With chronic mitral regurgitation, the dilated, thin walled, more compliant left atrium accommodates the large regurgitant volume from the left ventricle with little or no elevation of left atrial pressure. As a result, the pulmonary vessels remain relatively normal, and pulmonary hypertension and right ventricular hypertrophy are delayed in onset.

With acute mitral regurgitation, the left atrium is relatively small and not very compliant. The large regurgitant volume that is ejected by the left ventricle into the small left atrium causes markedly increased left atrial pressure. This results in early pulmonary vascular changes including marked pulmonary congestion, often pulmonary edema, and eventually pulmonary hypertension and right ventricular hypertrophy.