This study is being done to determine if apixaban is more safe, when compared to warfarin, at reducing the risk of bleeding during CIED procedures.

1. Signed informed written consent
2. Patients are taking warfarin or apixaban chronically
3. Patients are at risk of stroke or systemic embolism
4. Patients have NVAF
5. Patients will undergo CIED generator change
6. Age 18-90
7. Either gender
8. All eligibility criteria must be met

Exclusion:

1. Patient is critically ill, or in unstable condition
2. Patient is at high risk for bleeding, HAS-BLED score ≥ 3
3. Patient is required to take dual antiplatelet agents in addition to anticoagulant drug. (e.g. recent cardiac stent placement)
4. Patients have mechanical cardiac valve or another condition other than NVAF that requires chronic anticoagulation
5. Women of childbearing potential with positive pregnant test
6. Bilirubin >2x upper limit of normal, in association with AST/ALT/ALP >3x upper limit normal
7. Allergy to apixaban and warfarin
8. Subjects requiring taking strong dual inhibitors of CYP3A4 and P-gp who already meet the criteria to take 2.5 mg apixaban
9. Subjects requiring taking strong dual inducers of CYP3A4 and P-gp (.e.g., rifampin, carbamazepine, phenytoin, St. John's wort)
10. Patients who take any investigational drugs within 30 days of enrolling in study.
11. Prisoners or subjects who are involuntarily incarcerated.
12. Subjects who are compulsorily detained for treatment of either a psychiatric or physical (eg, infectious disease) illness.