Alveolar Gas Equation

All calculations should be independently verified prior to clinical use. These calculators are intended to supplement, not replace, clinical judgment.

Calculate the partial pressure of oxygen in the alveoli (P_AO₂) given F_iO₂, P_aCO₂, atmospheric pressure, and respiratory quotient. Use the helper below to determine atmospheric pressure from elevation or city.

F_iO₂ %

P_aCO₂ torr

Atmospheric pressure torr

Respiratory quotient (RQ)

P_AO₂ torr

Determine Atmospheric Pressure

Elevation ft m

— or select a city

Atmospheric pressure = torr

The alveolar gas equation calculates the partial pressure of oxygen in the alveoli:

P_AO₂ = ( F_iO₂ × (P_atmos − P_H₂O) ) − ( P_aCO₂ / RQ )

F_iO₂ is the fraction of inspired oxygen, entered here as a percentage (21% for room air, 100% for pure O₂). P_atmos is the ambient atmospheric pressure — 760 torr at sea level. P_H₂O is water vapor pressure at 37°C, equal to 47 torr. The respiratory quotient (RQ) is the ratio of CO₂ eliminated to O₂ consumed; it is typically 0.8 but can range from 0.7 to 1.0.

Atmospheric pressure decreases with elevation and is estimated here using:

P_atmos = 760 × e^{−elevation (ft) / 27200}

The alveolar gas equation is most commonly used to calculate the alveolar-arterial gradient. See the A-a Gradient calculator.

References

West JB. Respiratory Physiology: The Essentials. 9th ed. Lippincott Williams & Wilkins; 2012.
Weinberger SE, Cockrill BA, Mandel J. Principles of Pulmonary Medicine. 7th ed. Elsevier; 2019.
Levitzky MG. Pulmonary Physiology. 9th ed. McGraw-Hill Education; 2018.