MR can be caused by organic disease (eg, rheumatic fever, ruptured chordae tendineae, myxomatous degeneration, leaflet perforation) or a functional abnormality (ie, a normal valve may regurgitate [leak] because of mitral annular dilatation, focal myocardial dysfunction, or both). Congenital MR is rare but is commonly associated with myxomatous mitral valve disease. Alternatively, it can be associated with cleft of the mitral valve, as occurs in persons with Down syndrome, or a ostium primum atrial septal defect.

Acute mitral regurgitation
Acute MR is characterized by an increase in preload and a decrease in afterload causing an increase in end-diastolic volume (EDV) and a decrease in end-systolic volume (ESV). This leads to an increase in total stroke volume (TSV) to supranormal levels. However, forward stroke volume (FSV) is diminished because much of the TSV regurgitates as the regurgitant stroke volume (RSV). This, in turn, results in an increase in left atrial pressure (LAP). According to the Laplace principle, which states that ventricular wall stress is proportional to both ventricular pressure and radius, LV wall stress in the acute phase is markedly decreased since both of these parameters are reduced.

Chronic compensated mitral regurgitation
In chronic compensated MR, the left atrium (LA) and ventricle have sufficient time to dilate and accommodate the regurgitant volume. Thus LA pressure is often normal or only minimally elevated. Because of the left ventricular dilatation via the process of eccentric hypertrophy, TSV and FSV are maintained. Wall stress may be normal to slightly increased as the radius of the LV cavity increases but the end-diastolic LV pressure remains normal. As the LV progressively enlarges, the mitral annulus may stretch and prevent the mitral valve leaflets from coapting properly during systole, thus worsening the MR and LV dilatation.

Chronic decompensated mitral regurgitation
In the chronic decompensated phase, muscle dysfunction has developed, impairing both TSV and FSV (although ejection fraction still may be normal). This results in a higher ESV and EDV, which in turn causes a elevation of LV and LA pressure, ultimately leading to pulmonary edema and, if left untreated, cardiogenic shock.