# How do you interpret blood gases when patient is on oxygen

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1. For example if someone is on 4litres O2 how do you interpret the oxygen sats?
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2. 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.
3. (Original post by Number 9)
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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4. (Original post by Helenia)
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?*
5. (Original post by BreatheDeep)
*Do you look at oxygen litres or percentage and then subtract 10?*
Percentage FiO2. Which can be tricky to estimate with various oxygen delivery devices and respiratory patterns. But at the end of the day, it's a rule of thumb to see if there is significant V/Q mismatch going on.

(This is to estimate an appropriate PaO2, rather than sats, which are two very different things as mentioned above).

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