How do you interpret blood gases when patient is on oxygen

A registrar once told me that PaO2 should be about 10 lower than FiO2. So if someone just breathing room air, approximately 20-21% O2, then their O2 sats should be around 10-11mmHg, which is what we would expect in a normal person.

In your example 4 litres of oxygen gives an estimated FiO2 of 36% so you'd expect an PaO2 hovering around the mid-twenties. If it's notably different to what you'd expect then there's probably something wrong.

Not sure this is the most reliable way of interpreting the O2 but it's quick and easy.

It's a rough and ready application of the alveolar gas equation - look it up, I can't write it out on my phone! 4l is more like 28% than 36 though, but it's actually a variable percentage based on the patient's respiratory effort - Venturi masks are more accurate.

OP, first of all there's a difference between sats and pO2 - you don't need an ABG to measure the former. Secondly, what you're interpreting depends on the context. What do you want to know - is the patient on enough/too much oxygen? Are they getting better or worse?

In a healthy person, you'd expect the pO2 to be high teens/low 20s on 4l oxygen, but the sats may not change much - as they'd be 97-100% on air anyway.

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*Do you look at oxygen litres or percentage and then subtract 10?*