Mitral stenosis: pathogenesis and clinical consequences
THE CONTEXT
Progressive narrowing of the mitral valve impedes left ventricular filling, raises left atrial pressure and produces pulmonary and right heart complications.
Valve narrowing
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Rheumatic mitral stenosis develops through progressive fibrosis, calcification of the valve leaflets, and fusion of the cusps and subvalvular apparatus.
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The normal mitral valve orifice is about 5 cm² in diastole and may fall below 1 cm² in severe stenosis. Patients are usually asymptomatic until the orifice is below 2 cm².
Left atrial pressure and symptom triggers
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As stenosis progresses, left ventricular filling becomes increasingly dependent on left atrial contraction. Left atrial dilatation and hypertrophy accompany rising atrial pressure, pulmonary venous congestion and breathlessness.
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An increased heart rate shortens diastole and further raises left atrial pressure. Pregnancy and exercise are poorly tolerated because their increased cardiac output requirements also raise left atrial pressure.
Atrial fibrillation and pulmonary hypertension
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Atrial fibrillation is very common because of progressive left atrial dilatation. Its onset can cause sudden haemodynamic deterioration: loss of atrial contraction and tachycardia rapidly raise left atrial pressure and may produce pulmonary oedema.
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Without atrial fibrillation, the rise in left atrial pressure may be more gradual and better tolerated.
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Pulmonary hypertension may protect against pulmonary oedema but causes right ventricular hypertrophy and dilatation, tricuspid regurgitation and right heart failure.
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Fewer than 20% of patients remain in sinus rhythm; many of these patients have a small fibrotic left atrium and severe pulmonary hypertension.
Clinical associations
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Breathlessness reflects pulmonary congestion and low cardiac output; fatigue and mitral facies are associated with low cardiac output.
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Pulmonary congestion causes haemoptysis and cough. Pulmonary hypertension can cause chest pain.
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Oedema and ascites reflect right heart failure. Palpitations are associated with atrial fibrillation, while thromboembolism is associated with atrial stasis and atrial fibrillation.
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Auscultatory findings include a loud first heart sound, an opening snap and a mid-diastolic murmur. A right ventricular heave and loud pulmonary component of the second heart sound are associated with pulmonary hypertension.