RIvaroxaban for Valvular Heart diseasE and atRial Fibrillation Trial -RIVER Trial

Inclusion Criteria: 1. Male and female patients aged \>18 years at time of inclusion 2. Patients with Persistent or paroxysmal Atrial Fibrillation or flutter with bioprosthetic mitral valves. * The patient must be able to give informed consent Exclusion Criteria: 1. Cardiovascular-related conditions as known presence of cardiac thrombus or tumor * Active endocarditis * Uncontrolled hypertension 2. Hemorrhage risk-related criteria * Active internal bleeding * History of, or condition associated with, increased bleeding risk 3. Concomitant conditions and therapies * History of previous thromboembolism with high risk of bleeding: * Severe, disabling stroke (modified Rankin score of 4-5, inclusive) within 3 months * Acute thromboembolic events or thrombosis (venous/arterial) within the last 14 days prior to randomization * Acute MI within the last 14 days prior to randomization * Treatment with: Chronic aspirin therapy \> 100 mg daily or dual antiplatelet therapy; Intravenous antiplatelets; Fibrinolytics; Anticipated need for long-term treatment with a nonsteroidal antiinflammatory drug; Systemic treatment with a strong inhibitor of cytochrome P450 3A4, such as ketoconazole or protease inhibitors; Treatment with a strong inducer of cytochrome P450 3A4, such as rifampicin, phenytoin, phenobarbital, or carbamazepine. * Anemia * Pregnancy or breastfeeding or women of reproductive age not using effective contraceptive methods * Calculated creatinine clearance bellow 30 mL/min * Known significant liver disease or alanine aminotransferase N3× the upper limit of normal * Previous participation in this study.