A Study to Select the Dose and Evaluate the Effectiveness and Safety of the Drug Refralon®, Tablets, 1 mg for Long-term Use to Prevent Recurrence of Atrial Fibrillation/Flutter After Terminating Its Persistent Form

This trial is testing a heart rhythm medication called Refralon at different doses to find the best long-term dose for preventing atrial fibrillation (AFib) or atrial flutter from coming back after treatment has restored a normal heart rhythm. AFib is an irregular heartbeat that can cause stroke and heart failure. **You may be eligible if...** - You have persistent AFib or atrial flutter (lasting 7 days to 1 year) - Your heart's pumping function (left ventricular ejection fraction) is above 40% - You currently have AFib or flutter on your ECG at the time of enrollment **You may NOT be eligible if...** - You have had a heart attack or other structural heart disease - Your QT interval (a heart rhythm measurement) is prolonged over 500 ms - You have taken certain antiarrhythmic drugs recently (within 7 days, or 60 days for amiodarone) - You have severe kidney or liver disease - You are pregnant Talk to your doctor to see if this trial is right for you.