The Role of the AA Gradient in Critical Care

The AA gradient is this classic thing that we all learned about in med school where it's the difference between the inspired PO2 and the patients blood, PAO2. So I think teaching it to say if your PO2 on 100% is 300 or 150 or 50 is trying to tell you there's less and less surface area of the lung available for oxygenation. There are some niche applications where it can be helpful. When on the other side of that coin, is it not useful in critical care then? In order to get an accurate AA gradient, you ideally want the patient to be breathing room air.