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.

Video Alveolar gas equation

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
• 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

Maps Alveolar gas equation

Equation

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

If F_IO₂ is small, or more specifically if
${\displaystyle F_{I}O_{2}[1-RER]\ll 1}$
then the equation can be simplified to:
${\displaystyle p_{A}O_{2}\approx F_{I}O_{2}(P_{ATM}-pH_{2}O)-{\frac {p_{a}CO_{2}}{RER}}}$ where:

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

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See also

• Pulmonary gas pressures

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References

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External links

• Free interactive model of the simplified and complete versions of the alveolar gas equation (AGE)
• Formula at ucsf.edu
• Derivation and explanation from Continuing Care in Anaesthesia, Critical Care, and Pain
• Online Alveolar Gas Equation and iPhone application by Medfixation.
• A computationally functional Alveolar Gas Equation by vCalc.

Source of the article : Wikipedia