EMed

A-a gradient

Formula (kPa)

A-a = FiO2 - (1.25 x PaCO2) - PaO2

Normal A-a : young <3, >60 years <4

↑ A-a gradient = V/Q mismatch:

  • LVF/Pulm. oedema
  • PE
  • ARDS
  • LRTI
  • Other resp.membrane disease

A-a gradient (assume at sea level)

Result:

1 2 3 4 rule

Calc. metabolic compensation for resp. acid/alkalosis

HCO3 (N=24 mmol/L)
1kPa Δ in CO2 Acute Chronic
↑PaCO2 1 4
↓PaCO2 2 3

Content by Dr Íomhar O' Sullivan . Last review Dr ÍOS 20/07/22.