**Standards message here**

Calculate the pO_{2} 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_{A}O_{2} = ( F_{i}O_{2}
* (P_{atmos} - P_{H2O})) - (P_{a}CO_{2}
/ 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 CO_{2}
eliminated divided by the O_{2} 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_{A}O_{2} - P_{a}O_{2}

See A-a Gradient calculator.

*Created:* September 29,
2010

*Revised:* October 18, 2010