A manual synchronous low energy shock impedance as a predictor of successful defibrillation testing during subcutaneous ICD implantation

Background: Guidelines recommend defibrillation testing (DFT) during subcutaneous implantable cardioverter-defibrillator (S-ICD) implantation. Implant position, patient characteristics and device factors, such as shock impedance, influence defibrillation success. To evaluate the shock impedance, a manual synchronous 10J shock (low energy synchronous shock [LESS]) can be delivered, without the need to induce ventricular fibrillation (VF). Objective: To compare LESS and DFT impedance values and to evaluate the diagnostic accuracy of LESS impedance for predicting a successful DFT during S-ICD implantation. Methods: Consecutive S-ICD implantations were included. Shock impedances were compared by paired t-tests. Univariate analysis was performed to investigate factors associated with successful DFT. A prediction model of successful DFT based on LESS impedance was assessed by logistic regression. Receiver operating characteristic (ROC) curve, area under the ROC curve and the Hosmer-Lemeshow tests were used to evaluate the accuracy of LESS impedance. Results: Sixty patients were included (52 +/- 14 years; 69% male). LESS and DFT impedance values were highly correlated (r(2) = 0.97, p < .01). Patients with a failed first shock had higher body mass index (BMI) (30 +/- 3 vs. 25.7 +/- 4.3, p = .014), higher mean LESS (120 +/- 35 ohm vs. 86. +/- 23 ohm, p = .0013) and DFT impedance (122 +/- 33 ohm vs. 87 +/- 24 ohm, p = .0013). ROC analysis showed that LESS impedance had a good diagnostic performance in predicting a successful conversion test (AUC 84% [95% CI: 0.72-0.92]) with a cutoff value of <94 ohm to identify a successful DFT (sensitivity 71%, specificity 73%). Conclusion: LESS impedance values without the need to induce VF can intraoperatively predict a successful DFT.