Why is the A-a gradient increased in increased Fio2?

[tarsuc]

Why is the A-a gradient increased in increased Fio2? Whatever is the PAo2, it should adequately diffuse and increase the PaO2 also, right?

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[gettheleadout]

Potential explanation: O2 transport in the pulmonary capillaries is normally perfusion-limited, meaning alveolar oxygen tension equals pulmonary blood oxygen tension at the end of the pulmonary capillaries. With increasing FIO2, PAO2 increases such that O2 transport becomes diffusion-limited, meaning the blood oxygen tension < alveolar oxygen tension at the end of the capillaries. The higher the FIO2, the more right-shifted the O2 transport curve becomes, and the greater the difference between PAO2 and PaO2, thus, the greater the A-a gradient.

Edit: Also keep in mind the PaO2 is never exactly equal to PAO2 anyway, due to physiologic shunting (e.g. via the bronchial vasculature), plus of course any pathologic V/Q mismatch or diffusion impairment.

 

[tarsuc]

Thanks buddy!