Curriculum · Respiratory
Alveolar-arterial oxygen gradient in the assessment of hypoxaemia
What it is
The A-a gradient is the distance between the alveolar and the arterial O2, where A stands for alveolar and a for arterial or capillary. It is written A-a = PAO2 - PaO2. If an ABG is in hand, the gradient should always be measured, because the number that matters is not the PaO2 alone but the gap between what the alveolus holds and what the blood carries away.
How it is diagnosed
PaO2 is read straight off the ABG. PAO2 is calculated: PAO2 = 150 - PaCO2 x 1.25, at an FiO2 of 21%. The expected gradient rises with age and is given by 2.5 + (0.21 x age), with a normal of roughly 5 to 20 mmHg.