The A-a gradient compares the pressure of oxygen in the alveoli (PAO2, which is a value calculated by using the alveolar gas equation) to the pressure of oxygen in the arteries (PaO2, which is a value measured on ABG). It is essentially a “report card” of lung perfusion, showing how effectively oxygen is moved from inspired air, through the lungs, to the blood and helps to determine why the patient is hypoxemic.
How It Works
Anatomically, the only thing dividing alveolar air and deoxygenated blood that becomes arterial blood is the alveolar-capillary membrane. This layer is so thin that oxygen equalization between this “barrier” occurs readily, resulting in a normally very small A-a gradient. However, when oxygen is not effectively transferred from the alveoli to the blood, it will result in an elevated A-a gradient due to:
- Ventilation/perfusion mismatch (pneumonia, pulmonary embolism, COPD)
- Shunt (pulmonary edema, ARDS, congenital heart)
- Diffusion abnormalities (infiltrates, pulmonary fibrosis)
Calculation Parameters
A conservative estimate of the A-a gradient can be made by using the following calculation:
The normal A-a gradient (on room air) is 7 mm Hg in the young and 10 to 20 mm Hg in the adult. The value is variable and dependent on both age and FiO2 concentration.
For example:
- A-a gradient increases 5 to 7 mm Hg for every 10% increase in FiO2.
- At 100% FiO2, A-a gradient can be as high as 60 to 70 mm Hg.
- At sea level, at >60 years of age, the A-a gradient extends to 17 to 41 mm Hg and by >80 years of age extends to 25 to 38 mm Hg. (Both scenarios are related to decreased diffusion with increased age.)
It should be noted that a more onerous calculation can be used to determine the A-a gradient by determining the PAO2 (with the alveolar gas equation) and subtracting the PaO2 from that value. However, numerous apps are available on both the Internet and mobile phones to more readily accomplish this task.