The Alveolar-arterial gradient, commonly known as the A-a gradient, measures the difference in the oxygen concentration in the alveoli and the arteries across the capillary membrane in the lung. In an ideal system, the A-a gradient would be zero because there would be perfect equilibrium as oxygen diffuses and equalizes across the alveolar and arterial sides of the capillary membrane. However, there is a physiologic A-a gradient because of the differences in perfusion and ventilation in the apical and basilar regions of the lungs. Because this relationship exists, the changes in the A-a gradient have clinical utility in guiding the differential diagnosis of hypoxemia.
Nwonu, DO, Chioma; Dong, MD, Michael; and Kramer, MD, Daniel
"An Educational Case for Applying the Alveolar- Arterial Gradient in Hypoxemia: An Underutilized and Underappreciated Clinical Tool,"
The Medicine Forum: Vol. 24, Article 7.
Available at: https://jdc.jefferson.edu/tmf/vol24/iss1/7