A-a gradient =[PAO2 - PaO2]
where:
A-a gradient = difference between alveolar PO2 and arterial PO2
PAO2 = alveolar PO2 (calculated from the alveolar gas equation)
PaO2 = arterial PO2 (measured in arterial blood)
PAO2 =150 - PaCo2/0.8
Normal range for A-a gradient is
10-15 mm Hg
ALL causes of hypoxemia lead to ↑ A-a gradient, EXCEPT:
Hypoventilation, high altitude, upper airway obstruction (e.g. epiglottitis from Haemophilus influenzae, or croup from parainfluenza virus)
Everything else will cause ↑ A-a gradient (e.g. shunt, V/Q mismatch, etc.).
It's much better to remember the exceptions, then everything else becomes the rule!
Also to adjust for age, the thumb rule to calculate A-a gradient is :
Age /4 plus 4
A-a gradient >30 is considered elevated regardless of age.
Bhopalwala. H
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