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Alveolar-Arterial O2Gradient

A-a gradient is the difference in partial pressure of oxygen between arterial and alveolar blood. It can determine if hypoxia is a reflection of hypoventilation or due to deficiency in oxygenation.

 Arterial Oxygen Tension(mmHg)
 Arterial Carbon-Di-Oxide Tension(mmHg)
O2 Gradient


A normal A-a gradient is 10-20 mm Hg, with the normal gradient increasing within this range as the patient ages.
  • A normal A-a gradient in the face of hypoxemia suggests the hypoxemia is due to hypoventilation and not due to underlying lung disorders.
  • An increased A-a gradient identifies decreased oxygen in the arterial blood compared to the oxygen in the alveolus. This suggests a process that interferes with gas transfer, or in general terms, suggests ventilation-perfusion mismatch.


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