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A-a Gradient

 

Input
 

Age		mo yr
Patient Temp		Cel Degrees C Degrees F degC degF
Elevation		feet kilometre meters mile m
Percent Inspired O2		%O2 % fractionO2 fraction litresO2
p CO2		Pascal cmH2O mmHg mm[Hg] torr
Resp Quot		% fraction percent rate ratio
p aO2		Pascal cmH2O mmHg mm[Hg] torr

 
Results
 
Important: Inputs must be complete to perform calculation.

 

Expected AaG		Pascal cmH2O mmHg mm[Hg] torr
Measured AaG		Pascal cmH2O mmHg mm[Hg] torr
Decimal Precision   0 1 2 3

 

Data action

 
     
 
Include literature references: Yes   No

 

 

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. West JB. Respiratory physiology: the essentials. 7th edition, Philadelphia, Lippincott William & Wilkins 2005, p.170. NLMN ID: 101205971
2. 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
3. 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
4. Cole RB, Bishop JM. Variation in alveolar-arterial O2 tension difference at high levels of alveolar O2 tension. J Appl Physiol. 1967 Apr;22(4):685-93. PubMed ID: 6023181
5. Lilienthal JL, Riley RL, et al. An experimental analysis in man of the oxygen pressure gradient from alveolar air to arterial blood during rest and exercise at sea level and at altitude. Am J Physiol. 1946 Sep;147:199-216. PubMed ID: 21000737
6. Le Huu Thien B,Huynh Quang D. Equation des gaz alveolaires et difference alveolo-arterielle en oxygene. J Fran Viet Pneu. 2014; 05(14): 4-10.

 

Associated Medical Subject Headings [MeSH]
 

Adolescent,   Adult,   Aged,   Aging,   Air Pressure,   Altitude,   Arteries,   Blood,   Blood Pressure Determination,   Carbon Dioxide,   Exercise,   Female,   Humans,   Hydrogen-Ion Concentration,   Male,   Middle Aged,   Oximetry,   Oxygen,   Oxygen Consumption,   Partial Pressure,   Pulmonary Alveoli,   Pulmonary Circulation,   Regional Blood Flow,   Respiration,   Respiratory Function Tests,   Rest,   Spirometry

 

Associated Logical Observation Identifiers Names and Codes [LOINC]
 

Parameter	LOINC's and Suggested Units
Age	21612-7 : yr      21612-7 : yr      21612-7 : years      30525-0 : Years old      30525-0 : yr      30525-0 : years
Patient Temp	8329-5 : degC      8310-5 : degC      8331-1 : degC      8332-9 : degC      8333-7 : degC      8328-7 : degC      75539-7 : degC
Percent Inspired O2	3150-0 : %      19993-5 : %      19992-7 : %      19994-3 : %      19996-8 : %      19995-0 : %
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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