A 28-week pregnant woman has ABG: pH 7.44, PaCO2 30 mmHg, HCO3 20 mEq/L. How should this be interpreted in the context of normal pregnancy physiology?
- A Primary respiratory alkalosis with inadequate metabolic compensation — needs investigation
- B Normal arterial blood gas values for a 28-week pregnant woman ✓
- C Primary metabolic alkalosis with appropriate respiratory compensation
- D Mixed respiratory alkalosis and metabolic alkalosis
Correct answer: B. Normal arterial blood gas values for a 28-week pregnant woman
Explanation
Pregnancy is a state of chronic compensated respiratory alkalosis. Progesterone stimulates the respiratory center, increasing tidal volume and minute ventilation from early pregnancy, reducing PaCO2 to approximately 28-32 mmHg. The kidneys compensate by reducing HCO3 reabsorption (excreting HCO3), lowering serum HCO3 to ~20 mEq/L. The result is a mildly alkalotic pH (7.40-7.45). This ABG is entirely normal for this gestational age and reflects the well-known physiological hyperventilation of pregnancy.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.