The Alveolar Gas Equation is used to calculate the partial pressure of oxygen in the alveoli:

Alverolar Gas Equation Calculation: 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 mmHg at sea level. PH2O is vapour pressure of water at 37°C and is equal to 47 mmHg (760 mmHg – 47 mmHg). The respiratory quotient or respiratory coefficient (RQ) is the ratio of CO2 eliminated divided by the O2 consumed, and its value is typically 0.8 (RQ = CO2 eliminated / O2 consumed).

## Alveolar Gas Equation, PAO2

Atmoshpheric Pressure (PB): | |

Vapor Pressure (PH2O): | |

FiO2: | % |

PaCO2: | |

RQ: | |

Alveolar Gas Equation (PAO2): |
mmHg |

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**References:**

Interpretation of arterial oxygen tension . Retrieved June 27, 2012, from http://cmbi.bjmu.edu.cn/uptodate/critical care/Monitoring/Interpretation of arterial oxygen tension.htm

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