* Required


ExpectedAaG = 2.5 + (0.21 * Age)
MeasuredAaG = (FIO2 * (pAtm – pH2O)) – (pCO2 / RespQuot) + (pCO2 * FIO2 * (1 – RespQuot) / RespQuot) – paO2


  • 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.


  1. Chest.1979 Jun;75(6):748.The abbreviated alveolar air equation. Helmholz HF Jr.
  2. Am J Med. 1994 Jan;96(1):57-62.Use of the alveolar-arterial oxygen gradient in the diagnosis of pulmonary embolism. McFarlane MJ, Imperiale TF.

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