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

Inclusion Criteria: * Persistent form of AF/AFL lasting 7 days - 1 year (including patients who have previously undergone ineffective catheter or surgical ablation of AF or AFL and patients with a pacemaker or implanted cardioverter-defibrillator (ICD)); * Basic rhythm - AF and/or AFL according to the results of 12-lead ECG at randomization (Visit 2); * Left ventricular ejection fraction (LVEF) \>40% according to the Simpson method; Exclusion Criteria: * Intake of class IA, IC and class III antiarrhythmic drugs less than 7 days (for amiodarone less than 60 days) before the study drug intake; * History of myocardial infarction or other structural heart disease; * QT prolongation over 500 ms.