Monday, August 3, 2020

A-a gradient

Q: With higher FiO2, the A-a gradient will? (select one)

A) increase
B) decrease

Answer: A

The Alveolar–arterial gradient, popularly known as A–a gradient is simply a measure of the difference between the alveolar concentration (A) of oxygen and the arterial (a) concentration of oxygen. A normal A–a gradient is usually around 10 mmHg, and need to be adjusted for the age. Logic may imply that if a patient is breathing a very high FiO2, PaO2 will go up and A-a gradient will decrease. In actuality, when a patient receives a very high FiO2, both PAO2 and PaO2 increase, but the PAO2 increases disproportionately. This results in A-a gradient to increase.

In a clinical situation of hypoxemia, P/F (Po2/FiO2) ratio is a better determinant of hypoxemia



Kanber GJ, King FW, Eshchar YR, Sharp JT. The alveolar-arterial oxygen gradient in young and elderly men during air and oxygen breathing. Am Rev Respir Dis 1968; 97:376.

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