Optimal Pacing Rate for Cardiac Resynchronization Therapy

This study looks at the best pacing rate for patients with permanent atrial fibrillation (an irregular heart rhythm) who have a special heart device called cardiac resynchronization therapy (CRT). The goal is to find the ideal pacing speed that best helps the heart pump effectively. **You may be eligible if...** - You are 18 years or older with a history of persistent or permanent atrial fibrillation - You have a CRT or conduction system pacing device implanted in the last 3 months to 5 years - You have had a complete heart block or have undergone a procedure to block the electrical connection between the upper and lower heart chambers (AV node ablation) - You have a preserved heart pumping function (LVEF at or above 50%) but have symptoms of heart failure **You may NOT be eligible if...** - Your heart pumping function is reduced (LVEF below 50%) - You have severe valve disease, significant coronary artery disease, or end-stage kidney disease - You are on oxygen at home for lung disease - You are wheelchair-bound or unable to do a walking test - You are pregnant or plan to become pregnant - You are over 90 years old or in end-stage cancer or palliative care Talk to your doctor to see if this trial is right for you.