A-a gradient
A-a gradient is calculated as PAO2 – PaO2
1) PAO2 is the ‘ideal’ compartment alveolar PO2 determined from the alveolar gas equation
2) PAO2 = PiO2 – PaCO2/0.8
3) A normal A–a gradient for a young adult non-smoker breathing air, is between 5–10 mmHg.
4) However, the A–a gradient increases with age (see limitations)
classification of hypoxia based on a-a gradient
Normal A-a gradient
1) Alveolar hypoventilation (elevated PACO2)
2) Low PiO2 (FiO2 < 0.21 or barometric pressure < 760 mmHg)
Raised A-a gradient
1) Diffusion defect (rare)
2) V/Q mismatch
3) Right-to-Left shunt (intrapulmonary or cardiac)
4) Increased O2 extraction (CaO2-CvO2)
Limitations
1) Gradient varies with age and FiO2:
1. FiO2 0.21 – 7 mmHg in young, 14 mmHg in elderly
2. FiO2 1.0 – 31 mmHg in young, 56 mmHg in elderly
2) 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.
3) 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.
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