At typical cabin altitude of 8,000 feet, what is the approximate arterial oxygen tension in a healthy person breathing room air?

• A. About 90–100 mm Hg.
• B. About 40–50 mm Hg.
• C. About 70–75 mm Hg.
• D. About 80–85 mm Hg.

Answer: (C)

At 8,000 feet, the air contains less oxygen, so the amount of oxygen that reaches the blood drops. The alveolar gas equation links this inspired oxygen to the oxygen in the arteries: PAO2 ≈ PIO2 − (PaCO2 / R). Here, PIO2 is lower because barometric pressure is lower (about 565 mmHg at 8,000 ft), giving PIO2 ≈ (565 − 47) × 0.21 ≈ 109 mmHg. In a healthy person breathing room air, PaCO2 falls with altitude to around 30–35 mmHg due to hyperventilation, and with R ≈ 0.8, (PaCO2 / R) is about 38–44 mmHg. Thus PAO2 ≈ 109 − 38 to 44 ≈ 65–71 mmHg. The arterial PaO2 is slightly less than PAO2 because of a small A–a gradient, placing the typical value around 60–75 mmHg, i.e., roughly 70–75 mmHg. This is why the best estimate at 8,000 feet on room air is about 70–75 mmHg.