A-a Gradient

Patient Temp  
Percent Inspired O2  
p CO2  
Resp Quot  
p aO2  

Expected AaG  
Measured AaG  
Decimal Precision  

  • High A-a gradients are associated with oxygen transfer / gas exchange problems. These are usually associated with alveolar membrane diseases, interstitial diseases or V/Q mismatch.
  • Hypoxemia in the face of a normal A-a gradient implies hypoventilation with displacement of alveolar O2 by CO2 or other substance.
  • The FIO2 (fraction of inspired oxygen) is equal to the Percent Inspired O2 / 100.
  • In this calculator, pATM is estimated by elevation above sea level using this formula: pAtm = 760 * eTo(Elevation / -7000)
  • pH2O is dependent on body temperature and calculated with the following formula: pH2O = 47 * eTo((Patient Temp - 37) / 18.4)
  • The equation for expected A-a gradient assumes the patient is breathing room air; therefore, it is less accurate at higher percentages of inspired oxygen.

Equations used
pAtm = 760 * e(Elevation/-7000)
pH2O = 47 * e((PatientTemp-37)/18.4)
FIO2 = Percent Inspired O2 / 100
ExpectedAaG = 2.5 + (0.21 * Age)
MeasuredAaG = (FIO2 * (pAtm - pH2O)) - (pCO2 / RespQuot) + (pCO2 * FIO2 * (1 - RespQuot) / RespQuot) - paO2


  1. Kanber GJ, King FW, Eshchar YR, et al. 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. PubMed ID: 5638492 PubMed Logo
  2. Mellemgaard K. The alveolar-arterial oxygen difference: its size and components in normal man. Acta Physiol Scand. 1966 May;67(1):10-20. PubMed ID: 5963295 PubMed Logo



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