In a patient with severe obesity hypoventilation syndrome (OHS), the expected arterial blood gas pattern during an awake, resting state is:
- A Low PaO₂, low PaCO₂, low HCO₃⁻, and elevated pH indicating pure respiratory alkalosis
- B Normal PaO₂ and PaCO₂ with widened A-a gradient due to shunt from pleural effusions
- C Low PaO₂, elevated PaCO₂, elevated HCO₃⁻, and normal pH due to metabolic compensation ✓
- D Elevated PaO₂, low PaCO₂, and elevated HCO₃⁻ indicating chronic hyperventilation
Correct answer: C. Low PaO₂, elevated PaCO₂, elevated HCO₃⁻, and normal pH due to metabolic compensation
Explanation
OHS (Pickwickian syndrome) is defined as chronic hypoventilation in obesity (BMI ≥30) resulting in awake hypercapnia (PaCO₂ >45 mmHg) not explained by another cause. The kidneys compensate for chronic respiratory acidosis by retaining bicarbonate over days to weeks, raising HCO₃⁻ (typically 28–35 mEq/L) and normalizing pH. The A-a gradient is typically not markedly widened at rest unless there is coexisting lung disease. OHS patients differ from OSA-only patients who typically maintain normal awake blood gases.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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