Long-term low-voltage impedance measurements in subcutaneous implantable cardioverter-defibrillators

Background: High-voltage impedance (HVI), measured during subcutaneous implantable cardioverter-defibrillator (S-ICD) implantation via defibrillation testing or a synchronized shock, is associated with defibrillation efficacy. Recently, S-ICD systems have been upgraded to automatically perform long-term measurements of low-voltage impedance (LVI) using a 1-V subthreshold pulse. Objective: This study evaluated LVI as a surrogate for HVI and described its long-term trends in S-ICD recipients. Methods: We analyzed data from 1226 patients who underwent de novo S-ICD implantation across 15 Italian centers. Weekly averages of LVI and HVI were calculated, and agreement between simultaneous measurements was assessed. Results: Over a median follow-up of 37 months (interquartile range 20–57 months), 373 paired HVI and LVI measurements were analyzed. LVI strongly correlated with HVI (r = 0.90; 95% confidence interval 0.88–0.92; P < .001), with a mean bias of −3 Ω (limits of agreement −21 to 14 Ω). The mean LVI increased significantly during the first 3 months postimplantation (from 59 ± 14 to 76 ± 16 Ω; P < .001) before stabilizing (77 ± 17 Ω; P = .231). Higher LVI values were observed in overweight/obese patients when subcutaneous device positioning and the 3-incision lead deployment technique were used. Similarly, higher values were obtained when significant subcoil fat was observed and the Prospective, RAndomizEd comparison of subcuTaneOus and tRansvenous ImplANtable cardioverter-defibrillator therapy score was ≥90. Conclusion: LVI showed strong agreement with HVI during follow-up, supporting its potential use as a noninvasive surrogate for HVI. LVI increased during the initial postimplantation period and subsequently stabilized, possibly reflecting physiological changes.