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Difficulty: MediumNormal Pregnancy and Prenatal Care

A 32-year-old woman, gravida 3, para 2, at 36 weeks gestation is evaluated in the clinic for a routine prenatal visit. Her pregnancy has been uncomplicated. While lying flat on the examination table for fundal height measurement, she suddenly complains of dizziness, nausea, and shortness of breath. She appears pale and diaphoretic. Her blood pressure is 82/50 mm Hg, and her heart rate is 115/min. Upon rolling onto her left side, her symptoms resolve within a few minutes. Her repeat blood pressure is 112/70 mm Hg with a heart rate of 82/min. Which of the following is the most likely mechanism responsible for this patient's initial presentation?

  1. Decreased venous return to the right atriumAnswer
  2. B
    Progesterone-induced reduction in systemic vascular resistance
  3. C
    Gravid uterine compression of the descending aorta
  4. D
    Upward displacement of the diaphragm reducing vital capacity
  5. E
    Excessive parasympathetic nervous system activation

Answer

Decreased venous return to the right atrium
The correct answer accurately identifies the pathophysiology of supine hypotensive syndrome. In the third trimester (e.g., 36 weeks), the gravid uterus becomes large and heavy enough to physically compress the inferior vena cava (IVC) against the maternal spine when the patient lies flat on her back. This mechanical compression acts as a bottleneck, significantly reducing venous return to the right atrium (preload). Because cardiac output is heavily dependent on preload, the blood pressure plummets, causing cerebral hypoperfusion (dizziness, nausea, pallor). The body responds appropriately with a sympathetic reflex tachycardia (heart rate of 115/min) to try and maintain cardiac output. Rolling the patient onto her left side relieves the mechanical compression on the IVC, rapidly restoring venous return and normalizing her hemodynamics.

Step-by-Step Solution

1
Analyze the clinical presentation and vital signs.
The patient develops acute hypotension, compensatory tachycardia, and presyncopal symptoms exclusively while in the supine position.
Identifying the positional nature of the symptoms is the key to differentiating this from other causes of hypotension in pregnancy.
2
Determine the anatomical and physiological changes at 36 weeks gestation in the supine position.
The heavy gravid uterus mechanically compresses the inferior vena cava (IVC) against the vertebral column.
Understanding the physical relationship between the third-trimester uterus and major abdominal vessels explains the hemodynamic shifts.
3
Connect the mechanical compression to the hemodynamic consequences.
IVC compression impedes blood flow from the lower extremities back to the heart, severely reducing preload (venous return to the right atrium).
A sudden drop in preload leads to a proportional drop in cardiac output and blood pressure, triggering the symptoms of supine hypotensive syndrome.

Key Concept

Supine hypotensive syndrome occurs in late pregnancy when the gravid uterus mechanically compresses the inferior vena cava in the supine position, severely reducing venous return to the right atrium (preload) and consequently lowering cardiac output.
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Topics

Normal Pregnancy and Prenatal Care

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