Q. Pathology of mitral regurgitation?
During left ventricular systole as the pressure rises in left ventricle, blood is pumped simultaneously both into aorta and left atrium. The effective orifice that allows regurgitation is called effective regurgitant orifice. The fraction of total left ventricular stroke volume that regurgitates in to left atrium is regurgitant fraction. In chronic mitral regurgitation compliant left atrium dilates and accommodates the regurgitant blood thus not allowing the mean left atrial pressure to rise. As the blood flow across mitral valve increases during next diastole, end diastolic volume of left ventricle increases. With increased pre load, left ventricular ejection fraction increases and maintains forward stroke volume in spite of regurgitation. During systole as the left ventricle is open to low pressure left atrium, afterload decreases. Thus in chronic mitral regurgitation left ventricular ejection fraction is more than normal due to increased preload and decreased afterload. As the mitral regurgitation progresses, left ventricle and left atrium dilate. Dilated large left atrium will have areas of fibrosis leading to generation of atrial fibrillation. Dilated chambers with dilated mitral annulus and altered geometry of papillary muscles and posterior left atrial wall lead to aggravation of mitral regurgitation. Hence mitral regurgitation begets mitral regurgitation. Eventually changes take place in left ventricular myocardium causing myocardial dysfunction. As stated earlier, increased preload and decreased afterload of left ventricle allow maintained left ventricular ejection fraction even in the presence of myocardial dysfunction. Left ventricular ejection fraction is not a load independent index of left ventricular systolic function and is not suitable to assess left ventricular systolic function in patients with chronic mitral regurgitation. Removal of the regurgitant orifice through which left ventricle has a low pressure outlet by mitral valve replacement or repair will effectively increase the afterload. Failing myocardium will now face afterload mismatch and left ventricular ejection fraction actually decreases after surgery. Understanding this concept is of fundamental importance in timing of mitral valve surgery.