A-a Oxygen Gradient

Pa02 mmHg
PaC02 mmHg
Fi02(inspired O2 concentration)
21% (Room Air)


  • Alveolar-arterial (A-a) gradient
    • Difference between the alveolar and arterial concentration of oxygen
    • Can help identify the mechanism of hypoxemia especially when arterial O2 is measured before and after oxygen therapy
  • Normal values:
    • Young adult, non-smoker breathing room air: 5–10 mmHg
    • Older adults: approximated by (age / 4) + 4. Example: a 40 year old should have an A–a gradient < 14.
  • Increased A-a gradient can be caused by:
    • Shunt (low arterial O2 not corrected by oxygen therapy)
      • Alveolar collapse (atelectasis)
      • Filing of alveoli (pneumonia, pulmonary edema)
      • Intracardiac shunt (right-to-left shunt)
      • Vascular shunt (in lungs)
    • Ventilation/perfusion mismatching (low arterial O2 corrected by oxygen therapy)
      • Airway disease (COPD, asthma)
      • Interstitial lung disease
      • Alveolar disease
      • Pulmonary vascular disease


A-a oxygen gradient = PAO2 - PaO2


PAO2 = FiO2 × ( Patm - PH2O ) – ( PaCO2 ÷ 0.8 )

PaO2 = arterial PO2 (measured in arterial blood)

FiO2: inspired oxygen concentration OR
( oxygen administered in L/min × 3 ) + 21

Patm: atmospheric pressure, in mmHg (sea level = 760 mmHg)

PH2O: vapor pressure of water at body temperature (typically 47 mmHg)


  1. Deutsch AB et al: Assessment of the alveolar-arterial oxygen gradient as a screening test for pulmonary embolism in pregnancy. Am J Obstet Gynecol 203:373.e1, 2010  [PMID:20554265]
  2. Moammar MQ et al: Alveolar-arterial oxygen gradient, pneumonia severity index and outcomes in patients hospitalized with community acquired pneumonia. Clin Exp Pharmacol Physiol 35:1032, 2008  [PMID:18518885]
A-a Oxygen Gradient is a sample topic from the Calculators.

To view other topics, please or .

Emergency Central is a collection of disease, drug, and test information including 5-Minute Emergency Medicine Consult, Davis’s Drug, McGraw-Hill Medical’s Diagnosaurus®, Pocket Guide to Diagnostic Tests, and MEDLINE Journals created for emergency medicine professionals. .