The alveolar gas equation is the method for calculating
partial pressure of
alveolar oxygen (PAO2). The equation is used in assessing if the
lungs are properly transferring
oxygen into the
blood. The alveolar air equation is not widely used in clinical medicine, probably because of the complicated appearance of its classic forms.
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.[1]
Assumptions
The equation relies on the following assumptions:
Inspired gas contains no carbon dioxide (CO2)
Nitrogen (and any other gases except oxygen) in the inspired gas are in equilibrium with their dissolved states in the blood
PAO2, PEO2, and PiO2 are the partial pressures of oxygen in alveolar, expired, and inspired gas, respectively, and VD/VT is the ratio of physiologic dead space over tidal volume.[9]
^Curran-Everett D (June 2006). "A classic learning opportunity from Fenn, Rahn, and Otis (1946): the alveolar gas equation". Adv Physiol Educ. 30 (2): 58–62.
doi:
10.1152/advan.00076.2005.
PMID16709734.
S2CID42010762.
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^Riley RL, Lilienthal JL Jr, Proemmel DD, et al: On the
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^McNicol MW, Campbell EJM: Severity of respiratory failure: arterial blood gases in untreated patients.
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^Begin R, Renzetti AD Jr: Alveolar-arterial oxygen pressure gradient: I. Comparison between an assumed and actual respiratory quotient in stable chronic pulmonary disease; Relationship to aging and closing volume in normal subjects.
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^Suwa K, Geffin B, Pontoppidan H, et al: A nomogram for
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