What is the PAO2 of a patient on room air in Denver with Patm 630 mm Hg, FiO2 0.21, PH2O 47, PaCO2 34 mm Hg, and respiratory quotient 0.8?

What is the PAO2 of a patient on room air in Denver with Patm 630 mm Hg, FiO2 0.21, PH2O 47, PaCO2 34 mm Hg, and respiratory quotient 0.8?

• A. 40 mm Hg
• B. 50 mm Hg
• C. 60 mm Hg
• D. 80 mm Hg

Answer: (D)

Alveolar oxygen tension on room air is estimated with the alveolar gas equation: PAO2 = FiO2 × (Pb − PH2O) − PaCO2/R. Here, FiO2 is 0.21, Pb is 630 mmHg, PH2O is 47 mmHg, PaCO2 is 34 mmHg, and the respiratory quotient R is 0.8. Calculate: (Pb − PH2O) = 630 − 47 = 583; FiO2 × (Pb − PH2O) = 0.21 × 583 ≈ 122.4; PaCO2/R = 34/0.8 = 42.5. Subtract: 122.4 − 42.5 ≈ 80 mmHg. So the PAO2 is about 80 mmHg, which aligns with the given answer. This reflects how higher altitude lowers the available alveolar oxygen due to reduced barometric pressure, even on room air.

Alveolar oxygen tension on room air is estimated with the alveolar gas equation: PAO2 = FiO2 × (Pb − PH2O) − PaCO2/R. Here, FiO2 is 0.21, Pb is 630 mmHg, PH2O is 47 mmHg, PaCO2 is 34 mmHg, and the respiratory quotient R is 0.8. Calculate: (Pb − PH2O) = 630 − 47 = 583; FiO2 × (Pb − PH2O) = 0.21 × 583 ≈ 122.4; PaCO2/R = 34/0.8 = 42.5. Subtract: 122.4 − 42.5 ≈ 80 mmHg. So the PAO2 is about 80 mmHg, which aligns with the given answer. This reflects how higher altitude lowers the available alveolar oxygen due to reduced barometric pressure, even on room air.