Figure 2. A. During the narrow complex tachycardia, The RP’ interval(191ms)was longer than P’R interval (159ms). B. When ventricular pacing (SS=330ms) was delivered at a rate faster than the tachycardia, the ventricle was captured without changing the cycle length of the AA interval (345ms), which ruled out atrioventricular reentrant tachycardia. The earliest retrograde atrial activation recorded at the ostium of the coronary sinus was same to that recorded during the tachycardia, which implied the unlikely of atrial tachycardia. C. The tachycardia could be terminated by ventricular stimulation (SS=320ms) without retrograde ventricular-atrial conduction and so atrial tachycardia was excluded. D. After successful ablation near the CS ostium, no retrograde VA conduction was observed during ventricular stimulation (580ms).