A-a Gradient

oxygen pressure

The A-a gradient compares the pressure of oxygen in the alveoli (PAO2) to the pressure of oxygen in the arteries (PaO2). This pressure difference should range between 5 and 10 mm Hg in a healthy patient breathing room air.

The PAO2 must be calculated using the alveolar air equation. The PaO2 is measured during arterial blood gas analysis.

PAO2-PaO2

The A-a gradient helps determine why a patient is hypoxemic. For example, a hypoxemic patient with an elevated A-a gradient indicates a problem with gas exchange. This could include pulmonary shunting, a diffusion defect, or a V/Q mismatch. 

By contrast, a hypoxemic patient with a normal A-a gradient does not have a problem with gas exchange. Rather, their hypoxemia may be caused by hypoventilation (COPD, neurologic and neuromuscular disorders) or high altitude.

Hypoxemia CausesResponds to increases in FiO2A-a gradient statusPotential causes
High altitudeYesNormalLow barometric pressure
HypoventilationYesNormalNarcotics, neurologic or neuromuscular disorders, COPD
ShuntingNoIncreasedHeart defects, ATDS Pulmonary edema, atelectasis
DiffusionYesIncreasedFibrosis
V/A mistmatchYesIncreasedFibrosis, pneumonia, pulmonary embolism, COPD, asthma, pulmonary hypertension

For a more comprehensive breakdown of the A-a great checkout the article: Understanding oxygenation: The Alveolar-arterial gradient written by Lisa Shultis MAEd, RRT.

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Respiratory Therapist