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Difficulty: MediumCardiovascular System

A 29-year-old woman, gravida 1 para 0, at 28 weeks gestation is brought to the emergency department due to sudden-onset, severe shortness of breath and a cough productive of pink-tinged sputum. She immigrated to the United States two years ago and has not received routine prenatal care. Vital signs show a blood pressure of 110/70 mmHg, a pulse of 118/min, and a respiratory rate of 28/min. Oxygen saturation is 88% on room air. Physical examination reveals a gravid uterus consistent with dates. Auscultation of the lungs demonstrates bilateral basilar crackles. Cardiac examination reveals a loud first heart sound (S1) and a low-pitched, rumbling mid-diastolic murmur heard best at the cardiac apex. Which of the following underlying mechanisms best explains the acute onset of this patient's respiratory symptoms?

  1. Increased left atrial pressure secondary to shortened ventricular diastoleAnswer
  2. B
    Progesterone-mediated stimulation of central respiratory centers
  3. C
    Upward displacement of the diaphragm reducing total lung capacity
  4. D
    Bronchial smooth muscle constriction and mucosal inflammation
  5. E
    Systemic vasodilation leading to compensatory right ventricular overload

Answer

Increased left atrial pressure secondary to shortened ventricular diastole
This patient's physical examination findings of a loud S1 and a mid-diastolic rumbling murmur at the apex are classic for mitral stenosis (MS), most commonly a sequela of prior rheumatic fever. Patients with mild-to-moderate MS may remain asymptomatic until placed under physiologic stress. The hemodynamic changes of pregnancy—specifically the substantial expansion of plasma volume and the normal elevation in resting heart rate—frequently precipitate acute decompensation in these patients. Because flow across a stenotic mitral valve is highly dependent on the duration of diastole, the physiologic tachycardia of pregnancy shortens diastolic filling time. This prevents adequate emptying of the left atrium, resulting in a severe, acute increase in left atrial and pulmonary venous pressures. This elevated hydrostatic pressure causes transudation of fluid into the alveolar spaces (pulmonary edema), leading to her acute presentation of hypoxia, crackles, and pink-tinged sputum.

Step-by-Step Solution

1
Identify the cardiac pathology based on physical exam findings.
The loud S1 and low-pitched mid-diastolic rumbling murmur at the apex indicate mitral stenosis (MS), likely rheumatic in origin given her immigration history.
Accurate interpretation of the murmur is required to understand the underlying cardiovascular vulnerability.
2
Recognize the clinical syndrome.
Acute dyspnea, hypoxia, bilateral crackles, and pink-tinged frothy sputum strongly suggest acute pulmonary edema (decompensated heart failure).
Differentiating cardiogenic pulmonary edema from normal pregnancy dyspnea or primary lung disease is essential for identifying the mechanism.
3
Connect the cardiac pathology with the physiologic changes of pregnancy.
Pregnancy naturally increases blood volume and resting heart rate. In mitral stenosis, a faster heart rate shortens diastolic filling time, rapidly elevating left atrial pressure and causing fluid transudation into the alveoli.
The question requires identifying the specific physiologic trigger that caused the previously stable MS to decompensate into acute pulmonary edema.

Key Concept

Patients with mitral stenosis are highly susceptible to acute pulmonary edema during pregnancy because the physiologic increase in heart rate (tachycardia) shortens diastole, preventing adequate left atrial emptying and causing a severe backup of pressure into the pulmonary circulation.
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Topics

Cardiovascular System

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