Alveolar gas equation

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.

The equation relies on the following assumptions:

${\displaystyle p_{A}O_{2}=F_{I}O_{2}(P_{ATM}-pH_{2}O)-{\frac {p_{a}CO_{2}(1-F_{I}O_{2}[1-RER])}{RER}}}$

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