Inappropriate shocks in the subcutaneous ICD: Incidence, predictors and management.

Background: The entirely subcutaneous implantable cardioverter-defibrillator (S-ICD) eliminates the need for transvenous leads, and therefore has the potential to improve lead-longevity and reduce lead-related complications. The S-ICD has a morphology-based sensing algorithm of which inappropriate shocks have been reported.

Methods: We analyzed the incidence, predictors and management of inappropriate shocks in the EFFORTLESS S-ICD Registry, which collects S-ICD implantation information and follow-up data from clinical centers in Europe and New Zealand.

Results: During a follow-up of 21 ± 13 months, 48 out of 581 S-ICD patients (71% male, age 49 ± 18 years) experienced 101 inappropriate shocks (8.3%). The most common cause was cardiac signal oversensing (73%), such as T-wave oversensing. Eighteen shocks (18%) were due to supraventricular tachycardias (SVT), of which 15 occurred in the shock-only zone. Cox-proportional hazard modeling using time-dependent covariates demonstrated that patients with a history of atrial fibrillation (HR 2.4) and patients with hypertrophic cardiomyopathy (HR 4.6) had an increased risk for inappropriate shocks, while programming the primary vector for sensing (from xyphoid to V6) reduced the risk. Reprogramming or optimization of SVT treatment after the first clinical event of inappropriate shock was successful in preventing further inappropriate shocks for cardiac oversensing and SVT events.

Conclusions: Inappropriate shocks, mainly due to cardiac oversensing, occurred in 8.3% of the S-ICD patients. Patients with hypertrophic cardiomyopathy or a history of atrial fibrillation were at increased risk, warranting specific attention for sensing and programming in this population.