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In normal cardiac physiology, the mitral valve opens during left ventricular diastole, to allow blood to flow from the left atrium to the left ventricle. The reason the blood flows in the proper direction is that, during this phase of the cardiac cycle, the pressure in the left ventricle is less than the pressure in the left atrium, and the blood flows down the pressure gradient. In the case of mitral stenosis, the valve does not open completely, so the left atrium has to have a higher pressure than normal to have the blood overcome the increased gradient caused by the mitral valve stenosis.
Most cases of mitral stenosis are due to disease in the heart secondary to rheumatic fever and the consequent rheumatic heart disease . Less common causes of mitral stenosis are calcification of the mitral valve leaflets, and as a form of congenital heart disease.
The normal area of the mitral valve orifice is about 4 to 6 cm2. Under normal conditions, a normal mitral valve will not impede the flow of blood from the left atrium to the left ventricle during (ventricular) diastole, and the pressures in the left atrium and the left ventricle during diastole will be equal. The result is that the left ventricle gets filled with blood during early diastole, with only a small portion of extra blood contributed by contraction of the left atrium (the "atrial kick") during late ventricular diastole.
| Pressure tracings in the left atrium (LA) and the left ventricle (LV) in an individual with severe mitral stenosis. Blue areas represent the diastolic pressure gradient due to the stenotic valve. |
When the mitral valve area goes below 2 cm2, the valve causes an impedement to 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 left ventricle requires the atrial kick to fill 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.
Since the normal left ventricular diastolic pressures is about 5 mmHg, a pressure gradient across the mitral valve of 20 mmHg due to severe mitral stenosis will cause a left atrial pressure of about 25 mmHg. This left atrial pressure is transmitted to the pulmonary vasculature. Pulmonary capillary pressures in this level cause an imballance between the hydrostatic pressure and the oncotic pressure, leading to extravasation of fluid from the vascular tree and pooling of fluid in the lungs (pulmonary congestion).
Increases in the heart rate will allow less time for the left ventricle to fill, also causing an increase in left atrial pressure and pulmonary congestion.
When the mitral valve area goes less than 1 cm2, there will be an increase in the left atrial pressures (required to push blood through the stenotic valve). This increase in left atrial pressures will impede venous return to the left atrium from the lungs, causing pulmonary congestion ( congestive heart failure).
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 fibrillationAtrial fibrillation AF is an electrical rhythm disturbance of the heart affecting the atria. Abnormal electrical impulses in the atria cause the muscle to contract erratically and pump blood inefficiently. The atrial chambers are thus not able to complete. When atrial fibrillation develops, the atrial kick is lost (since it is due to the normal atrial contraction).
In individuals with severe mitral stenosis, the left ventricular filling is dependant 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.
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