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Cornell Pediatrics

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.

Input FiO2 %
Input PaCO2 torr
Input Atmospheric Pressure   torr
Input Respiratory Quotient 
PAO2   =  

Determine Atmospheric Pressure
Elevation feet   meters
Atmospheric Pressure  =  


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

PAO2 = ( FiO2 * (Patmos - PH2O)) - (PaCO2 / RQ)

The FiO2 is the fraction of inspired oxygen (usually as a fraction, but entered here as a percentage for ease of use). Patmos is the ambient atmospheric pressure, which is 760 torr at sea level. PH2O 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 = PAO2 - PaO2

See A-a Gradient calculator.


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

Weill Medical College of Cornell University
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