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.
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 Causes||Responds to increases in FiO2||A-a gradient status||Potential causes|
|High altitude||Yes||Normal||Low barometric pressure|
|Hypoventilation||Yes||Normal||Narcotics, neurologic or neuromuscular disorders, COPD|
|Shunting||No||Increased||Heart defects, ATDS Pulmonary edema, atelectasis|
|V/A mistmatch||Yes||Increased||Fibrosis, 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.