Author: Tanner Marshall, MS
Editor: Rishi Desai, MD, MPH, Tanner Marshall, MS
The mitral valve has two leaflets—the anterior and posterior leaflet, and together they separate the left atrium from the left ventricle. During systole, the valve closes, which means blood has just one option—to be ejected out the aortic valve and into circulation.
If the mitral valve doesn’t shut all the way, blood can leak back into the left atrium, called mitral valve regurgitation. During diastole, the mitral valve opens and lets blood fill into the ventricle. If the mitral valve doesn’t open enough, it gets harder to fill the left ventricle, called mitral valve stenosis.
Let’s start with mitral valve regurgitation - the leading cause of mitral valve regurgitation in the United States and the most common of all valvular conditions is mitral valve prolapse. When the left ventricle contracts during systole, a ton of pressure is generated so the blood can be pumped out the aortic valve, so a lot of pressure pushes on that closed mitral valve, but normally the papillary muscles and connective tissue called chordae tendineae, or heart strings, keep the valve from prolapsing or falling back into the atrium.
With mitral valve prolapse the connective tissue of the leaflets and surrounding tissue are weakened, called myxomatous degeneration. Why this happens isn’t well understood, but it is sometimes associated with connective tissue disorders like Marfan syndrome and Ehlers-Danlos syndrome. Myxomatous degeneration results in a larger valve leaflet area and elongation of the chordae tendineae which can sometimes rupture, something that typically happens to the ones on the posterior leaflet. That can allow the posterior leaflet to fold up into the left atrium.
Patients with a mitral valve prolapse are usually asymptomatic, but often have a classic heart murmur which includes a mid-systolic click, which is sometimes followed by a systolic murmur.
The click is a result of the leaflet folding into the atrium and being suddenly stopped by the chordae tendineae. Although mitral valve prolapse doesn’t always cause mitral regurgitation, it oftentimes does. If the leaflets don’t make a perfect seal, a little bit of blood leaks backward from the left ventricle into the left atrium and causes a murmur.
The mitral valve prolapse murmur is somewhat unique in that when patients squat down, the click comes later and the murmur is shorter, but when they stand or do a valsalva maneuver the click comes sooner and the murmur lasts longer.
The reason this happens is that squatting increases venous return, which fills the left ventricle with slightly more blood, and this means that the left ventricle gets just a little bit larger. The larger leaflets therefore have more space to hang out, and as the ventricle contracts and gets smaller, it takes just a little longer for the leaflet to get forced into the atrium. Standing on the other hand reduces venous return, meaning a little less blood in the ventricle and so, a little less room to hang out, and so the leaflet gets forced out earlier during contraction. The other heart murmur that follows this pattern is like the one in hypertrophic cardiomyopathy.
So in addition to mitral valve prolapse other causes of mitral regurgitation include damage to the papillary muscles from a heart attack. If these papillary muscles die, they can’t anchor the chordae tendineae which then allows the mitral valve to flop back and allow blood to go from the left ventricle to the left atrium.
Also, left-sided heart failure that leads to ventricular dilation can lead to mitral regurgitation as well, because as the left ventricle dilates, it stretches the mitral valve annulus or ring open and lets blood leak into the left atrium.
Mitral regurgitation can also be caused by rheumatic fever, an inflammatory disease that can affect the heart tissue and lead to chronic rheumatic heart disease. The chronic inflammation leads to leaflet fibrosis, which makes it so that they don’t form a nice seal and instead let blood leak through. Patients with mitral valve regurgitation typically have a holosystolic murmur, meaning it lasts for the duration of systole.
Now even though we said that left-sided heart failure can cause mitral regurgitation, it also goes the other way, mitral regurgitation can be a cause of left-sided heart failure. With mitral regurgitation, every time the left ventricle contracts, some blood is inadvertently pumped out into the left atrium, which leads to increased preload as that blood is again drained back into the left ventricle after contraction. It’s kind of like if you were digging a hole and every time you shoveled some dirt out, half of it would fall back in, lots of wasted work, right?
In this way, both the left atrium and ventricle experience volume overload. To better manage this larger volume, the left side of the heart undergoes eccentric hypertrophy, where new sarcomeres are added in series to existing ones, and so it grows larger. This compensation works for awhile, but eventually the left ventricle might not be able to keep up and it can lead to left-sided heart failure.
Alright now let’s switch gears to mitral valve stenosis which is most often again caused by rheumatic fever. In this case, instead of having inflammation causing the leaflets to not form a good enough seal, the leaflets can fuse together, called commissural fusion. When this happens, the normal mitral valve opening which is about 4 to 6 cm2 can narrow down to 2cm2. This makes it a lot harder for blood to flow from the left atrium to left ventricle and so the volume of blood in the left atrium increases, leading to higher pressures in the left atrium. Higher pressures flowing through a fibrotic valve make a “snap” sound when it opens, which is followed by a diastolic rumble as blood is forced through the smaller opening.
A constant elevation in both blood volume and pressure in the left atrium causes it to dilate and can allow blood to back up into the pulmonary circulation, causing pulmonary congestion and pulmonary edema, which can lead to symptoms like dyspnea, or difficulty breathing. All this extra blood volume in the pulmonary circulation causes higher pressures in the pulmonary circulation, or pulmonary hypertension, which can ultimately make it harder for the right ventricle to pump blood to the lungs, and over time the right ventricle can hypertrophy, and ultimately fail, which is called right-sided heart failure.
In addition, when the left atrium dilates the muscle walls stretch and the pacemaker cells that run through the walls become more irritable, increasing the risk of atrial fibrillation. During atrial fibrillation, the right and left atrium don’t contract properly anymore, which allows more blood to stagnate. Since the left atrium is already pretty dilated, static pools of blood can become a major risk for thrombus formation, and if a blood clot forms it can immediately get into the systemic circulation.
Finally, thinking about the heart in relation to other structures in the chest. If the atrium dilates and gets really large it can compress its neighbor, the esophagus, and patients might have difficulty swallowing solid foods, called dysphagia. For patients with severe problems related to mitral regurgitation or stenosis, treatment involves valve repair or surgical replacement of the valve.
Sources edit
https://en.wikipedia.org/wiki/Mitral_valve_stenosis