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. 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.