<jats:title>Abstract</jats:title> <jats:sec> <jats:title>Background</jats:title> <jats:p>We describe a case series of patients for a gradual rise in daily, low-voltage sub-threshold measurement (LVSM) of shock (high-voltage, HV) impedance in a group of patients with Boston Scientific implantable cardioverter-defibrillators (ICDs) and investigate the cause of the abnormality.</jats:p> </jats:sec> <jats:sec> <jats:title>Case summary</jats:title> <jats:p>Six patients presented with a gradual rise in HV impedance above normal range (132.5 ± 20.8 Ω). Patients were young with a mean age of 29 ± 11 years, four patients had hypertrophic cardiomyopathy, one left ventricular non-compaction, and one long QT. All lead designs were silicon body with GORE polytetrafluoroethylene (ePTFE) coated coils, and a lower true shock impedance (TSI) was seen in all cases with full output synchronized shock. We compared the rate of HV impedance rise with our historical cohort of Boston ICDs using an unpaired t-test. The change in impedance per month was significantly higher amongst our six patients when compared with our cohort of Boston Scientific ICDs (3.2 ± 1.9 Ω/month vs. 0.0008 ± 0.005 Ω/month, P &lt; 0.001). Patients were individually investigated and management discussed in a dedicated device multi-disciplinary team meeting (MDT).</jats:p> </jats:sec> <jats:sec> <jats:title>Discussion</jats:title> <jats:p>There are distinct differences between TSI and LVSM. The TSI is derived from a full output shock, whilst LVSM is calculated from a small current output. These cases highlight the inaccuracies of the LVSM measurement. The gradual rise in LVSM is significantly higher than the value for TSI in these patients we propose the most likely mechanism is encapsulation fibrosis surrounding the right ventricular shock coil. Management for these patients requires vigorous testing to rule out electrical failure, and replacement maybe necessary.</jats:p> </jats:sec>