inability of the mitral valve to prevent the flow of blood back from the left ventricle, or lower chamber of the heart, into the left atrium, or upper chamber. Normally, the valve permits blood to flow from the atrium to the ventricle but prevents its return. Most often, the inability of the mitral valve to close adequately is caused by scarring from rheumatic heart disease; it may also be due to a congenital defect of the valve or may arise from defects in the muscles and tendons (the papillary muscles and chordae tendineae) that operate the valve. Less frequently it may be due to endocarditis (inflammation of the heart lining) or cardiac tumour. The condition is recognized from characteristic heart sounds and from patterns that show up in echocardiography or electrocardiography.

Persons with mitral insufficiency may not be conscious of any effect of the condition or may be easily fatigued and may experience difficulty in breathing after exertion or while lying down. The left atrium may become greatly enlarged, and left ventricular failure eventually develops.

Medical treatment is restriction of vigorous exercise, reduction of sodium intake and increase in sodium excretion, and administration of anticoagulants to avoid the formation of blood clots in the veins. Some persons with severe defects are treated surgically by replacement of the valve.