A-a Gradient

 

Input
 

Age		mo yr
Patient Temp		degC degF
Elevation		meters
Percent Inspired O2		%O2 fractionO2 litresO2
p CO2		Pascal atm bar cmH2O cmHg ftH2O gm/sqcm inH2O inHg kPa mbar mmHg psi torr
Resp Quot		% fraction ratio
p aO2		Pascal atm bar cmH2O cmHg ftH2O gm/sqcm inH2O inHg kPa mbar mmHg psi torr

 

Results
 

Expected AaG		Pascal atm bar cmH2O cmHg ftH2O gm/sqcm inH2O inHg kPa mbar mmHg psi torr
Measured AaG		Pascal atm bar cmH2O cmHg ftH2O gm/sqcm inH2O inHg kPa mbar mmHg psi torr
Decimal Precision   0 1 2 3

 

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.

 
Equations used
 

pAtm = 760 * e(Elevation/-7000)

pH2O = 47 * e((PatientTemp-37)/18.4)

FIO2 = PercentInspiredO2 / 100

ExpectedAaG = 2.5 + (0.21 * Age)

MeasuredAaG = (FIO2 * (pAtm - pH2O)) - (pCO2 / RespQuot) + (pCO2 * FIO2 * (1 - RespQuot) / RespQuot) - paO2

 

 

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 ID: 5638492
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