Rahul Sharma (Editor)

Alveolar gas equation

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The partial pressure of oxygen (pO2) in the pulmonary alveoli is required to calculate both the alveolar-arterial gradient of oxygen and the amount of right-to-left cardiac shunt, which are both clinically useful quantities. However it is not practical to take a sample of gas from the alveoli in order to directly measure the partial pressure of oxygen. The alveolar gas equation allows the calculation of the alveolar partial pressure of oxygen from data that is practically measurable. It was first characterized in 1946.

Contents

Assumptions

The equation relies on the following assumptions:

  • Inspired gas contains no carbon dioxide (CO2) or water
  • Nitrogen (and any other gases except oxygen) in the inspired gas are in equilibrium with their dissolved states in the blood
  • Inspired and alveolar gases obey the ideal gas law
  • Carbon dioxide (CO2) in the alveolar gas is in equilibrium with the arterial blood i.e. that the alveolar and arterial partial pressures are equal
  • The alveolar gas is saturated with water

    • Equation

    p A O 2 = F I O 2 ( P A T M − p H 2 O ) − p a C O 2 ( 1 − F I O 2 [ 1 − R E R ] ) R E R

    If FIO2 is small, or more specifically if
    F I O 2 [ 1 − R E R ] ≪ 1
    then the equation can be simplified to:
    p A O 2 ≈ F I O 2 ( P A T M − p H 2 O ) − p a C O 2 R E R where:

    Sample Values given for air at sea level at 37°C.

    References

    Alveolar gas equation Wikipedia
    (Text) CC BY-SA


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