Alveolar gas equation example: PAO2 = [PB - PH2O]FIO2 - PaCO2 (1.25). With PB = 760 mmHg, PH2O = 47 mmHg, FIO2 = 0.21, PaCO2 = 40 mmHg, what is PAO2 approximately?

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Multiple Choice

Alveolar gas equation example: PAO2 = [PB - PH2O]FIO2 - PaCO2 (1.25). With PB = 760 mmHg, PH2O = 47 mmHg, FIO2 = 0.21, PaCO2 = 40 mmHg, what is PAO2 approximately?

The main idea is using the alveolar gas equation to estimate PAO2 by accounting for inspired oxygen, water vapor, and the CO2 term scaled by the respiratory quotient. Compute step by step: - Subtract water vapor from barometric pressure: 760 − 47 = 713. - Multiply by the inspired oxygen fraction: 713 × 0.21 ≈ 150. - Adjust for CO2 with the usual R ≈ 0.8, so PaCO2/R = 40/0.8 = 50 (equivalently PaCO2 × 1.25 = 50). - Subtract the CO2 term: 150 − 50 ≈ 100. Thus PAO2 is about 100 mmHg. The closest option is 100 mmHg.

The main idea is using the alveolar gas equation to estimate PAO2 by accounting for inspired oxygen, water vapor, and the CO2 term scaled by the respiratory quotient.

Compute step by step:

  • Subtract water vapor from barometric pressure: 760 − 47 = 713.

  • Multiply by the inspired oxygen fraction: 713 × 0.21 ≈ 150.

  • Adjust for CO2 with the usual R ≈ 0.8, so PaCO2/R = 40/0.8 = 50 (equivalently PaCO2 × 1.25 = 50).

  • Subtract the CO2 term: 150 − 50 ≈ 100.

Thus PAO2 is about 100 mmHg. The closest option is 100 mmHg.

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