Abbreviation: A-a gradient
The difference between the partial pressure of oxygen in the alveoli (A) and the partial pressure of oxygen in the arteries (a). The normal range varies with age, altitude, and the concentration of inhaled oxygen. Normal range for a young person breathing room air at sea level is 5–10 mm Hg and increases with physical exercise. In diseased states, A-a gradient increases with a ventilation-perfusion mismatch and/or impaired gas diffusion across the alveoli but remains normal or low with alveolar hypoventilation.