the mitral valve has two leaflets—the anteriorand posterior leaflet, and together they separate the left atrium from the left ventricle. duringsystole, the valve closes, which means blood has just one option—to be ejected out theaortic valve and into circulation. if the mitral valve doesn’t shut all the way, bloodcan leak back into the left atrium, called mitral valve regurgitation. during diastole,the mitral valve opens and lets blood fill in to the ventricle. if the mitral valve doesn’topen 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 and the most common of all valvular conditionsis mitral valve prolapse. when the left ventricle
contracts during systole, a ton of pressureis generated so the blood can be pumped out the aortic valve, so a lot of pressure pusheson that closed mitral valve, but normally the papillary muscles and connective tissuecalled chordae tendineae, or heart strings, keep the valve from prolapsing or fallingback into the atrium. with mitral valve prolapse the connective tissue of the leaflets andsurrounding tissue are weakened, called myxomatous degeneration. why this happens isn’t wellunderstood, but it is sometimes associated with connective tissue disorders like marfansyndrome and ehlers-danlos syndrome. myxomatous degeneration results in a larger valve leafletarea and elongation of the chordae tendineae which can sometimes rupture, something thattypically happens to the ones on the posterior
leaflet. that can allow the posterior leafletto 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 sometimesfollowed by a systolic murmur. the click is a result of the leaflet folding into the atriumand being suddenly stopped by the chordae tendineae. although mitral valve prolapsedoesn’t always cause mitral regurgitation, it oftentimes does. if the leaflets don’tmake a perfect seal, a little bit of blood leaks backward from the left ventricle intothe left atrium and causes a murmur. the mitral valve prolapse murmur is somewhatunique 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 lastslonger. the reason this happens is that squatting increases venous return, which fills the leftventricle with slightly more blood, and this means the left ventricle gets just a littlebit larger. the larger leaflets therefore have more space to hang out, and as the ventriclecontracts and gets smaller, it takes just a little longer for the leaflet to get forcedinto the atrium. standing on the other hand reduces venous return, meaning a little lessblood in the ventricle and a little less room to hang out, and so the leaflet gets forcedout earlier during contraction. the other murmur that follows this pattern is like theone in hypertrophic cardiomyopathy. so in addition to mitral valve prolapse othercauses 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 allowsthe mitral valve to flop back and allow blood to go from the left ventricle to the leftatrium. also, left-sided heart failure that leads to ventricular dilation can lead tomitral regurgitation as well, because as the left ventricle dilates, it stretches the mitralvalve annulus or ring open and lets blood leak into the left atrium. mitral regurgitationcan also be caused by rheumatic fever, an inflammatory disease that can affect the hearttissue and lead to chronic rheumatic heart disease. the chronic inflammation leads toleaflet fibrosis, which makes it so that they don’t form a nice seal and instead let bloodleak through. patients with mitral valve regurgitation
typically have a holosystolic murmur, meaningit lasts for the duration of systole. now even though we said that left-sided heartfailure can cause mitral regurgitation, it also goes the other way, mitral regurgitationcan be a cause of left-sided heart failure. with mitral regurgitation, every time theleft ventricle contracts, some blood is inadvertently pumped up into the left atrium, which leadsto increased preload as that blood is again drained back into the left ventricle aftercontraction. it’s kind of like if you were digging a hole and every time you shoveledsome 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 largervolume, the left side of the heart undergoes
eccentric hypertrophy, where new sarcomeresare added in series to existing ones, and so it grows larger. this compensation worksfor awhile, but eventually the left ventricle may not be able to keep up and it can leadto left-sided heart failure. alright now let’s switch gears to mitralvalve stenosis which is most often again caused by rheumatic fever. in this case, insteadof having inflammation causing the leaflets to not form a good enough seal, the leafletscan fuse together, called commissural fusion. when this happens, the normal mitral valveopening which is about 4 to 6 cm2 can narrow down to 2cm2. this makes it a lot harder forblood to flow from the left atrium to left ventricle and the volume of blood in the leftatrium increases, leading to higher pressures
in the left atrium. higher pressures flowingthrough a fibrotic valve make a “snap†sound when it opens, which is followed bya diastolic rumble as blood is forced through the smaller opening. a constant elevation in both blood volumeand pressure in the left atrium causes it to dilate and can allow blood to back up intothe pulmonary circulation, causing pulmonary congestion and pulmonary edema, which canlead to symptoms like dyspnea, or difficulty breathing. all this extra blood volume inthe pulmonary circulation causes higher pressures in the pulmonary circulation, or pulmonaryhypertension, which can ultimately make it harder for the right ventricle to pump bloodto the lungs, and over time the right ventricle
can hypertrophy, and ultimately fail, whichis called right-sided heart failure. in addition, when the left atrium dilates the muscle wallsstretch and the pacemaker cells that run through the walls become more irritable, increasingthe risk of atrial fibrillation. during atrial fibrillation, the right and left atrium don’tcontract properly anymore, which allows blood to stagnate. since the left atrium is alreadypretty 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 left atriumdilates and gets really large it can compress its neighbor, the esophagus, and patientsmight have difficulty swallowing solid foods,
called dysphagia. for patients with severe problems relatedto mitral regurgitation or stenosis, treatment involves valve repair or surgical replacementof the valve.
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