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

High-voltage impedance (HVI) measurement is commonly performed during subcutaneous implantable cardioverter-defibrillator (S-ICD) implantation, either through 65 J/80 J defibrillation testing or with a synchronized 10 J shock. HVI has been associated with defibrillation efficacy. Recently, S-ICD systems have been upgraded to provide automatic long-term measurements of low-voltage impedance (LVI) using a subthreshold 1 V pulse. This study aimed to evaluate the performance of LVI as a surrogate for HVI and to describe the long-term trend of LVI in S-ICD recipients.

Consecutive patients undergoing de novo S-ICD implantation were enrolled across 13 Italian centers. Data were collected from the LATITUDE remote monitoring database, where the S-ICD recorded LVI measurements every three days. To assess agreement between LVI and HVI, HVI values recorded at the time of each 80 J shock delivered during follow-up were compared with the nearest LVI values stored by the device.

Data from 729 patients (mean age 49±15 years, 80% male, BMI 26±5 kg/m²) were analyzed between 2015 and 2023. The median follow-up duration was 39 months [IQR: 21–56 months]. During follow-up, 130 patients (18%) received shocks. A total of 225 paired HVI and LVI measurements were available for analysis. LVI values were closely correlated with HVI values (r = 0.91; 95% CI: 0.88–0.93; P < 0.001). Bland-Altman analysis, accounting for repeated measurements per patient, showed a bias of -4 Ohm between LVI and HVI, with limits of agreement (bias ± 2SD) from -24 to 15 Ohm. Figure 1 illustrates the long-term trend of average LVI in the cohort: mean LVI rose significantly within the first 10 weeks post-implantation (reaching 90% of the long-term value), stabilizing at a long-term mean of 79±1 Ohm (values measured after 52 weeks) without further variation over time.

LVI measurements by S-ICD demonstrated strong agreement with HVI values measured during 80 J shock delivery, suggesting that LVI may serve as a surrogate marker for defibrillation efficacy at implantation, without requiring shock delivery. Long-term LVI monitoring could provide continuous insight into system integrity. The observed progressive increase in LVI during the initial weeks post-implantation likely reflects inflammatory and healing processes, with stable values thereafter.