Alveolar Gas (Air) Equation
The alveolar air equation is a way to calulate the amount of oxygen available for diffusion from the alveolus into the blood. This helps us to determine the cause of low oxygen levels in the blood. The alveolar gas equation looks at two things - the amount of oxygen that would be available after water vapor is accounted for and if there were no carbon dioxide in the alveolus, and the amount that is being displaced by carbon dioxide in the alveolus.
The potential amount of alveolar oxygen is equal to the barometric pressure minus the water vapor pressure (always 47 mm Hg) multiplied by the fraction (or percentage) of oxygen inspired. (PB - PH2O)FIO2
The alveolar carbon dioxide is calculated by multiplying the arterial carbon dioxide partial pressure by the constant 1.25 which is a function of the respiratory exchange ratio of 0.8. The amount of oxygen that moves into the blood stream (250 ml) exceeds the amount of carbon dioxide that diffuses out (200 ml).
PaCO2(1.25)
The potential amount of alveolar oxygen available minus the amount of alveolar carbon dioxide thus gives us the actual amount of oxygen available for diffusion.
PAO2 = (PB-PH20)FIO2 - PaCO2(1.25)
This could also be written as below (multiplying by 1.25 will give the same results as dividing by 0.8)
PAO2 = [PB – PH2O]FIO2 – PaCO2 / 0.8
When the PaCO2 is less than 60 mm Hg, AND the FIO2 is greater than 0.60 (or 60%) then the constant can be dropped as its effect is not significant.