A-a gradient is calculated as PAO2 – PaO2
- PAO2 is the ‘ideal’ compartment alveolar PO2 determined from the alveolar gas equation
- PAO2 = PiO2 – PaCO2/0.8
- A normal A–a gradient for a young adult non-smoker breathing air, is between 5–10 mmHg.
- However, the A–a gradient increases with age (see limitations)
CLASSIFICATION OF HYPOXIA BASED ON A-a GRADIENT
Normal A-a gradient
- Alveolar hypoventilation (elevated PACO2)
- Low PiO2 (FiO2 < 0.21 or barometric pressure < 760 mmHg)
Raised A-a gradient
- Diffusion defect (rare)
- V/Q mismatch
- Right-to-Left shunt (intrapulmonary or cardiac)
- Increased O2 extraction (CaO2-CvO2)
- Gradient varies with age and FiO2:
- FiO2 0.21 – 7 mmHg in young, 14 mmHg in elderly
- FiO2 1.0 – 31 mmHg in young, 56 mmHg in elderly
- For every decade a person has lived, their A–a gradient is expected to increase by 1 mmHg – a conservative estimate of normal A–a gradient is < [age in years/4] + 4.
- an exaggerated FiO2 dependence in intrapulmonary shunt (PAO2 vs PAO2/PaO2 difference diagram with regard to increasing percentage of shunt) and even more so in V/Q mismatch.