Abstract Body (Do not enter title and authors here): Introduction
Patients with rTOF are at elevated risk of ventricular tachycardia (VT) and sudden cardiac death (SCD), primarily due to slowly conducting anatomic isthmuses (SCAI) formed during surgical repair. Although existing ACHD risk calculators like Risk of SCD in ACHD (RSACHD) and PREVENTION-ACHD (PACHD) help predict SCD in broad ACHD populations, their performance in rTOF—particularly in predicting presence of SCAI—is unknown.

Objective
This study aims to evaluate the ability of these calculators to predict the presence of SCAI, inducible VT during programmed ventricular stimulation (PVS), or clinical arrhythmic events in a large rTOF population.

Methods
We retrospectively studied patients with repaired TOF (rTOF) in a single academic center, dividing them into 3 groups: those who underwent prophylactic electrophysiology study (EPS) with PVS and electroanatomic mapping for ablation of SCAI (Group 1; n=58), those with EPS with PVS only (Group 2; n=35), or those with a clinical diagnosis of rTOF with no EPS (Group 3; n=269). Outcomes included the presence of SCAI (Group 1), inducibility of VT (Groups 1-2), and a composite arrhythmic outcome of sustained VT, SCD, or appropriate ICD shock (Groups 2-3). RSACHD and PACHD scores were analyzed using logistic regression to assess associations with outcomes; significance was defined as α ≤ 0.05.

Results
The cohort included 361 patients (46% male, mean age 36 years). Neither RSACHD nor PACHD scores were associated with SCAI (n = 58 patients, p = 0.17 and 0.93, respectively; Fig. 1a) or inducible VT (n =93 patients, p = 0.09 and 0.62; Fig. 1b). Both scores, however, were significantly associated with the composite outcome (n = 278 patients, p < 0.01 for both; Fig. 1c).

Conclusion
While RSACHD and PACHD scores are associated with clinical arrhythmic outcomes in rTOF patients, they do not reliably predict SCAI or inducible VT. These findings suggest that while useful for long-term risk stratification, current ACHD risk calculators do not capture the anatomic mechanisms underlying VT in rTOF or guide timing of “prophylactic EPS” (commonly performed prior to pulmonary valve replacement). Further work is needed to develop targeted tools for substrate-level risk prediction.