A-a Gradient

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