Master the board step 3

Master the Boards: USMLE Step 3
Mitral Stenosis (MS)
Rheumatic fever is the most common cause of MS. Look for an immigrant
patient because of the low rates of rheumatic fever in the United States. Also
look for a pregnant patient because of the large increase in plasma volume
with pregnancy. Special features of MS are as follows:
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Dysphagia: Large left atrium pressing on the esophagus
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Hoarseness: Pressure on recurrent laryngeal nerve
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Atrial fibrillation leading to stroke
Basic Science Correlate
Mechanism of Increased MS Symptoms in Pregnant Women
Pregnant women have a 50 percent increase in plasma volume. More volume
with the same valve diameter means more pressure, backflow, and symptoms.
Pregnancy also changes the hypothalamic osmolar receptors. ADH levels stay
higher in pregnancy, so the collecting duct absorbs more free water.
Physical Exam
Choose the cardiovascular (CV) exam, chest, and extremities.
The murmur of MS is a diastolic rumble after an opening snap, which can be
described as an “extra sound” in diastole. The S1 is louder. As the mitral stenosis
worsens, the opening snap moves closer to S2. The murmur will increase
in intensity with leg raising, squatting, and expiration.
Basic Science Correlate
Mechanism of Opening Snap Earlier in Worsening MS
The mitral valve opens when LA pressure > LV pressure. Worse MS = Higher LA
pressure. Higher LA pressure pushes the mitral valve open earlier.
Diagnostic Testing
TTE is the best initial diagnostic test. TEE is more accurate. Left heart catheterization
is the most accurate test. For CCS cases, also choose an EKG and
a chest x-ray, which will show left atrial hypertrophy. On chest x-ray, there
is straightening of the left heart border and elevation of the left mainstem
bronchus. There may also be a description of a double density in the cardiac
silhouette (from left atrial enlargement).
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