narrowing of the mitral valve, the function of which is to permit blood to flow from the atrium, or upper chamber, to the ventricle, or lower chamber, of the left side of the heart and to prevent its backflow. Narrowing of the mitral valve is usually a result of rheumatic fever; rarely, the narrowed valve is a congenital defect. The condition, most common in women under 45, is diagnosed by recognition of typical heart sounds and confirmed by certain patterns that appear in echocardiography or electrocardiography.

Narrowing of the valve increases pressure in the left atrium and in the pulmonary veins and capillaries (oxygenated blood from the lungs enters the left atrium by way of the pulmonary veins). The increased pressure in the pulmonary vessels may lead to congestion of the lungs and the collection of fluid in the pulmonary tissues. Difficulty in breathing, particularly after exercise, is one consequence. If the small vessels of the lungs develop resistance, possibly by the thickening of their walls, accumulation of fluid in the lungs decreases, but increased back pressure in the right ventricle of the heart (from which blood is pumped to the lungs) may lead to early failure of the right side of the heart.

Medical treatment includes the regulation of exercise so as to avoid fatigue and to minimize difficulty in breathing; reduction of sodium intake and increase in sodium excretion to reduce accumulation of fluids in the tissues; and administration of anticoagulants to reduce the possibility of clot formation. Surgical treatment is replacement of the valve with one of Dacron, stainless steel, or some other special material, or with a transplant of a valve from a pig heart.