Alveolar gas equation: FiO2 = 0.60, PaCO2 = 50, Pb = 760, PH2O = 47, R = 0.8. What is PAO2?

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Alveolar gas equation: FiO2 = 0.60, PaCO2 = 50, Pb = 760, PH2O = 47, R = 0.8. What is PAO2?

The alveolar gas equation estimates PAO2 from inspired oxygen, barometric pressure, water vapor, and the CO2 correction: PAO2 = FiO2 × (Pb − PH2O) − ( PaCO2 / R ). With FiO2 0.60, Pb 760, PH2O 47, PaCO2 50, and R 0.8, first find the inspired-dry portion of the atmosphere: Pb − PH2O = 760 − 47 = 713 mmHg. Then adjust for oxygen: 0.60 × 713 ≈ 428 mmHg. Next account for CO2 with the respiratory quotient: 50 / 0.8 = 62.5 mmHg. Subtracting gives PAO2 ≈ 428 − 62.5 ≈ 365 mmHg. So the correct value is about 365 mmHg, which matches the first option.

The alveolar gas equation estimates PAO2 from inspired oxygen, barometric pressure, water vapor, and the CO2 correction: PAO2 = FiO2 × (Pb − PH2O) − ( PaCO2 / R ). With FiO2 0.60, Pb 760, PH2O 47, PaCO2 50, and R 0.8, first find the inspired-dry portion of the atmosphere: Pb − PH2O = 760 − 47 = 713 mmHg. Then adjust for oxygen: 0.60 × 713 ≈ 428 mmHg. Next account for CO2 with the respiratory quotient: 50 / 0.8 = 62.5 mmHg. Subtracting gives PAO2 ≈ 428 − 62.5 ≈ 365 mmHg. So the correct value is about 365 mmHg, which matches the first option.

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