A patient with severe kyphoscoliosis has a TLC of 2.8 L (predicted 5.2 L), FEV1/FVC ratio of 82%, and normal DLCO. Arterial blood gas shows PaO₂ = 62 mmHg, PaCO₂ = 48 mmHg. The primary mechanism of hypoxemia in this patient is:
- A Diffusion limitation across the alveolar-capillary membrane
- B True right-to-left anatomical shunt through collapsed lung segments
- C Hypoventilation with a normal A-a gradient, reflecting pure ventilatory pump failure ✓
- D Hypoventilation reducing alveolar PO₂ with an elevated A-a gradient
Correct answer: C. Hypoventilation with a normal A-a gradient, reflecting pure ventilatory pump failure
Explanation
Severe kyphoscoliosis restricts chest wall expansion causing ventilatory pump failure (hypoventilation). The PaCO₂ elevation (48 mmHg) and reduced PaO₂ are consistent with alveolar hypoventilation. Using the alveolar gas equation: PAO₂ = FiO₂(PB-47) - PaCO₂/RQ ≈ 150 - 48/0.8 = 90 mmHg; A-a gradient = 90 - 62 = 28 mmHg — slightly widened but this is expected with age/positional atelectasis. Predominantly this is hypoventilation with near-normal A-a gradient. Diffusion limitation and shunt produce an elevated A-a gradient with normal or low PaCO₂.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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