Mitral regurgitation: a leak the atrium absorbs
The mitral valve is the inflow door of the left ventricle, and it is held shut during the squeeze by delicate cords, the chordae tendineae, anchored to the papillary muscles in the ventricle wall. In mitral regurgitation, the valve fails to seal during systole, so each time the ventricle squeezes, part of its blood is squirted backward into the low-pressure left atrium instead of forward into the aorta.
This makes a systolic murmur — heard best at the apex and often radiating toward the armpit — that lasts the whole of systole, because the valve is leaking the entire time it should be shut. The left atrium, never built for pressure, stretches to absorb the regurgitant volume. A big, stretched atrium is electrically irritable, which is why long-standing mitral regurgitation so often ends in atrial fibrillation. Slowly, the volume load is passed back to the lungs and the ventricle tires.
Mitral valve prolapse: the common, mostly benign one
Mitral valve prolapse deserves its own mention because it is one of the most common valve findings and is usually harmless. The leaflets are a little too floppy, so during systole one or both billow back into the atrium like a sail catching wind. The classic sound is a mid-systolic click (the leaflet reaching the end of its tether) sometimes followed by a soft murmur if a little leak follows. Most people with prolapse live entirely normal lives and need only occasional follow-up.
Mitral stenosis, and a word on the right-sided valves
Mitral stenosis is the mirror image: the valve stiffens and its opening narrows, so blood struggles to flow from atrium to ventricle during diastole. The bottleneck makes a diastolic murmur (a low rumble) and dams blood up behind the valve. Pressure climbs in the left atrium and backs up into the lungs, causing breathlessness and, when severe, pulmonary congestion. Almost all mitral stenosis worldwide is a late consequence of rheumatic heart disease — the subject of the next guide.
The right-sided valves follow the same rules at lower pressure. Tricuspid regurgitation — usually not from the valve itself but because a strained right ventricle has stretched its ring — pushes blood back toward the body's veins, raising the neck veins and causing leg swelling and a large, tender liver. Pulmonary stenosis, the rarest of the four, is most often congenital and makes a systolic murmur as the right ventricle pushes against the narrow exit to the lungs.