PULMONARY FUNCTION TEST-Aa GRADIENT

SUMMARY

1. The alveolar-arterial gradient (A-a gradient) is the difference between the partial pressure of oxygen in alveoli (A) and that in arterial blood (a): D_A-aO₂ = P_AO₂ - P_aO₂ = 149 - [P_aO₂ + (1.25 * P_aCO₂)]

3. D_A-aO₂ is 5-15 mmHg in healthy young patients & increases with age.

3. This exists due to dead-space which is present within the body.

4. D_A-aO₂ is increased in all causes of hypoxemia except in hypoventilation and high altitude.

5. Very useful as it helps you quickly identify whether hypoxemia exists because of a problem in the alveolar-capillary unit (e.g. pneumonia, PE) or if some other cause is to blame (hypoventilation).

Reference(s)

Wilkinson, I. (2017). Oxford handbook of clinical medicine. Oxford: Oxford University Press.
Hannaman, R. A., Bullock, L., Hatchell, C. A., & Yoffe, M. (2016). Internal medicine review core curriculum, 2017-2018. CO Springs, CO: MedStudy.
Therapeutic Guidelines. Melbourne: Therapeutic Guidelines Limited. https://www.tg.org.au [Accessed 2021].