Mitral valve stenosis is a heart disease in which the mitral valve—which regulates blood flow on the left side of the heart—fails to open as wide as it should. Although it has no immediate effect on health, eventually mitral valve stenosis can cause the left atrium to work harder to pump blood through the narrowed valve, leading to heart failure and complications such as stroke, heart rhythm problems (such as atrial fibrillation), fluid buildup in the lungs, and stroke.