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Alveolar gas equation
Formula for the partial pressure of alveolar oxygen From Wikipedia, the free encyclopedia
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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]
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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
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Equation
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If FiO2 is small, or more specifically if then the equation can be simplified to:
where:
Sample Values given for air at sea level at 37 °C.
Doubling FiO2 will double piO2.
Other possible equations exist to calculate the alveolar air.[2][3][4][5][6][7][8]
Abbreviated alveolar air equation
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]
Respiratory quotient (R)
Physiologic dead space over tidal volume (VD/VT)
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V D V T = p A CO 2 − p E CO 2 p A CO 2 {\displaystyle {\frac {V_{D}}{V_{T}}}={\frac {p_{A}{\ce {CO2}}-p_{E}{\ce {CO2}}}{p_{A}{\ce {CO2}}}}} Intuitive Explanation
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As 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.
Firstly, the partial pressure of inhaled oxygen is simply the fraction of inhaled oxygen multiplied by the atmospheric pressure . Once oxygen enters the airways, we must account for the partial pressure of water vapor which is assumed to reach 100% saturation, hence . Once the humidified atmospheric air reaches the alveoli, gas exchange takes place so we need to consider the amount of that enters the blood and that leaves the blood. Conveniently, the arterial blood equals the alveolar blood and so this is a value we know. It would also be convenient if the same number of and molecules were exchanged, in which case the alveolar gas equation would simply be . However in reality the number of molecules exchanged differs slightly from the number of molecules, to correct for this the respiratory exchange ratio is used which is the ratio of produced by the body to consumed by the body. Hence the alveolar gas equation becomes:
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See also
References
External links
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