Determinants of Subcutaneous Implantable Cardioverter-Defibrillator Efficacy

OBJECTIVES: This study determined the impact of subcutaneous implantable cardioverter-defibrillator (S-ICD) coil and generator position on defibrillation threshold (DFT). BACKGROUND: S-ICD implantation can occasionally result in unacceptably high DFT. Implant position characteristics associated with high DFTs in S-ICD patients have not been fully elucidated. METHODS: A 3.8-million-element computer model built from magnetic resonance images was used to simulate the electric fields that occur during defibrillation. Generator positions were tested from posterior to anterior in 4-cm increments. The left parasternal coil was tested with 0, 5, and 10 mm of underlying subcutaneous fat and the generator with 20 mm of underlying fat. The estimated DFT for the S-ICD was defined as the energy delivered when producing an electric field of 4 volts/cm in at least 95% of the ventricular myocardium. RESULTS: Estimated DFTs were 22, 29, 64, and 135 joules for posterior, standard (lateral), mid-anterior, and anterior generator locations, respectively. Defibrillation thresholds were 29, 58, and 95 joules with 0, 5, and 10 mm subcoil fat, respectively, and 45 joules with 20 mm subgenerator fat. Combining anterior generator position with subcoil fat resulted in a very high DFT (379 joules). Shock impedance increased with both subcoil and subgenerator fat but was minimally affected by anterior/posterior generator position. CONCLUSIONS: The model suggests that an S-ICD implantation strategy involving posterior generator location and coil and generator directly over the fascia without underlying fat is likely to markedly lower DFTs with the S-ICD and assist in troubleshooting of patients with unacceptably high DFTs.