A pregnant woman with rheumatic heart disease and mitral stenosis (mitral valve area 1.0 cm²) presents at 28 weeks with NYHA Class III symptoms and pulmonary oedema. Which of the following best describes the reason why mitral stenosis is particularly poorly tolerated in pregnancy?
- A The physiological tachycardia and increased cardiac output of pregnancy reduce diastolic filling time, increasing left atrial and pulmonary venous pressures across the fixed mitral stenosis ✓
- B Pregnancy causes aortic root dilatation that worsens mitral leaflet coaptation
- C Increased progesterone in pregnancy causes mitral annular dilatation worsening stenosis
- D Pregnancy-associated thrombocytopenia increases the risk of left atrial thrombus formation
Explanation
Mitral stenosis is fixed obstruction at the mitral valve; blood must cross the narrowed orifice during diastole to fill the left ventricle. In pregnancy, physiological tachycardia (heart rate increases by 10–20 bpm) shortens diastolic filling time per beat, while the 40–50% increase in cardiac output demands greater stroke volume. Both effects increase left atrial pressure and pulmonary venous hypertension, precipitating pulmonary oedema — even moderate MS can become severely symptomatic by the third trimester. This is the mechanism, not valve anatomy change. The Gorlin formula: MVA = CO / (37.7 × HR × √PCWP−LVEDP).
Reference: Williams Obstetrics, 26th ed.
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Written and medically reviewed by the StethoPrep medical team.