Dapagliflozin Effect on FunctiOnal Mitral Regurgitation and Myocardial Fibrosis (DEFORM)

The goal of this clinical trial is to learn about the effect of dapagliflozin on reducing the degree of mitral regurgitation and myocardial fibrosis in patients with functional mitral regurgitation (FMR) of more than moderate and left ventricular ejection fraction (LVEF) more than 40%. The main question\[s\] it aims to answer are: * For FMR patients with EF\>40%, whether adding SGLT2 to guideline directed medical therapy (GDMT) can reduce MR degree and myocardial fibrosis, as well as improve patient's cardio-pulmonary function and quality of life is unknown. * For FMR patients with EF\>40%, whether adding SGLT2 to GDMT can decelerate the progression of FMR to end-stage heart failure, reduce the demand for percutaneous mitral valve repair and improve the prognosis is unknown. Participants in dapagliflozin arm will add dapagliflozin of 10 mg/d orally for 6 months on the basis of regular GDMT for FMR. Researchers will compare the dapagliflozin arm to the GDMT arm who only accept the regular GDMT to see if the effective regurgitant orifice area was decreased. The primary endpoint was the effective regurgitant orifice area(EROA) change from baseline to 6 months. The secondary endpoints include the cardiac magnetic resonance assessment of myocardial fibrosis, the N-terminal fragment of the pro brain natriuretic peptides(NT-pro BNP), maximal exercise capacity by cardiopulmonary exercise testing, distance in the 6 minutes walking test, Kansas City cardiomyopathy questionnaire and Cardiovascular events.