MITRAL VALVE PROLAPSE. In MVP, one or both valve flaps are enlarged, and some of their supporting "strings" may be too long. When the heart pumps (contracts), the mitral valve flaps don't close smoothly or evenly. Instead, part of one or both flaps collapses backward into the left atrium. Sometimes, this allows a small amount of blood to leak backward through the valve and cause a heart murmur.