The LEADLESS II IDE Study for the Nanostim Leadless Pacemaker

Inclusion Criteria: 1. Subject must have one of the clinical indications before device implant in adherence with Medicare, ACC/AHA/HRS/ESC single chamber pacing guidelines including: * Chronic and/or permanent atrial fibrillation with 2 or 3° AV or bifascicular bundle branch block (BBB block), including slow ventricular rates (with or without medication) associated with atrial fibrillation; or * Normal sinus rhythm with 2 or 3° AV or BBB block and a low level of physical activity or short expected lifespan (but at least one year); or * Sinus bradycardia with infrequent pauses or unexplained syncope with EP findings; and 2. Subject ≥18 years of age; and 3. Subject has life expectancy of at least one year; and 4. Subject is not enrolled in another clinical investigation; and 5. Subject is willing to comply with clinical investigation procedures and agrees to return for all required follow-up visits, tests, and exams; and 6. Subject has been informed of the nature of the study, agrees to its provisions and has provided written informed consent, approved by the IRB; and 7. Subject is not pregnant and does not plan to get pregnant during the course of the study. Exclusion Criteria: 1. Subject has pacemaker syndrome, has retrograde VA conduction or suffers a drop in arterial blood pressure with the onset of ventricular pacing; or 2. Subject is allergic or hypersensitive to \<1 mg of dexamethasone sodium phosphate; or 3. Subject has a mechanical tricuspid valve prosthesis; or 4. Subject has a pre-existing endocardial pacing or defibrillation leads; or 5. Subject has current implantation of either conventional or subcutaneous implantable cardioverter defibrillator (ICD) or cardiac resynchronization therapy (CRT); or 6. Subject has an implanted vena cava filter; or 7. Subject has evidence of thrombosis in one of the veins used for access during the procedure; or 8. Subject has an implanted leadless cardiac pacemaker; or 9. Subject had recent cardiovascular or peripheral vascular surgery within 30 days of enrollment