Impact of anatomical factors and implantation position on defibrillation threshold in extravascular implantable cardioverter-defibrillator

Background The extravascular implantable cardioverter-defibrillator (EV-ICD) has demonstrated a high defibrillation success rate. However, device repositioning due to defibrillation threshold (DFT) test failure and late-phase DFT elevation have been reported. The currently recommended implantation position is based on small simulation studies and may not fully reflect anatomical variability in real-world practice. Objective To identify anatomical and implantation position factors associated with successful defibrillation at 15 J during EV-ICD implantation. Although defibrillation at ≤30 J (with a 10-J safety margin) is considered clinically acceptable, we evaluated 15-J success to clarify factors associated with lower DFT. Methods Among 44 patients who underwent EV-ICD implantation, 42 consecutive patients who underwent postoperative computed tomography (CT) were retrospectively analyzed. Patients were divided into two groups according to the success or failure of the 15-J DFT test performed intraoperatively or postoperatively. Univariate logistic regression analysis was conducted to identify factors associated with 15-J success, followed by receiver operating characteristic (ROC) analysis to determine optimal cutoff values. Results Device implantation was successful in all patients. DFT testing was successful at ≤40 J in 42/42 patients (100%), at ≤30 J in 39/42 (93%), and at ≤15 J in 26/42 (62%). Five patients (12%) required intraoperative or postoperative repositioning to achieve successful DFT testing. Univariate logistic regression analysis showed that lower body mass index (BMI) (odds ratio [OR] 0.78, 95% confidence interval [CI] 0.609–0.999, p=0.050), shorter Ring2–anterior RV wall distance (OR 0.73, 95% CI 0.552–0.960, p=0.027), thinner sub-generator tissue thickness (OR 0.58, 95% CI 0.370–0.906, p=0.017), and shorter vertical distance between the leftmost cardiac border and the generator upper edge (OR 1.11, 95% CI 1.011–1.160, p=0.022) were significantly associated with 15-J success. ROC analysis demonstrated that the Ring2–anterior RV wall distance had the highest discriminative ability (cutoff 5.6 mm, area under the curve [AUC] 0.88, sensitivity 82.3%, specificity 73.8%). Sub-generator tissue thickness (cutoff 6.5 mm, AUC 0.82, sensitivity 93.5%, specificity 72.4%) and vertical distance between the leftmost cardiac border and the generator upper edge (cutoff 3.9 mm, AUC 0.80, sensitivity 100%, specificity 74.1%) also showed good predictive performance. Conclusion Lower BMI, shorter Ring2–anterior RV wall distance, thinner sub-generator tissue thickness, and shorter vertical distance between the leftmost cardiac border and the generator upper edge were significantly associated with successful defibrillation at 15 J. Optimization of lead position, minimization of sub-generator tissue thickness, and appropriate generator positioning may facilitate lower DFT in EV-ICD implantation.