Ideal alveolar gas equation for PAO2 is PAO2 = [PB - PH2O]FIO2 - PaCO2 (1.25). Which of the following expresses PAO2 with the 1.25 factor included?

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Multiple Choice

Ideal alveolar gas equation for PAO2 is PAO2 = [PB - PH2O]FIO2 - PaCO2 (1.25). Which of the following expresses PAO2 with the 1.25 factor included?

This question tests the alveolar gas equation and how the CO2 term is adjusted by the respiratory quotient. The idea is that PAO2 comes from the oxygen added by inspired air, [PB − PH2O] × FiO2, minus the CO2 term corrected for ventilation per metabolic needs. The CO2 term is PaCO2 multiplied by 1.25 because the inverse of the typical respiratory quotient (R ≈ 0.8) is 1/R ≈ 1.25. So the correct form is PAO2 = [PB − PH2O]FiO2 − PaCO2(1.25). Options that omit the 1.25 factor fail to account for that CO2 correction, and using a different multiplier (like 0.5) would not reflect physiologic R values. The expression with the 1.25 factor mirrors the standard adjustment for CO2 in the alveolar gas equation.

This question tests the alveolar gas equation and how the CO2 term is adjusted by the respiratory quotient. The idea is that PAO2 comes from the oxygen added by inspired air, [PB − PH2O] × FiO2, minus the CO2 term corrected for ventilation per metabolic needs. The CO2 term is PaCO2 multiplied by 1.25 because the inverse of the typical respiratory quotient (R ≈ 0.8) is 1/R ≈ 1.25. So the correct form is PAO2 = [PB − PH2O]FiO2 − PaCO2(1.25).

Options that omit the 1.25 factor fail to account for that CO2 correction, and using a different multiplier (like 0.5) would not reflect physiologic R values. The expression with the 1.25 factor mirrors the standard adjustment for CO2 in the alveolar gas equation.

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