alveolar-arteriolar (Aa) oxygen gradient

Reference interval

• FiO2 of 21%: 5-25 mm Hg
• FiO2 of 100%: < 150 mm Hg

Increases

• hypoxemia
  • excluding hypoventilation

Interpretation

Calculated: [[[A3796|FiO2]] [(0.2* x barometric pressure - PH2O*) - PaCO2#)] - PaO2

* fraction of O2 in air & water vapor pressure, respectively

# alveolar pressure of CO2 (generally 40-45 mm Hg)

For room air FiO2 (0.2 x barometric pressure - PH2O) generally assumed to be 150.

Thus Aa gradient (room air): 150 - PaCO2 - PaO2

In the elderly, paO2 is generally less than that in younger patients. (see paO2)

More general terms

• general chemistry test