Thursday, March 21, 2019

A-a Gradient

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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