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 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 |
| Diffusion | Yes | Increased | Fibrosis |
| V/A mistmatch | Yes | Increased | Fibrosis, pneumonia, pulmonary embolism, COPD, asthma, pulmonary hypertension |
