Integration of PRAETORIAN Score, Defibrillation Testing and Follow-Up Outcomes for the Assessment of Conversion Failure in Patients with Subcutaneous Implantable Cardioverter Defibrillator: results from the i-SUSI registry

Background: Although reports of good clinical outcomes after avoiding defibrillation testing (DFT) during subcutaneous implantable cardioverter defibrillator (S-ICD) implantation have increased over recent years, current guidelines still recommend performing DFT at the time of implant. Objective: We aimed to identify predictors of conversion failure in a real-world S-ICD population and assess whether PRAETORIAN score performance is improved by additional clinical variables. Methods: We analyzed 1063 patients from the international iSUSI registry with available imaging and complete follow-up. The primary endpoint was a composite of DFT failure and ineffective appropriate shocks during follow-up. PRAETORIAN score and individual components were assessed, and ROC curve analysis with DeLong's test was used to evaluate model performance. Results: Patients were classified into low (76.6%), intermediate (16.3%), and high-risk (7.2%) PRAETORIAN score categories. Among 748 patients who underwent DFT, 64 (8.6%) experienced DFT failure; 17 ineffective shocks occurred during follow-up. Primary outcome occurred in 4.5% of low, 6.7% of intermediate-, and 16.8% of high-risk patients. PRAETORIAN score Step-1, anterior generator position at PRAETORIAN score Step-2 (OR 2.3 [1.2-4.2], p=0.006), high BMI and post-shock impedance, independently predicted conversion failure. Overall PRAETORIAN score (AUC 0.71) outperformed individual steps (Step 1-3 AUCs: 0.68-0.70; p>0.05 for all comparisons). A multivariable model combining PRAETORIAN score, BMI, and impedance significantly improved predictive accuracy (AUC 0.78vs.0.71; p=0.0078). Conclusion: PRAETORIAN score, BMI, and impedance are independent predictors of defibrillation failure. Their combined use may improve risk stratification and may help guide clinical decision-making, especially when DFT is omitted.