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A-a Gradient

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

 
Input:
 
Age  
Patient Temp  
Elevation  
Percent Inspired O2  
p CO2  
Resp Quot  
p aO2  
Results:
 
Expected AaG  
Measured AaG  
 
Decimal Precision:

 
     
Notes
  • 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.

 
Related Results
 
          
 

 
References
  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 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 Logo

 
Associated Medical Subject Headings [MeSH] Terms
 
         Adolescent, Adult, Aged, Aging, Arteries, Blood Pressure Determination, Carbon Dioxide, Female, Humans, Hydrogen-Ion Concentration, Male, Middle Aged, Oximetry, Oxygen, Oxygen Consumption, Partial Pressure, Pulmonary Alveoli, Respiration, Respiratory Function Tests

 
Associated Logical Observation Identifiers Names and Codes [LOINC]
 
Parameter LOINC's and Units
Age 21612-7 : Years old      21612-7 : yr      21612-7 : years      30525-0 : Years old      30525-0 : yr      30525-0 : years      
Patient Temp 8310-5 : degC      
Percent Inspired O2 19993-5 : %O2      19992-7 : %O2      19994-3 : %O2      19996-8 : %O2      19995-0 : %O2      
p CO2 2019-8 : mmHg      34705-4 : mmHg      
p aO2 11556-8 : mmHg      2703-7 : mmHg      19254-2 : mmHg      
Measured AaG 19991-9 : mmHg      53809-0 : mmHg      

 
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All information contained in and produced by the MedCalc 3000 system is provided for educational purposes only. This information should not be used for the diagnosis or treatment of any health problem or disease. THIS INFORMATION IS NOT INTENDED TO REPLACE CLINICAL JUDGMENT OR GUIDE INDIVIDUAL PATIENT CARE IN ANY MANNER. Click here for full notice and disclaimer.
 
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