A patient breathing room air at sea level has the following arterial blood gases: PaO2 = 62 mm Hg; PCO2 = 75 mm Hg. When the FIO2 is raised to 0.28, the PaO2 rises to 95 mm Hg. What is the most likely cause of the hypoxemia?
a. Hypoventilation
b. Impaired diffusion
c. Right-to-left shunt
d. V/Q imbalance
ANS: A
In the case of simple hypoventilation, a rise in the alveolar PCO2 is always accompanied by a proportionate fall in alveolar PO2. The P(A–a)O2 is normal in such cases. The hypoxemia will re-spond readily to O2 therapy.
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