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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.
The alveolar partial pressure of oxygen, PAO2 is given as:
Otherwise the A-a gradient may be estimated by the formula:
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.
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