The alveolar-arterial gradient is the difference between the oxygen concentration in the alveolus and the oxygen concentration in the arterial blood. This is a reflection of the adequacy of oxygen transport across the alveolar membrane. The value is calculated by subtracting the arterial oxygen concentration, which is obtained with the ABG, from the alveolar oxygen concentration, which is calculated from a room air ABG in the following manner:

Alveolar O2= 150 - ( 1.25 x PaCO2 )

The alveolar oxygen concentration for a ventilated patient is calculated via a different equation:

Alveolar gas equation = ( PB - PH2O ) x ( FIO2 ) - PaCO2/R

= ( 760 - 47 ) x ( FIO2 ) - PaCO2/0.8

The normal value for the A-a gradient may be calculated via the following equation:

Expected A -a gradient < 4 + patients age/4 or < 20 whichever value is higher.

The A-a gradient is useful in the work up of hypoxia. Hypoxia with a normal A-a gradient is caused by either hypoventilation or a decrease in the O2 concentration of inspired air. When hypoxia is associated with an elevated A-a gradient the differential includes ventilation-perfusion mismatching, intrapulmonary or intracardiac right to left shunting of blood or diffusion impairment of gases across the alveolar membrane.