Mitral valve replacement is a surgical procedure in which a diseased or defective mitral valve in the heart is replaced with a mechanical or tissue valve. Medical conditions which may require this surgery include mitral stenosis (a narrowing of the valve) and mitral insufficiency (in which the valve does not close properly). In most cases, this surgery can improve or relieve symptoms caused by a defective mitral valve, such as chest pain, fainting, and fatigue.