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

## Alveolar Gas Equation Calculation

PAO2 = ( FiO2 * (Patm – PH2O)) – (PaCO2 / RQ).

The partial pressure of oxygen in the alveoli (PAO2) is required to calculate the **alveolar-arterial gradient (A-a gradient)**.

The FiO2 is the fraction of inspired oxygen (usually as a fraction, but entered here as a percentage for ease of use). Patm is the ambient atmospheric pressure, which is 760 mmHg at sea level. PH2O is vapor pressure of water at 37°C and is equal to 47 mmHg. The respiratory quotient or respiratory coefficient (RQ) is the ratio of CO2 produced divided by the O2 consumed, and its value is typically 0.8 (RQ = CO2 eliminated / O2 consumed).

## How to Calculate Alveolar Gas Equation, PAO2

Atmospheric 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