizations (VPDs) delivered from the right ventricular (RV) apex failed to affect the tachycardia; the responses to earlier VPDs and coronary sinus (CS) atrial premature depolarizations (APDs) are shown in Figure 1, top and bottom, respectively.