Optimal Pacing Rate for Cardiac Resynchronization Therapy

Who May Qualify: 1. Age 18 years or older 2. History of persistent or permanent atrial fibrillation 3. Implantation of CRT or conduction system pacing in prior 3 months to 5 years of study start 4. History of intrinsic AVN block or have undergone AVN ablation in prior 3 months to 5 years of study start 5. LVEF ≥ 50% 6. N-terminal pro-B-type natriuretic peptide (NT-proBNP) \>400 pg/mL in the last 24 months 7. Clinical HF diagnosis or NYHA class II or higher 8. Able to provide willing to sign a consent form Who Should NOT Join This Trial: 1. LVEF \<50% 2. Wide QRS (greater than 150ms) 3. Isolated RV pacing 4. Severe valvular disease 5. Severe coronary artery disease as defined by one of the following: 1. ACS or PCI within 1 year 2. Any angina (CCS class 1+) 3. Unrevascularizable severe CAD (\>70% stenosis in 1+ major vessels and/or based on functional assessment) 6. ESRD 7. Significant primary pulmonary disease on home oxygen 8. Major orthopedic issues, such as being wheelchair bound and/or unable to perform a six-minute walk test 9. Ventricular ectopy \>15% premature ventricular contractions (PVC) 10. End stage cancer diagnosis 11. Life expectancy less than one year 12. Palliative or hospice care 13. Hypertrophic cardiomyopathy (HCM) 14. Uncorrected ventricular septal defect 15. Infiltrative cardiomyopathy (CM) 16. Uncontrolled hypertension as defined by blood pressure \>160/100 mm Hg on two measurements ≥15 minutes apart 17. Hemoglobin \<7 g/dL 18. Age \>90 years old 19. Pregnant or intends to become pregnant Always talk to your doctor about whether this trial is right for you.