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Difficulty: MediumNormal Pregnancy and Prenatal Care

During a routine prenatal visit, a 30-year-old woman at 34 weeks gestation reports experiencing episodes of lightheadedness, nausea, and a feeling of impending fainting over the past two weeks. She states that these episodes occur exclusively when she is lying flat on her back. The symptoms resolve rapidly when she turns onto her side. Her pregnancy has been uncomplicated. Her blood pressure is 112/68 mm Hg and her heart rate is 85/min while seated in the clinic. Fetal heart monitoring shows a baseline rate of 145/min with moderate variability and no decelerations. Which of the following is the most likely underlying mechanism for this patient's symptoms?

  1. A
    Progesterone-mediated decrease in systemic vascular resistance
  2. B
    Disproportionate expansion of plasma volume relative to red blood cell mass
  3. C
    Upward displacement of the diaphragm by the enlarging uterus
  4. Decreased venous return due to inferior vena cava compressionAnswer
  5. E
    Increased venous capacitance leading to blood pooling in the lower extremities

Answer

The symptoms are caused by decreased venous return due to inferior vena cava compression.
The correct answer accurately describes the pathophysiology of supine hypotensive syndrome. In the third trimester, the heavy, gravid uterus mechanically compresses the inferior vena cava when the patient lies in the supine position. This compression impedes venous return to the right atrium, leading to a transient decrease in cardiac output and subsequent cerebral hypoperfusion. Patients characteristically present with lightheadedness, nausea, and presyncope when lying flat. Symptoms rapidly resolve when the patient turns to a lateral decubitus position, which displaces the uterus off the inferior vena cava and restores normal venous return.

Step-by-Step Solution

1
Identify the patient's key clinical presentation and its positional nature.
The patient is in her third trimester and experiences lightheadedness and nausea exclusively when lying flat, which resolves upon turning to her side.
To establish that the symptoms are strictly positional and related to the supine posture.
2
Correlate the symptoms with the anatomic and physiologic changes of late pregnancy.
In the third trimester, the enlarged, heavy gravid uterus occupies significant space in the abdomen and can mechanically compress retroperitoneal structures depending on maternal posture.
To determine the underlying pathophysiology of the positional presyncope.
3
Determine the specific vascular structure affected and its hemodynamic consequence.
The supine position causes the gravid uterus to compress the inferior vena cava, impeding venous return to the right atrium and thereby transiently lowering cardiac output.
To accurately identify the mechanism of supine hypotensive syndrome.

Key Concept

Supine hypotensive syndrome in pregnancy is caused by mechanical compression of the inferior vena cava by the gravid uterus, leading to decreased venous return and transient cerebral hypoperfusion.
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Topics

Normal Pregnancy and Prenatal Care

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