Assessment of a Chronically Implanted Parasternally Delivered EV-ICD Lead (ASCEND EV) Study

Inclusion Criteria: 1. At least 18 years old 2. Class I or IIa indication for implantation of an ICD according to the ACC/AHA/HRS Guidelines or ESC guidelines Exclusion Criteria: 1. Participation in any concurrent clinical study without prior written approval from the Sponsor 2. Inability or unwillingness to provide informed consent to participate in the Study 3. Any known conditions which may complicate the AtaCor EV-ICD Lead System implant procedure or prevent the subject from completing protocol testing through the 3-month follow-up visit 4. Circumstances that may prevent data collection or completion of specified follow-up visits 5. Allergies to any device materials listed in the Instructions for Use (IFU) 6. Subjects on anticoagulation therapy that cannot be temporarily discontinued for the procedure 7. Known history of lung disease with FEV1 \< 1.0 Liter Device Related: 8. Presence or planned use of medical devices that introduce current into the body (e.g., pacemaker, LVAD, neurostimulator, etc.) 9. Implanted with or planned implantation of any device which delivers current in the body, that may interfere with therapy delivery, including, but not limited to a pacemaker, or neurostimulator 10. Presence or planned use of a subcutaneous ICD lead, subcutaneous coils/arrays, epicardial patches or epicardial pace/sense leads 11. Any known need for future MRI Anatomy Related: 12. BMI ≥ 35 kg/m2 13. Structural abnormalities of the heart that may increase risk of the study procedure or an obstructed/restricted pathway into the mediastinum 14. Prior sternotomy of any type, including but not limited to median, mini or clamshell 15. Prior surgery with disruption of the lung, pericardium or connective tissue between the sternum and pericardium 16. Known significant anatomic derangement of or within the thorax (e.g., pectus excavatum, significant scoliosis) and associated with displacement of the heart or lungs or impeded mediastinal access. 17. History of thoracic radiation therapy, pneumothorax, pneumomediastinum or other medical treatments/conditions which may complicate the AtaCor EV-ICD Lead System implant procedure 18. Known adhesions in the thorax or history of medical treatments, surgeries or conditions that increase the potential for adhesions in the thorax 19. Surgically corrected congenital heart disease (not including catheter-based procedures) Cardiac Related: 20. Subjects who require permanent bradycardia pacing or cardiac resynchronization therapy 21. NYHA IV functional class in past 90 days 22. Inotropic therapy in past 180 days 23. Known history of pericardial disease, pericarditis or mediastinitis 24. Patients with a medical condition that precludes them from undergoing defibrillation testing: * Severe aortic stenosis * Current Intracardiac LA or LV thrombus * Severe proximal three-vessel or left main coronary artery disease without revascularization * Hemodynamic instability * Unstable angina * Recent stroke or transient ischemic attack (within the last 6 months) * Known inadequate external defibrillation * LVEF \< 20% * LVEDD \>70 mm