Terminal S Wave Pattern in Lead V3 as a Simple Marker for Ventricular Tachycardia Risk Stratification in Patients With Repaired Tetralogy of Fallot

Introduction Patients with repaired tetralogy of Fallot (rTOF) remain at risk of ventricular tachycardia (VT). This risk is most commonly related to slow conduction within the septal isthmus, which is defined as the region between the ventricular septal defect (VSD) patch and adjacent valvular annuli. While assessment of septal isthmus conduction currently requires invasive electrophysiological mapping, the 12‐lead electrocardiogram (ECG) may offer a noninvasive alternative. Therefore, the present study aimed to identify ECG markers reflecting septal isthmus conduction properties and evaluate their ability to predict isthmus conduction characteristics and VT inducibility. Methods and Results In five patients with rTOF undergoing septal isthmus ablation, 12‐lead ECGs obtained before and after ablation were compared to identify candidate ECG markers. The identified marker was validated in an independent cohort of 23 patients stratified according to septal isthmus conduction properties into normal conduction (Normal, n = 5), slow‐conducting isthmus (SCI, n = 10), and conduction block (Block, n = 8) groups. The results indicated that the terminal S wave (TSW) in lead V3 became more prominent after ablation compared with that before ablation. In the validation cohort, V3 TSW was categorized as absent, small (< 0.1 mV), or large (≥ 0.1 mV), and the TSW patterns differed significantly among the groups ( p < 0.001). Specifically, absent TSW was dominant in the Normal group (4/5, 80%), small TSW was the most frequent in the SCI group (7/10, 70%), and large TSW was predominant in the Block group (7/8, 87.5%). Small TSW predicted SCI with 70% sensitivity and 85% specificity ( p = 0.013), whereas large TSW predicted Block with 88% sensitivity and 80% specificity ( p = 0.006). Moreover, large TSW predicted VT non‐inducibility with high specificity and high positive predictive value ( p = 0.019). Conclusion This study indicates that V3 TSW is a simple, noninvasive ECG marker reflecting septal isthmus conduction properties in rTOF, suggesting its potential utility for VT risk stratification.