Alveolar-arterial gradient

The Alveolar-arterial gradient (A-a gradient), is a measure of the difference between the alveolar concentration of oxygen and the arterial concentration of oxygen. It is used in diagnosing the source of hypoxemia.^[1]

A-a gradient = PA_O2 – Pa_O2^[2]

Where:

• PA_O2 = alveolar PO₂ (calculated from the alveolar gas equation)

• Pa_O2 = arterial PO₂ (measured in arterial blood A-a gradient)

The A-a gradient is useful in determining the source of hypoxemia. The measurement helps isolate the location of the problem as either intrapulmonary (within the lungs) or extrapulmonary (somewhere else in the body).

A normal A-a gradient is less than 10 mmHg, but can range from 5-20 mmHg. Normally, the A-a gradient increases with age. For every decade a person has lived, their A-a gradient is expected to increase by 1 mmHg. An abnormally increased A-a gradient suggests a defect in diffusion, V/Q (ventilation/perfusion ratio) defect, or right-to-left shunt.^[3]

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