Tehran University of Medical Sciences The Research in Heart Yield and Translational Medicine (RHYTHM) 3115-7270 21 4 2026 07 05 286 296 Amr Hassan cardiovascular medicine department, Cairo University Mohamed Rashad cardiovascular medicine department, Cairo University Emmanuel Aziz cardiovascular medicine department, Cairo University Ahmed Al Damaty cardiovascular medicine department, Cairo University 2026 06 05 2026 07 05 Background: Radiofrequency (RF) catheter ablation is the standard treatment for symptomatic patients with Wolff-Parkinson-White (WPW) syndrome. Successful RF ablation of an accessory pathway (AP) depends on accurate localization of its site. Methods: This retrospective, observational, cross-sectional study included all consecutive patients who underwent successful RF ablation of a single manifest posteroseptal AP at the Cardiac Electrophysiology Laboratory of the Cardiovascular Department, Cairo University Hospitals. Twelve-lead electrocardiograms were collected and analyzed to identify electrocardiographic criteria that differentiate epicardial from endocardial posteroseptal APs to guide mapping and ablation strategies. Results: Seventy-three patients with a posteroseptal AP were included: 47 were ablated from the right posteroseptal endocardium; 6 from the left posteroseptal endocardium; and 20 were ablated epicardially (from the coronary sinus or its branches). Epicardial posteroseptal APs were predicted by a negative delta wave in lead II, a positive delta wave in lead aVR, a positive delta wave in lead V1, and/or QRS complex notching in lead II. Endocardial posteroseptal APs were predicted by a negative delta wave in lead V1 and/or an R/S ratio less than 1 in lead V1. Double QRS transition was not useful for differentiation unless combined with a Q wave or predominantly negative QRS complex in lead V1. Conclusion: Epicardial and endocardial posteroseptal APs can be differentiated based on delta wave polarity in leads II, aVR, and V1; R/S ratio in lead V1; and QRS complex notching in lead II. https://rhythm.tums.ac.ir/index.php/jthc/article/view/2483 https://rhythm.tums.ac.ir/index.php/jthc/article/download/2483/1288