The primary study objective is to evaluate the feasibility, initial clinical safety and potential effectiveness of the Calyan Pacemaker system.

This is an early feasibility study, designed to evaluate the initial clinical safety, device functionality and stability and potential effectiveness of the Calyan pacemaker. The Calyan device will be implanted, and if acute success criteria are met, chronic pacing will be delivered over a 3-month primary follow-up period, with subsequent extended follow-up of all patients. The data from this study will be used to justify the design of a larger prospective pivotal study that will assess the safety and effectiveness of the Calyan pacemaker device.

Inclusion Criteria:

1. Age 18 or above
2. Willing and capable of providing informed consent
3. Class I or IIa indication for implantation of a single-chamber ventricular pacemaker, according to ACC/AHA/HRS guidelines
4. A life expectancy of at least one year, and is a suitable candidate based on overall health and well-being

Exclusion Criteria:

1. Patients with complete AV block or other pacemaker-dependent conditions

2. Patients in whom a substernal device implant should be avoided:

   1. Any prior sternotomy;
   2. Any prior medical condition or procedure that leads to adhesions in the anterior mediastinal space;
   3. Any marked sternal abnormality, such as pectus excavatum or pectus carinatum
   4. Prior abdominal surgery in the epigastric region
   5. Planned sternotomy
   6. Prior or planned chest radiotherapy
   7. Hiatal hernia that distorts mediastinal anatomy
   8. Adhesions in the anterior mediastinal space
   9. Severe obesity so that subxiphoid/substernal tunneling cannot be safely performed

3. Patients with severe RV dilation, gross hepatosplenomegaly, or severe obesity such that subxiphoidal/substernal tunneling cannot be safely performed

4. Patients with a class III indication for a permanent pacemaker

5. Patients with decompensated heart failure not due to bradycardia and expected to worsen with chronic RV pacing

6. Patients with an implanted cardiac pacemaker, cardioverter defibrillator, cardiac resynchronization device, or neurostimulator device, or planned implantation of a cardioverter defibrillator, cardiac resynchronization device, or neurostimulator device

7. Patients with a current or planned implantation of a substernal device, or any implanted device that would interfere with the implantation or operation of a substernal device

8. Patients who have previously undergone an open-heart surgical procedure.

9. Patients with an active infection

10. Patient on chronic oral anticoagulation which cannot be temporarily discontinued for surgery

11. Patients with a condition in which pericardial pacing would be difficult or impossible, such as acute pericarditis, chronic pericardial effusion or pericardial thickening or calcification, cardiac tamponade, or chronic restrictive pericarditis

12. Patients who have tested positive for the COVID-19 in the past 3 months, or are currently showing symptoms consistent with COVID-19

13. Women of childbearing potential who are or might be pregnant at the time of the study or breastfeeding

14. Subjects with a life expectancy of less than 12 months

15. Patients who are currently enrolled or planning to participate in any concurrent clinical study with an investigational therapy

16. Patients with decompensated heart failure expected to worsen with chronic RV pacing

17. Patients with COPD with oxygen dependence