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): DA-aO2 = PAO2 - PaO2 = 149 - [PaO2 + (1.25 * PaCO2)]
3. DA-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. DA-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., Furmedge, D. and Sinharay, R. (2017). Oxford handbook of clinical medicine. Oxford: Oxford University Press. Get it on Amazon.
Feather, A., Randall, D. and Waterhouse, M. (2020). Kumar And Clark’s Clinical Medicine. 10th ed. S.L.: Elsevier Health Sciences. Get it on Amazon.
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].
