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M itral insufficiency, or mitral regurgitation, is the inadequate closure of the mitral valve, which interferes with expulsion of cardiac output from the left ventricle. The mitral valve is located between the left atrium and the left ventricle. When the heart contracts, blood is moved forward from the left ventricle out through the aortic valve and into the aorta. During the high pressures that are generated during contraction, blood flows backward through the regurgant valve into the left atrium. Cardiac output, therefore, is separated into forward systemic flow into the aorta and backward regurgitant flow into the left atrium. The amount of forward versus backward flow depends on the severity of the mitral insufficiency and the afterload (impedance to flow against which the left ventricle pumps).

Mitral insufficiency causes an increase in blood volume in both the left atrium and ventricle. This situation occurs because of the regurgitant blood that flows from left atrium to left ventricle to left atrium again. The increased volume of blood in chronic mitral insufficiency accumulates slowly, allowing the left atrium and left ventricle to increase in size. The heart, therefore, tolerates the regurgitant blood flow without engorgement of the pulmonary circulation or reduction of cardiac output. In acute mitral insufficiency, the left atrium and ventricle are not able to tolerate the dramatic increase in blood volume, so cardiac output decreases and blood backs up quickly into the pulmonary circulation. Pulmonary congestion and acute illness follow.

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