Alveolar gas equation calculation: FiO2 0.50, Pb 760, PH2O 47, PaCO2 40, RQ 0.8. What is PAO2?

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Alveolar gas equation calculation: FiO2 0.50, Pb 760, PH2O 47, PaCO2 40, RQ 0.8. What is PAO2?

The alveolar gas equation links inspired oxygen, airway humidification, and CO2 to the oxygen tension in the alveoli. Use PAO2 = FiO2 × (Pb − PH2O) − (PaCO2 / RQ). Compute step by step: - Pb − PH2O = 760 − 47 = 713 - FiO2 × (Pb − PH2O) = 0.50 × 713 = 356.5 - PaCO2 / RQ = 40 / 0.8 = 50 - PAO2 = 356.5 − 50 = 306.5 mmHg So the alveolar oxygen tension is about 306.5 mmHg. In healthy lungs, PAO2 roughly reflects PaO2, so this value aligns with expected arterial oxygen in the absence of a large A–a gradient.

The alveolar gas equation links inspired oxygen, airway humidification, and CO2 to the oxygen tension in the alveoli. Use PAO2 = FiO2 × (Pb − PH2O) − (PaCO2 / RQ).

Compute step by step:

  • Pb − PH2O = 760 − 47 = 713

  • FiO2 × (Pb − PH2O) = 0.50 × 713 = 356.5

  • PaCO2 / RQ = 40 / 0.8 = 50

  • PAO2 = 356.5 − 50 = 306.5 mmHg

So the alveolar oxygen tension is about 306.5 mmHg. In healthy lungs, PAO2 roughly reflects PaO2, so this value aligns with expected arterial oxygen in the absence of a large A–a gradient.

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