|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.
- Chest.1979 Jun;75(6):748.The abbreviated alveolar air equation. Helmholz HF Jr.
- 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.
ThePhysician.org includes risk tools and content intended for use by healthcare professionals. These tools do not give professional advice; physicians and other healthcare professionals who use these tools or databases should exercise their own clinical judgment as to the information they provide.
Please read Disclaimer for additional information.