Correct!
2. Hypoventilation
All are causes of hypoxemia although alveolar-capillary block as a cause of hypoxemia at rest is controversial. The Alveolar-arterial (A-a) gradient in this patient is normal (1). This is calculated by first determining the PAO2 (partial pressure of O2 in the alveoli) obtained from the Alveolar Gas equation. The atmospheric pressure in Phoenix is about 747 mm Hg and the pressure of water vapor is 47 mm Hg at body temperature. Subtracting 47 from 747 conveniently gives 700 and multiplying 700 by 0.21, the atmospheric percentage of oxygen, gives 147 mm Hg. From this subtract 1.25 X PaCO2 and the PaO2 which gives an A-a gradient of 4 (147-(103 + 40) = 4). This is normal based on normal being patient age/4 +4. Hypoventilation is the only cause of cause of hypoxemia which results in a normal A-a gradient. The A-a gradient rises with age and oxygen administration.
Given the clinical situation what is the appropriate treatment? (Click on the correct answer to proceed to the fourth of five pages)