### Description

The alveolar oxygen tension refers to the partial pressure in the alveolar gas. This is less than the oxygen tension in the inspired air, due to removal of oxygen by passing blood and the addition of carbon dioxide.

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))