Efficacy and Safety Study of F373280

Inclusion Criteria: * Men or women aged more than 18 years (inclusive) * Patients with current episode of persistent Atrial Fibrillation (AF) between 7 days and 6 months duration for whom electrical cardioversion is warranted * Previous history of first documented episode of persistent AF. * Previous history of ischemic or non ischemic heart failure * New York Heart Association (NYHA) class I or II chronic heart failure at selection and at inclusion * Left ventricular systolic dysfunction defined at selection and at inclusion by a reduced left ventricular ejection fraction (LVEF) ≥ 30% and ≤ 45% or for patients with a LVEF \> 45%: * an increased left ventricular end-diastolic size (diameter ≥ 60 mm and/or \> 32 mm/m² and/or volume \> 97 ml/m²) * and/or an increased left ventricular end-systolic size (diameter \> 45 mm and/or \> 25 mm/m² and/or volume \> 43 ml/m²) * and/or a reduced left ventricular outflow tract velocity time integral \< 15 cm * On appropriate, stable medical treatments for heart failure, including a diuretic and/or angiotensin-converting enzyme, and/or angiotensin-receptor blocker and/or mineralocorticoid receptor (MR) antagonists, and/or betablockers * Left atrial area ≤ 40 cm² at selection and at inclusion * Patients treated or having to be treated by vitamin K antagonist * For female patient of child-bearing potential: * In all the countries except Italy: * Use of an effective method of contraception (hormonal contraception or intra-uterine device) assessed by the investigator, for at least 2 months before the selection in the study, and agreement to go on using it during the whole duration of the study and up to 1 month after the last dose of the study treatment * Documented as surgically sterilized * In Italy only: * Absolute abstention from sexual intercourse during the whole duration of the study and for a month after the end of the study or * Use of double barrier contraception method (use of effective medical contraception method) from at least 2 months before the start of the study to the entire duration of the study and for a month after the end of the study or * Documented as surgically sterilized. * For female patient of child-bearing potential: negative urine pregnancy test at inclusion * For male with a child-bearing potential partner (In Italy only): * Absolute abstention from sexual intercourse during the whole duration of the study and for 3 months after the end of the study or * Use of double barrier contraception method (use of condom for male and effective contraception method for the partner) from the entire duration of the study to 3 months after the end of the study. Ethical / legal considerations: * Having signed his/her written informed consent, * Affiliated to a social security system, or is beneficiary (if applicable in the national regulation) Exclusion Criteria: * No previous history of first documented episode of persistent AF * More than two successful cardioversions (electrical or pharmacological) in the last 6 months * Secondary Atrial Fibrillation due to alcohol or severe valvular heart disease (grade III to IV) * NYHA class III or IV heart failure at selection or at inclusion * Thyroid disease uncontrolled by treatment: TSH ± T4L ± T3L to be checked in case of treatment for thyroid disease * Myocardial infarction or unstable angina or presence of unstable ischemic coronaropathy assessed by coronarography or cardiac stress test (Echo stress, exercise stress test, nuclear or MR perfusion evaluation methods) within 6 months before selection * Severe chronic kidney disease (creatinine ≥ 25 mg/L or estimated glomerular filtration rate \< 30 ml/min) at selection * Bradycardia (HR ≤ 50 bpm) * Hyperkalemia or hypokalemia (according to the standards of local laboratories) at selection * Cardiac surgery within 3 months before selection or planned during the study duration Criteria related to treatments: * Previously ineffective pharmacological or electrical cardioversion * Concomitant treatment with ranolazine or any antiarrhythmic drug (within 7 days prior to selection), except amiodarone, dronedarone and stable dose of digoxin, betablockers, calcium-blockers * Concomitant treatment with oral amiodarone or dronedarone from selection * Concomitant treatment with intravenous amiodarone from selection * Patient requiring a cardiac resynchronization therapy (CRT) or having undergone CRT implantation within the last 6 months * Treatment with any Polyunsaturated Fatty Acid (PUFA) within the last 3 months * Dietary supplement with ω 3 or ω 6 according to investigator's judgement * Having undergone any form of ablation therapy for AF * Patient treated with oral anticoagulant treatment other than vitamin K antagonist: new oral anticoagulants (dabigatran, rivaroxaban, apixaban), or treated with irreversible antiplatelet agents P2Y12 inhibitors such as ticlopidine, clopidogrel or prasugrel Other criteria: * Patient liable not to comply with protocol instructions and/or with treatment, in the investigator's opinion * Patient having taken part in a clinical trial in the preceding 2 months or taking part in a trial at the time of selection * Patient linguistically or mentally unable to understand the nature, objectives and possible consequences of the trial, or refusing to patient himself/herself to its constraints * Patient family member or work associate (secretary, nurse, technician,..) of the Investigator * Patient having forfeited his / her freedom by administrative or legal award or being under guardianship * Breastfeeding female patient