A patient with diffuse alveolar damage has impaired CO2 elimination despite a normal PaCO2 on arterial blood gas. This apparent paradox is explained by:
- A Increased functional residual capacity dilutes CO2 in alveoli, preventing equilibration failure
- B CO2 diffuses 20x faster than O2, so CO2 elimination is maintained even with significant membrane damage; hypoxemia (from impaired O2 transfer) stimulates hyperventilation that keeps PaCO2 normal ✓
- C Compensatory metabolic alkalosis normalizes PaCO2 through HCO3- retention
- D Preferential blood flow to unaffected alveoli prevents CO2 accumulation
Explanation
CO2 has a diffusing capacity approximately 20-25x greater than O2 (due to its much higher solubility in aqueous media, as per Fick's law where diffusion capacity ∝ solubility/MW^0.5). Therefore, while O2 transfer across damaged alveolar membranes fails (causing hypoxemia), CO2 transfer remains adequate or is compensated by increased ventilation. In diffuse alveolar damage, hypoxemia is the dominant early abnormality; hypercapnia typically occurs only in severe disease when ventilatory reserve is exhausted or when hypoventilation/respiratory muscle failure supervenes. The hypoxemia-driven hyperventilation (from carotid body stimulation) also helps maintain or even lower PaCO2.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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