Voltage gradients in transvenous and subcutaneous defibrillation and their risk of myocardial damage.

INTRODUCTION: Transvenous implantable cardioverter-defibrillator (ICD) shocks have been associated with cardiac biomarker elevations and are thought in some cases to contribute to adverse clinical outcomes and mortality, possibly from myocardium exposed to excessive shock voltage gradients. Currently, there are only limited data for comparison with subcutaneous ICDs. We sought to compare ventricular myocardium voltage gradients resulting from transvenous (TV) and subcutaneous defibrillator (S-ICD) shocks to assess their risk of myocardial damage. METHODS: A finite element model was derived from thoracic magnetic resonance imaging (MRI). Voltage gradients were modeled for an S-ICD with a left-sided parasternal coil and a left-sided TV-ICD with a mid-cavity, a septal right ventricle (RV) coil, or a dual coil lead (TV mid, TV septal, TV septal + superior vena cava [SVC]). High gradients were defined as > 100 V/cm. RESULTS: The volumes of ventricular myocardium with high gradients > 100 V/cm were 0.02, 2.4, 7.7, and 0 cc for TV mid, TV septal, TV septal + SVC, and S-ICD, respectively. CONCLUSION: Our models suggest that S-ICD shocks produce more uniform gradients in the myocardium, with less exposure to potentially damaging electrical fields, compared to TV-ICDs. Dual coil TV leads yield higher gradients, as does closer proximity of the shock coil to the myocardium.

Magnetic field interactions between current consumer electronics and cardiac implantable electronic devices.

BACKGROUND: Electronic products, including the iPhone 12, Apple Watch Series 6, and 2nd Generation AirPods, contain magnets to facilitate wireless charging. Permanent magnets may affect CIED magnet mode features by causing pacemakers to pace asynchronously and defibrillators to suspend arrhythmia detection. This study determined if CIEDs are affected by static magnetic fields from commonly used portable electronics (PE) at any distance and intends to reinforce FDA recommendations concerning consumer PE which contain permanent magnets. METHODS: The maximum magnet field measurement was evaluated by a Gauss meter. The interaction between PE and CIEDs from Boston Scientific and Medtronic were tested ex vivo using a body torso model. The CIED was placed in physiologic saline, and the PE was placed at the surface and at increasing distances of 0.5, 1.0, and 1.5 cm. Interactions were recorded by assessment of magnet mode status. RESULTS: The iPhone 12 had almost three times the static magnetic field measured at the surface as the iPhone XR, but magnetic field strength decreased dramatically with increasing distance. At the surface of the model, PE triggered magnet mode in all CIEDs. The maximum interaction distance for all combinations of CIEDs and Apple products was 1.5 cm. CONCLUSIONS: The iPhone 12 produces a stronger static magnetic field than previous iPhone models. Magnets in PE tested will not interact with CIEDs when they are 15 cm from the implanted device. Since no interaction was observed beyond 1.5 cm, it is unlikely that magnet mode activation will occur during most daily activities.

Factors Associated With High-Voltage Impedance and Subcutaneous Implantable Defibrillator Ventricular Fibrillation Conversion Success.

BACKGROUND: The ability to predict defibrillation efficacy at the time of subcutaneous implantable cardioverter-defibrillator implantation without the need to induce ventricular fibrillation might eliminate the need for defibrillation testing. The purpose of this study was to determine the association of high-voltage impedance and system implant position on ventricular fibrillation conversion success with a submaximal 65-J shock. METHODS: In the subcutaneous implantable cardioverter-defibrillator IDE study (Investigational Device Exemption), a successful conversion test required 2 consecutive ventricular fibrillation conversions at 65 J in either shock vector. Chest radiographs were obtained after implantation. Patients with imaging and impedance data were included. Suboptimal device position was defined as an inferior electrode or pulse generator or electrode coil depth >3 mm anterior to the sternum. Absence of suboptimal positional parameters was defined as appropriate position. Conversion success rate was calculated among all 65-J tests. RESULTS: Of 314 patients who underwent subcutaneous implantable cardioverter-defibrillator implantation, 282 patients were included in this analysis. There were 637 inductions to test defibrillation at 65 J. Sixty-two conversion failures (9.7%) occurred in 42 (14.9%) patients. Lower body mass index and lower shock impedance were associated with higher conversion success rate, whereas white race was associated with lower conversion success rate. Suboptimal position was more common in obese patients. Inferior electrode and greater distance between the lead and sternum were associated with a higher impedance. When appropriate system position was achieved, conversion failure was not associated with high body mass index. CONCLUSIONS: Subcutaneous implantable cardioverter-defibrillator shock efficacy is associated with system position and high-voltage system impedance. A high impedance is associated with inferiorly placed pulse generator and electrode system, inadequate coil depth, and a lower rate of defibrillator success. CLINICAL TRIAL REGISTRATION: URL: https://www.clinicaltrials.gov . Unique identifier: NCT01064076.

Determinants of Subcutaneous Implantable Cardioverter-Defibrillator Efficacy: A Computer Modeling Study.

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.