alveolar oxygen tension (PAO2) =
= ((fractional concentration of oxygen in the inspired gas) * ((atmospheric pressure) - (water vapor pressure))) - ((PACO2) * ((fractional concentration of oxygen in the inspired gas) + ((1 - (fractional concentration of oxygen in the inspired gas)) / (respiratory quotient ))))
since PaCO2 can be used to approximate PACO2
alveolar oxygen tension (PAO2) =
= ((fractional concentration of oxygen in the inspired gas) * ((atmospheric pressure) - (water vapor pressure))) - ((PaCO2) / (respiratory quotient))
where:
• Fractional concentration of oxygen in the inspired gas = FIO2 = (per cent inspired oxygen) / (100%)
• An alternative equation replaces (1 / (respiratory quotient)) with (1.25 - (0.25 * (FIO2))); this is derived from using 0.8 as the respiratory quotient and the relation in the top equation ((fractional concentration of oxygen in the inspired gas) + ((1 - (fractional concentration of oxygen in the inspired gas)) / (respiratory quotient )))
• Water vapor pressure is 47 mm Hg at sea level and 37 °C, but can be calculated by the equations given previously.
If the respiratory quotient is 0.8, and if the (barometric pressure-water vapor pressure) is assumed to be about 700, then the equation can be simplified to:
alveolar oxygen tension (PAO2) =
= (7 * (per cent inspired oxygen as whole number from 0 to 100)) - (1.25 * (PaCO2))
For persons breathing room air at sea level, this can further simplified to:
alveolar oxygen tension (PAO2) in mm Hg =
= 150 - (1.25 * (PaCO2))
