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Difficulty: MediumNormal Pregnancy and Prenatal Care

A healthy pregnant patient at 2828 weeks of gestation comes to the obstetric clinic for a scheduled evaluation. She is 2424 years old and this is her first pregnancy. The patient mentions occasionally feeling as though she cannot take in a full breath, especially when resting. She confirms she has no chest pain, cough, or difficulty speaking in full sentences, and she continues to walk her dog daily without limitation. Vital signs reveal a heart rate of 8585/min, respiratory rate of 1616/min, blood pressure of 112/68112/68 mmHg, and an oxygen saturation of 99%99\% on room air. Pulmonary auscultation reveals clear breath sounds bilaterally. A comprehensive metabolic panel and arterial blood gas analysis are drawn as part of a voluntary physiologic study she enrolled in. Which of the following profiles most accurately reflects the expected acid-base status in this patient?

  1. A
    pH 7.407.40; PaCO 2_2 4040 mmHg; HCO 3−_3^- 2424 mEq/L
  2. B
    pH 7.327.32; PaCO 2_2 5050 mmHg; HCO 3−_3^- 2525 mEq/L
  3. pH 7.447.44; PaCO 2_2 2828 mmHg; HCO 3−_3^- 1919 mEq/LAnswer
  4. D
    pH 7.487.48; PaCO 2_2 4040 mmHg; HCO 3−_3^- 2929 mEq/L
  5. E
    pH 7.357.35; PaCO 2_2 2828 mmHg; HCO 3−_3^- 1515 mEq/L

Answer

pH 7.447.44; PaCO 2_2 2828 mmHg; HCO 3−_3^- 1919 mEq/L
During a normal pregnancy, elevated levels of circulating progesterone directly stimulate the central respiratory center in the medulla. This stimulation leads to a substantial increase in tidal volume and overall minute ventilation, even though the respiratory rate typically remains within normal limits. Consequently, pregnant patients often experience a subjective sensation of breathlessness, known as physiologic dyspnea of pregnancy.

Because minute ventilation is increased, pregnant women normally 'blow off' more carbon dioxide, resulting in a primary respiratory alkalosis characterized by a decreased PaCO 2_2 (typically between 2727 and 3232 mmHg). In response, the kidneys initiate a compensatory metabolic acidosis by increasing the excretion of bicarbonate, leading to a decreased serum HCO 3−_3^- (typically between 1818 and 2222 mEq/L). The resulting arterial pH is slightly basic, usually ranging from 7.407.40 to 7.457.45. The profile demonstrating a pH of 7.447.44, a PaCO 2_2 of 2828 mmHg, and an HCO 3−_3^- of 1919 mEq/L perfectly matches this expected chronic compensated respiratory alkalosis.

Step-by-Step Solution

1
Analyze the patient's clinical presentation.
The patient reports a sensation of incomplete breaths with normal vital signs, a normal respiratory rate, and clear lungs.
This presentation is consistent with the physiologic dyspnea of pregnancy, a normal finding.
2
Identify the primary mechanism of respiratory changes in pregnancy.
Elevated progesterone levels directly stimulate the medullary respiratory center, causing an increase in tidal volume and minute ventilation.
This physiological hyperventilation leads to an expected decrease in arterial PaCO2, creating a primary respiratory alkalosis.
3
Determine the body's compensatory response.
To maintain near-normal blood pH, the kidneys increase the excretion of bicarbonate (HCO3-).
This metabolic compensation lowers serum bicarbonate levels to balance the low PaCO2.
4
Select the laboratory profile that matches this physiology.
The expected profile must demonstrate a slightly alkalemic pH, decreased PaCO2, and decreased HCO3-.
Only the profile with pH 7.44, PaCO2 28 mmHg, and HCO3- 19 mEq/L fits chronic compensated respiratory alkalosis.

Key Concept

Normal pregnancy is characterized by a progesterone-mediated increase in minute ventilation, which results in a chronic, compensated respiratory alkalosis (decreased PaCO2, decreased HCO3-, and slightly elevated pH).
Estimated Time:1m 15s
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Topics

Normal Pregnancy and Prenatal Care

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