Fig. 1

a Surface EKG shows ventricular pre-excitation with negative delta wave in limb lead II consistent with posteroseptal AP. b The AP potential can be seen between atrial and ventricular electrograms (long white arrow). The disappearance of the AP potential and loss of ventricular pre-excitation during RF ablation is demonstrated. The tip of ablation catheter is shown to be in the CS diverticulum where the earliest local ventricular EGM in relation to the delta wave was detected. c Retrograde mapping of the AP during ventricular pacing. The distal ablation catheter was in the CS diverticulum. The local atrial EGM detected on ablation catheter was earlier than atrial EGM on the HIS catheter. MCV: middle cardiac vein