Alveolar Gas Equation

Calculate the pO₂ of alveolar gas.

Atmospheric pressure is 760 torr at sea level or in costal cities, but can be estimated from elevation. Elevations of various cities in the United States are listed.

The alveolar gas equation is usually used to calculate the partial pressure of oxygen in the alveoli:

P_AO₂ = ( F_iO₂ * (P_atmos - P_H2O)) - (P_aCO₂ / RQ)

The FiO2 is the fraction of inspired oxygen (usually as a fraction, but entered here as a percentage for ease of use). P_atmos is the ambient atmospheric pressure, which is 760 torr at sea level. P_H2O is water vapor pressure at 37°C and is equal to 47 mmHg. The respiratory quotient (RQ) is the ratio of CO₂ eliminated divided by the O₂ consumed, and its value is typically 0.8 but can range from 0.7 to 1.0.

The alveolar gas equation is most commonly used in the calculation of the Alveolar - arterial gradient:

A-a gradient = P_AO₂ - P_aO₂

See A-a Gradient calculator.

Created: September 29, 2010
Revised: October 18, 2010