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Difficulty: MediumNormal Pregnancy and Prenatal Care

Routine evaluation of a patient at 1111 weeks gestation is notable for her reporting a sensation of "not getting enough air." She has no history of asthma or cardiac disease, and her pregnancy has been uncomplicated. Current vital signs include a blood pressure of 112/70112/70 mm Hg, a pulse of 8888/min, and a respiratory rate of 1818/min. Pulse oximetry shows 99%99\% on room air. Auscultation of the chest reveals clear breath sounds bilaterally with no wheezes or crackles. Which of the following arterial blood gas profiles is most consistent with the expected physiologic adaptations in this patient?

  1. pH: 7.447.44, PaCO 2_2: 3030 mm Hg, HCO 3−_3^-: 2020 mEq/LAnswer
  2. B
    pH: 7.407.40, PaCO 2_2: 4040 mm Hg, HCO 3−_3^-: 2424 mEq/L
  3. C
    pH: 7.327.32, PaCO 2_2: 5050 mm Hg, HCO 3−_3^-: 2626 mEq/L
  4. D
    pH: 7.487.48, PaCO 2_2: 4242 mm Hg, HCO 3−_3^-: 3030 mEq/L
  5. E
    pH: 7.357.35, PaCO 2_2: 3030 mm Hg, HCO 3−_3^-: 1616 mEq/L

Answer

pH: 7.44, PaCO2: 30 mm Hg, HCO3-: 20 mEq/L
During pregnancy, increased levels of progesterone directly stimulate the medullary respiratory center. This leads to an increase in tidal volume and minute ventilation, which frequently presents as subjective dyspnea (a feeling of not getting enough air) despite normal physical exam findings and oxygenation. The hyperventilation increases CO2 elimination, resulting in a primary respiratory alkalosis characterized by a decreased PaCO2 (typically 28-32 mm Hg) and a slightly elevated pH (around 7.44). To maintain homeostasis, the kidneys compensate by increasing bicarbonate excretion, causing a decrease in serum HCO3- (typically 20-22 mEq/L). The profile showing a pH of 7.44, low PaCO2, and low HCO3- accurately reflects this expected physiologic state.

Step-by-Step Solution

1
Analyze the patient's clinical presentation.
The patient is at 11 weeks gestation with subjective dyspnea, normal vital signs, and clear lungs, which is consistent with physiologic dyspnea of pregnancy.
To differentiate normal physiologic changes from pathological respiratory conditions like asthma or pulmonary embolism.
2
Identify the hormonal driver of respiratory changes in pregnancy.
Elevated progesterone levels directly stimulate the central respiratory center in the medulla.
To understand the physiological mechanism underlying the patient's altered breathing pattern.
3
Determine the expected effect on pulmonary physiology and arterial blood gases.
Progesterone increases tidal volume and minute ventilation, leading to increased CO2 expiration (primary respiratory alkalosis). The kidneys compensate by excreting bicarbonate.
To predict the exact laboratory values reflecting this compensated state.

Key Concept

During normal pregnancy, elevated progesterone levels stimulate the respiratory center, increasing minute ventilation and causing a chronic, compensated respiratory alkalosis.
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Topics

Normal Pregnancy and Prenatal Care

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