PAO2 at sea level with FiO2 = 0.21, PaCO2 = 40, R = 0.8. PAO2 is approximately:

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Multiple Choice

PAO2 at sea level with FiO2 = 0.21, PaCO2 = 40, R = 0.8. PAO2 is approximately:

Alveolar oxygen tension is estimated with the alveolar gas equation. PAO2 ≈ PIO2 − (PaCO2 / R), where PIO2 = FiO2 × (Pb − PH2O). At sea level, Pb ≈ 760 mmHg and PH2O ≈ 47 mmHg, with FiO2 = 0.21. So PIO2 = 0.21 × (760 − 47) ≈ 0.21 × 713 ≈ 149.7 mmHg. Then PaCO2 / R = 40 / 0.8 = 50 mmHg. Therefore PAO2 ≈ 149.7 − 50 ≈ 99.7 mmHg. So PAO2 is about 99.7 mmHg, which matches the provided value.

Alveolar oxygen tension is estimated with the alveolar gas equation. PAO2 ≈ PIO2 − (PaCO2 / R), where PIO2 = FiO2 × (Pb − PH2O).

At sea level, Pb ≈ 760 mmHg and PH2O ≈ 47 mmHg, with FiO2 = 0.21. So PIO2 = 0.21 × (760 − 47) ≈ 0.21 × 713 ≈ 149.7 mmHg. Then PaCO2 / R = 40 / 0.8 = 50 mmHg. Therefore PAO2 ≈ 149.7 − 50 ≈ 99.7 mmHg.

So PAO2 is about 99.7 mmHg, which matches the provided value.

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