Follow up: All patients will receive mitral valve replacement later. The prevalence of operation mortality and post-OP atrial fibrillation will be assessed. Otherwise, patient will receive regular follow-up at our outpatient clinic for at least 1 year and any heart failure event necessary for hospitalization will be assessed by telephone interview, chart review or personal visit interview if needed. Those events, including operation mortality, post-OP atrial fibrillation and post-OP hospitalization for heart failure, will account for patients' prognosis.