In rheumatic heart disease, an autoimmune attack on the mitral valve produces thickening of the valve leaflets. The mitral valve is often described as having a "fish-mouth" appearance. Whether the worsening of mitral stenosis over time is secondary to a smoldering rheumatic process or simply mitral valve damage due to high pressures and turbulence is unknown. Both most likely contribute to the progressive nature of mitral stenosis.
Over time as the mitral stenosis worsens and the mitral valve area decreases, a pressure gradient develops between the left atrium and the left ventricle to help assist diastole. The elevated left atrial pressures are transmitted to the pulmonary veins and the right side of the heart. On exertion as cardiac output increases and the velocity of flow through the mitral valve increases, the transmitted pressure gradient becomes exponentially larger. This is explained by the modified Bernoulli equation:
Pressure gradient = 4v2
Thus if the velocity (v) of flow is doubled, the transmitral pressure gradient increases by 4 times. The resultant large increase in LA pressure is primarily responsible for the exertional symptoms seen in mitral stenosis.
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