A-a gradient

A-a gradient = Alveolar Oxygen – arterial Oxygen

This is “relatively” simple way of working out if the paO2 on a ABG is normal, and demonstrates V/Q mismatch well. V/Q mismatch is simple terms is either an area of the lung either under ventilated(pneumonia) or under perfused (PE).

Step 1. Alveolar Gas Equation

pAO2 = Fi02(%) – (paCO2/0.8)

luckily as we use kPa and work at around 100m above sea-level we don’t need to calculate of FiO2

(so air = 21%, FiO2 0.6 = 60%, FiO2 0.8 =80%)

Step 2. A-a gradient

A-a gradient = pAO2 – paO2

Step 3. Is it normal?

Normal varies with both age and the FiO2 so there is no simple answer, and as we use kPa in the UK, commonly quoted calculations need to be modified [i.e.(age/4)+4 OR (age*0.21)+2.5 for age and 7.5 per 10% increase in FiO2]

Age calculation for kPa

(age * 0.03) + 0.3

Age: 20yrs(0.9), 30yrs(1.2), 40yrs(1.5), 50yrs(1.7), 60yrs(2.0), 70yrs(2.3), 80yrs(2.6), 90yrs(2.9)

FiO2 calculation for kPa

A-a gradient increase = 0.1 * % increase in FiO2 (over 21%)

So some examples of expected A-a gradients:

  • 20yr old on air = 0.9
  • 20yr old on 35% = 2.3
  • 60yr old on 24% = 2.3
  • 60yr old on 80%(NRB) = 7.9

However be warned that a normal A-a gradient doesn’t exclude PE data suggests that 10%  of PE’s have a normal A-a gradient



Kanber GJ, The alveolar-arterial oxygen gradient in young and elderly men during air and oxygen breathing. Am Rev Respir Dis. 1968 Mar;97(3):376-81