Double advancement of his potential for atrial scan pacing: What is the mechanism?

A 48‐year‐old man was referred for radiofrequency ablation of a narrow QRS tachycardia. At baseline, the atrio‐His (AH) and His‐ ventricular (HV) intervals were 70 and 70 ms, respectively. A dual atrioventricular nodal physiology was observed by programed atrial extra‐stimulation. A narrow QRS tachycardia was induced by atrial burst pacing with double ventricular response (Figure 1). The AH, HA, and HV interval during the tachycardia were 360, 40, and 70 ms, respectively. During the tachycardia (SVT‐1), the earliest atrial site was His region. A V‐A‐V sequence was observed after RV entrainment pacing and the difference between the post pacing interval (PPI) and TCL corrected by AH interval (cPPI‐TCL) was 196 ms. Atrial scan pacing (A‐scan) from the coronary sinus (CS) ostium terminated SVT‐1. SVT‐1 was diagnosed as a slow‐fast atrioventricular nodal reentrant tachycardia (AVNRT). After slow pathway ablation, SVT‐2 was induced after isoproterenol infusion. The AH, HA and HV interval during SVT‐2 were 465, 40, and 70 ms, respectively. Atrial sequence was identical to SVT‐1. A V‐A‐V sequence was observed after RV entrainment pacing and cPPI‐TCL was 140 ms. A‐scan from CS advanced His potential immediately after stimulation and the following His potential (Figure 2). What is the mechanism of SVT‐2? 2 | COMMENTARY