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A Narrow QRS Complex Tachycardia With Eccentric Retrograde Atrial Activation Sequence: What is the Mechanism?

A 37-year-old man with a 3-year history of recurrent episodes of paroxysmal supraventricular tachycardia was referred to our institution for an electrophysiological study and catheter ablation. Resting 12-lead surface ECG revealed normal sinus rhythm without preexcitation. An electrophysiologic study was performed after obtaining informed consent, in the fasting state, and free of medications by positioning multipolar catheters at the His bundle, in the coronay sinus (CS) and in the right ventricular (RV) apex. Baseline sinus cycle length, atrial-His (AH) interval, and His-ventricular (HV) interval were 820, 130, and 43 ms, respectively. Programmed atrial stimulation established the presence of anterograde dual AV nodal physiology. Eccentric atrial activation during ventricular pacing was observed. Earliest retrograde atrial activation during ventricular pacing was noted in the distal CS. This location corresponded to the region of the anterolateral mitral annulus. A narrow QRS complex supraventricular tachycardia with a cycle length of 410 ms was reproducibly induced with programmed stimulation (Fig. 1A). The tachycardia exhibited a retrograde atrial activation sequence identical to that observed during ventricular pacing. A premature ventricular complex stimulus from the RV apex during His refractory period failed to advance the atrium. The response to overdrive ventricular pacing during the tachycardia is shown in Figure 1B. What is the mechanism of the tachycardia?