Patients with prosthetic mitral valves need a long-term anticoagulation therapy. After mitral valve replacement using mechanical prostheses warfarin is administrated during the whole life. Biological prostheses allows do discontinue anticoagulation 3 months after surgery in the absence of other risk factors of thrombosis especially atrial fibrillation. The common approach to patients with mitral valve disease and atrial fibrillation is mitral valve prosthetic replacement concomitant to Cox-Maze procedure. This allows to maintain sinus rhythm in 65-80% of patients. In the study the safety and efficacy of dabigatran in comparison with warfarin will be evaluated after the mitral valve surgery and Cox-Maze procedure.