Case presentation
A 72-year-old man without overt structural heart disease was referred to our institution for catheter ablation of a narrow QRS tachycardia. A 12-lead electrocardiogram recorded during sinus rhythm showed no ventricular preexcitation. Multielectrode catheters were placed in the high right atrium, His bundle (HB) region, coronary sinus (CS), and right ventricular (RV) apex. During sinus rhythm, the cycle length (CL), atrio-His interval, and His-ventricular interval were 758, 102, and 52 ms, respectively. A narrow QRS complex tachycardia with a CL of 270 ms was induced without atrio-His jump by atrial extrastimulation. The intracardiac electrograms with RV overdrive pacing during the tachycardia are shown in Figure 1A-B. What is the mechanism of this tachycardia?