ASCEND CSP IDE Study

Inclusion Criteria: 1. Subject must meet current clinical practice guidelines for implantation of ICD or CRT-D and will undergo one of the following: 1. de novo Abbott ICD system implant (single or dual chamber) 2. de novo Abbott CRT-D system implant and is intended to also undergo left ventricle coronary sinus lead implant (for patients indicated for CRT-D according to locally approved indications) 2. Subject is ≥ 18 years of age or age of legal consent, whichever age is greater. 3. Subject is willing to undergo implant defibrillation testing if requested. Subjects with AF in whom defibrillation testing is required must have been on or willing to be on uninterrupted anticoagulation therapy before, during, and after the implant procedure. 4. Subject is willing to comply with clinical investigation procedures and agrees to return to clinic for all required follow-up visits, tests, and exams. 5. Subject has been informed of the nature of the clinical investigation, agrees to its provisions and has provided a signed written informed consent, approved by the IRB/EC. Exclusion Criteria: 1. Subject has had myocardial infarction or unstable angina within 40 days prior to signing consent 2. Subject has had recent cardiac revascularization (angioplasty, stent or bypass graft) in the 4 weeks prior to signing consent or planned within 3 months following consent 3. Subject has had any other therapeutic cardiovascular procedure (such as transcatheter aortic valve replacement, MitraClip, cardiac surgery, left atrial appendage closure, patent foramen ovale closure, or any ablation procedures) in the 3 months prior to signing consent or planned within 3 months following consent 4. Subject has had a recent Cerebrovascular Accident (CVA) or Transient Ischemic Attack (TIA) within 3 months of prior to signing consent 5. Subject has a life expectancy of less than 12 months 6. Subject has contraindications for standard RV transvenous and/or LBBAP lead placement (e.g., mechanical right heart valve) 7. Subject is contraindicated for ≤1mg Dexamethasone sodium phosphate. 8. Subject is pregnant or breastfeeding women, or women of childbearing potential and who are not on a reliable form of birth regulation method or abstinence 9. Subject has an existing pacemaker (including a temporary pacing system), ICD, CRT, or cardiac contractility modulation (CCM) device or leads 10. Subject has any evidence of active infection or undergoing treatment for an infection 11. Subject is enrolled or planning to enroll in another clinical trial that might confound the results of the present study 12. Subject has moderate or severe aortic stenosis 13. Subject has ventricular septal defect (VSD) or had prior surgery on the interventricular septum that could impact LBBAP implant, such as septal myomectomy, ethanol septal ablation, VSD repair, etc 14. Subject has end-stage renal disease 15. Subject has NYHA IV classification 16. Subject has undergone cardiac transplantation or have a classification of Status 1 for cardiac transplantation or consideration for transplantation during the study follow-up period 17. Subject has had previously extracted leads 18. Subject has had an LV Assist Device 19. Subject has had a failed LBBAP lead implant Defibrillation testing exclusion criteria (these are part of general exclusion criteria when defibrillation testing is requested): 1. Subject has pre-existing or suspected pneumothorax during implant 2. Subject has current known intracardiac left atrial or Left Ventricular thrombus 3. Subject has severe proximal three-vessel or left main coronary artery disease without revascularization 4. Subject has Ejection Fraction less than 20% 5. Subject has recent stroke or transient ischemic attack (within the last 6 months) 6. Subject has known inadequate external defibrillation 7. Subject has any other known medical condition not listed that precludes their participation in the opinion of the investigator