What is the alveolar arterial gradient?

alveolar-arterial gradient (A-a gradient). This tells us about the difference in oxygen partial pressure between the alveolar and arterial blood. It is a widely used variable for assessing intrapulmonary oxygen exchange and can be calculated by subtracting PaO2 from alveolar oxygen tension (PAO2). Normally it equals 1 as there is little difference in alveolar and arterial PO2 in the normal healthy lung. However in disease such as emphysema and pulmonary fibrosis loss of alveoli and thickening of the respiratory membrane lead to a difference in the oxygen breathed in (alveoli) and that that reaches the blood (arterial).

alveolar-arterial gradient (A-a gradient). This tells us about the difference in oxygen partial pressure between the alveolar and arterial blood. It is a widely used variable for assessing intrapulmonary oxygen exchange and can be calculated by subtracting PaO2 from alveolar oxygen tension (PAO2). Normally it equals 1 as there is little difference in alveolar and arterial PO2 in the normal healthy lung. However in disease such as emphysema and pulmonary fibrosis loss of alveoli and thickening of the respiratory membrane lead to a difference in the oxygen breathed in (alveoli) and that that reaches the blood (arterial).