The alveolar-arterial oxygen gradient

The Alveolar : arterial oxygen gradient is an index of the efficiency of gas transfer across the alveolar membrane. It is the difference in oxygen tension across the alveolar membrane.

• A-a (gradient) = PAO2 (partial pressure alveolar O2) - PaO2 (partial pressure arterial O2)

The alveolar partial pressure of oxygen, PAO2 is given as:

• PAO2 = (FiO2 - PCO2)/R where R is approximately 0.8 and FiO2 is 20 kPa on room air.

Otherwise the A-a gradient may be estimated by the formula:

• PiO2 (inspired PO2) - ( PaO2 (arterial PO2) + PaCO2 (arterial PCO2) )

The partial pressure of inspired O2 is approximately its percentage in kPa.

The upper limit for this gradient varies with age between 1.5 kPa and 3.0 kPa. A gradient in excess of 3.0 kPa suggests impaired gas exchange.