Parasternal Extravascular ICD System Pivotal Clinical Investigation (ALARION 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. Patients who cannot read or write 2. Expected survival \< 1 year 3. Pregnant or nursing subjects and those who plan pregnancy during the clinical investigation follow-up period 4. Participation in any concurrent clinical study without prior approval from the Sponsor 5. Inability or unwillingness to provide informed consent to participate in the study 6. Any known circumstances which may prevent the completion of protocol testing and data collection through the 6-month follow-up visit 7. Circumstances that may prevent data collection or completion of specified follow-up visits 8. Allergies to any device materials listed in the Instructions for Use (IFU) 9. Contraindication for temporary suspension of oral/systemic anticoagulation 10. Known history of lung disease with FEV1 \< 1.0 Liter Device Related: 11. 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, LVAD or neurostimulator 12. Implanted with a subcutaneous ICD lead, subcutaneous coils/arrays, epicardial patches or epicardial pace/sense leads 13. Any known need for MRI prior to approval of MRI conditional labeling Anatomy Related: 14. Known structural abnormalities of the heart that may increase risk of the Parasternal EV-ICD System procedure 15. Known obstructed or restricted pathway into the mediastinum for the Atala™ lead 16. Prior surgery with disruption of the lung, pericardium or connective tissue between the sternum and pericardium, including sternotomy of any type 17. Known significant anatomic derangement of or within the thorax (e.g., pectus excavatum, significant scoliosis) 18. History of thoracic radiation therapy or other medical treatments/conditions which may complicate the Parasternal EV-ICD System implant procedure 19. Known adhesions in the thorax or history of medical treatments, surgeries or conditions that increase the potential for adhesions in the thorax 20. Surgically corrected congenital heart disease (not including catheter-based procedures) Cardiac Related: 21. Indication for permanent bradycardia pacing or cardiac resynchronization therapy 22. Decompensated heart failure 23. Currently on inotropic therapy 24. Known history of pericardial disease, pericarditis or mediastinitis 25. Risk factors associated with 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 3 months) * Known inadequate external defibrillation * LVEF \< 20% (most recent assessment; must be within 180 days of consent) * LVEDD \>70 mm (most recent assessment; must be within 180 days of consent)