Equations for Calculating Alveolar Oxygen Pressure and Alveolar to Arterial Oxygen Gradient

PAO2 calculation

FIO2 is the fraction of inspired oxygen (eg, 0.21 in room air), Patm is the ambient barometric pressure (eg, 760 mm Hg at sea level), PH2O is the partial pressure of water vapor (eg, usually 47 mm Hg), PaCO2 is the measured partial pressure of arterial carbon dioxide, and R is the respiratory quotient, which is assumed to be 0.8 in a patient who is resting and eating a normal diet.

Alveolar-arterial PO2 gradient (A-a)DO2 simplification

(A-a)DO2 is the alveolar-arterial PO2 gradient. (A-a)DO2 is typically < 20 but increases with age (because of age-related decline in pulmonary function) and with increasing FIO2 (because, although hemoglobin becomes 100% saturated at a PaO2 of about 150 mm Hg, oxygen is soluble in blood, and the oxygen content of plasma continues to increase at increasing FIO2).