MedCourseFinder

Pulmonology

Alveolar-arterial Oxygen Gradient

AI-assisted content.

Enter the values and press Calculate.

Formula

PAO2 = FiO2 x (Patm - 47) - PaCO2/0.8; A-a = PAO2 - PaO2. The upper limit of normal on room air is approximately age/4 + 4.

Limitations

  • Age/4 + 4 is a rule of thumb for adults breathing room air; the expected gradient rises with FiO2, so no age-based classification is given when FiO2 is above 0.21.
  • Assumes a respiratory quotient of 0.8 and a water vapour pressure of 47 mmHg (37 C).
  • The age/4 + 4 rule was seen in two secondary sources; its original derivation was not located.

References

  1. Ideal alveolar air and the analysis of ventilation-perfusion relationships in the lungs. Riley RL, Cournand A. J Appl Physiol. 1949;1(12):825-847. PMID 18145478
  2. The alveolar-arterial oxygen difference: its size and components in normal man. Mellemgaard K. Acta Physiol Scand. 1966;67(1):10-20. PMID 5963295