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Difficulty: EasyNormal Pregnancy and Prenatal Care

A primigravid patient is evaluated at 22 weeks gestation during a scheduled check-up. She mentions occasionally feeling a 'need to breathe deeper' but denies any cough, wheezing, or chest pain. Her physical examination is unremarkable. Physiologic evaluation reveals a normal respiratory rate, increased tidal volume, and a decreased arterial partial pressure of carbon dioxide (PaCO2PaCO_2) compared to expected pre-pregnancy baseline values. Which of the following best explains these normal respiratory changes in this patient?

  1. Progesterone-mediated stimulation of the central respiratory centerAnswer
  2. B
    Diaphragmatic elevation leading to decreased functional residual capacity
  3. C
    Estrogen-induced relaxation of bronchial smooth muscle
  4. D
    Increased maternal oxygen consumption causing primary tachypnea
  5. E
    Dilutional anemia resulting in compensatory hyperventilation

Answer

Progesterone-mediated stimulation of the central respiratory center
During normal pregnancy, progesterone levels increase substantially. Progesterone acts as a direct stimulant on the medullary respiratory center, lowering the threshold and increasing the sensitivity to carbon dioxide. To maintain a new, lower set-point for PaCO2PaCO_2, the body increases minute ventilation. This increase is accomplished primarily by increasing tidal volume rather than respiratory rate. The result is a physiologic, mild respiratory alkalosis with a compensatory decrease in serum bicarbonate.

Step-by-Step Solution

1
Identify the patient's respiratory presentation.
The patient has a normal respiratory rate, increased tidal volume, and a decreased PaCO2PaCO_2, representing a mild respiratory alkalosis.
Recognizing this pattern is the first step in differentiating normal physiologic adaptations of pregnancy from pathological dyspnea.
2
Determine the primary physiological driver for these changes during pregnancy.
Progesterone levels rise significantly during pregnancy and directly stimulate the medullary respiratory center.
This central stimulation increases the sensitivity to carbon dioxide, forcing an increase in minute ventilation (via larger tidal volumes) to clear CO2CO_2.

Key Concept

Normal physiologic respiratory changes in pregnancy include a progesterone-driven increase in central respiratory sensitivity to carbon dioxide, which leads to increased tidal volume, increased minute ventilation, and a mild, compensated respiratory alkalosis.
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Topics

Normal Pregnancy and Prenatal Care

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