BACKGROUNDThe SMART passTM algorithm for subcutaneous implantable cardioverter-defibrillator (S-ICD) prevents inappropriate shocks due to oversensing. The mechanisms and significance of SMART pass deactivation remain unclear. OBJECTIVETo assess whether SMART pass deactivation is associated with inappropriate shocks and elucidate the underlying mechanism. METHODSWe retrospectively investigated 115 patients who underwent S-ICD implantation between 2016 and 2021. SMART pass deactivation and inappropriate shocks during follow-up were assessed. The QRS amplitudes of the subcutaneous (S-ECG) and 12-lead electrocardiogram (ECG) at the time of implantation (pre) and SMART pass deactivation (post) were measured. The patients were divided into the SP-ON group with SMART pass consistently on and the SP-OFF group with the experience of SMART pass deactivation. RESULTSThree of twelve patients in the SP-OFF group experienced inappropriate shocks during a median of 1094 (IQR, 887–1502)-day follow-up compared with four of 87 patients in the SP-ON group. Pre- and post-S-ECG QRS amplitude were significantly lower in the SP-OFF than in the SP-ON group (both, p < 0.05), despite similar 12-lead ECG QRS amplitude in both groups. A significant temporary drop in the QRS amplitude of the S-ECG led to SMART pass deactivation (pre vs. at deactivation p = 0.015, 95% confidence interval: 0.3–1.9). CONCLUSIONThe rate of inappropriate shocks was numerically higher following SMART pass deactivation. A low QRS amplitude in S-ECG was a potential risk factor for SMART pass deactivation. Careful follow-up and suitable management are required for managing patients with risks of SMART pass deactivation.