Mitral stenosis pathophysiology
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Mitral stenosis (MS) is most commonly secondary to acute rheumatic fever. Generally, the initial valvulitis is associated with valvular regurgitation but over a period of 2 or more years, the commissures fuse and the valves thicken and calcify. The chordal supporting structure also calcifies and retracts. The result is the typical “fish mouth deformity”. 70% of the time; the mitral valve is involved in isolation, and 25% of the time; the aortic valve is involved as well. The tricuspid and pulmonic valves are involved less commonly. Patients develop symptoms when the mitral vavle area is 2 to 2.5 cm2.
Almost all cases of mitral stenosis are due to disease in the heart secondary to rheumatic fever and the consequent rheumatic heart disease (a condition that may develop after strep throat or scarlet fever). Around 90% of cases of rheumatic heart disease are associated with mitral stenosis.
The valve problems develop 5 - 10 years after the rheumatic fever, a tiny nodule forms along the valve leaflets , the leaflets eventually thicken with deposition of fibrin. The cusps may become fibrosed, calcified and thickened over a span of a decade. Chronic turbulent flow through a deformed valve appears to cause these changes and as a result the valve looses it's normal morphology. The degree of leaflet thickening and calcification and the severity of chordal involvement are variable. Rheumatic fever is becoming rare in the United States, so mitral stenosis is also less common.
Hemodynamic Perturbations in Mitral Stenosis
- The severity of mitral stenosis depends on the pressure gradient between the left atrium and ventricle which depends on the cross sectional area of the mitral valve.
- The normal mitral valve orifice has a cross sectional area of about 4.0 cm2.
- Mitral stenosis is mild if the cross sectional area is about 2 cm2 and the pressure gradient is small.
- Mitral stenosis is moderate if the cross sectional area is about 1.0 to 1.5 cm2.
- Mitral stenosis is severe if the cross sectional area is ≤1.0 cm2 and the pressure gradient between the left atrium and left ventricle is significant.
Normal Mitral Valve Anatomy
- The normal mitral valve orifice area is 4-6 cm2.
- Mitral stenosis occurs when the orifice area is reduced to at least 2.2 cm2.
- This degree of narrowing results in a gradient across the mitral valve.
- The opening is surrounded by a fibrous ring known as the mitral valve annulus.
- The anterior cusp covers approximately two-thirds of the valve area (imagine a crescent moon within the circle, where the crescent represents the posterior cusp).
- These valve leaflets are prevented from prolapsing into the left atrium by the action of tendons attached to the posterior surface of the valve, the chordae tendineae.
- The inelastic chordae tendineae are attached at one end to the papillary muscles and at the other end to the valve cusps.
- Papillary muscles are fingerlike projections that extend from the wall of the left ventricle.
- Chordae tendineae from each muscle are attached to both leaflets of the mitral valve.
- Thus, when the left ventricle contracts, the intraventricular pressure forces the valve to close, while the tendons keep the leaflets coapting together and prevent the valve from opening in the wrong direction; thus preventing blood to flow back to the left atrium.
- Each chord has a different thickness. The thinnest ones are attached to the free leaflet margin, whereas thickest ones are attached quite away from the free margin. This disposition has important effects on systolic stress distribution physiology.
Mild Mitral Stenosis
When the mitral valve cross sectional area is about 2 cm2 and the pressure gradient is small.
Moderate Mitral Stenosis
When the mitral valve area goes below 2 cm2, the valve causes an impediment to the flow of blood into the left ventricle, creating a pressure gradient across the mitral valve. This gradient may be increased by increases in the heart rate or cardiac output. As the gradient across the mitral valve increases, the amount of time necessary to fill the left ventricle with blood increases. Eventually, the valve is so tight and the gradient is so high that the atrial kick is required to fill the left ventricle with blood. As the heart rate increases, the amount of time that the ventricle is in diastole and can fill up with blood (called the diastolic filling period) decreases. When the heart rate goes above a certain point, the diastolic filling period is insufficient to fill the ventricle with blood and pressure builds up in the left atrium, leading to pulmonary congestion. The patient may experience dyspnea on exertion at this point.
Severe Mitral Stenosis
When the mitral valve area goes less than 1 cm2, there will be a further increase in the left atrial pressures. Since the normal left ventricular diastolic pressures is about 5 mm Hg, a pressure gradient across the mitral valve of 20 mm Hg due to severe mitral stenosis will cause a left atrial pressure of about 25 mm Hg. This left atrial pressure is transmitted to the pulmonary vasculature and causes an elevated pulmonary capillary wedge pressure. Pulmonary capillary pressures in this level cause an imbalance between the hydrostatic pressure and the oncotic pressure, leading to extravasation of fluid from the vascular tree and pooling of fluid in the lungs (congestive heart failure causing pulmonary edema). Increases in the heart rate will allow less time for the left ventricle to fill, also causing an increase in left atrial pressure and further pulmonary congestion causing Hemoptysis may develop. The constant pressure overload of the left atrium will cause the left atrium to increase in size. As the left atrium increases in size, it becomes more prone to develop atrial fibrillation. When atrial fibrillation develops, the atrial kick is lost.
In individuals with severe mitral stenosis, the left ventricular filling is dependent on the atrial kick. The loss of the atrial kick due to atrial fibrillation can cause a precipitous decrease in cardiac output and sudden congestive heart failure. Mitral stenosis may cause left ventricular dysfunction if it is associated with mitral regurgitation.
Right Heart Failure
The elevated pressures in the left atrium are transmitted into the pulmonary circuit, and pulmonary hypertension may develop. Due to hypoxemia, there may be pulmonary vasoconstriction as well that further elevates right heart pressures. The elevated pulmonary capillary wedge pressure leads to a rise in interstitial edema which also increases the load on the right ventricle. Finally, intimal hyperplasia and medial hypertrophy develop in the pulmonary vascular bed.
All the aforementioned changes lead to a rise in the pulmonary arterial pressure and the right ventricle begins to dilate and fail. As a result of the dilation of the right ventricle, tricuspid regurgitation develops. The jugular venous pressure may be elevated. Other signs of right heart failure such as hepatic congestion and pedal edema may also eventually develop.
A chronic smouldering rheumatic myocarditis may further reduce left ventricular function. Patients with mitral stenosis also often have aortic stenosis. Some patients will also have mixed mitral regurgitation/stenosis.
Mitral Stenosis in Pregnancy
In pregnancy, the pressure gradient between the left atrium and ventricle is usually increased due to the increase in the heart rate and cardiac output during pregnancy. This can lead to the diagnosis of previously asymptomatic case of mitral stenosis, or worsening of the symptoms of previously diagnosed case.
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