Every once in a while this comes up and it bugs me.
I have read in a few different studies that really high pao2 is harmful (after cardiac arrest, MI, etc). So to me it seems logical to try and minimize fio2 as long as the saturation is adequate.
and typically our orders are to wean 02 for spo2>=90%
occasionally when I ask the RT to frm down the fio2 because the patients spo2 is in the high 90s, they ask me for a blood gas because they are concerned about the pao2.
who cares what the pao2 is as long as the sat is good?
o2 delivery equation is CO x sao2 x hgb x 1.34 + pao2(.003)
pao2 barely matters n terms of arterial o2 content.
Maybe that's just what they are taught to follow?
Mar 25, '16
You can't keep the patient O2 dependent forever so trying to wean off as much as possible is a logical intervention. AS long as the saturations are above 92% I wouldn't worry about the PO2. Unless specified by the MD. Just remind the RT what the orders are.
Some doctors even give orders to keep saturations above 88% to some chronic COPD'ers, with CO2 retention.
Mar 25, '16
Interesting. This may be a practice in your facility or area. My experience (as an RT) was different. We tried to minimize arterial sticks and relied heavily on pulse oximetry to titrate oxygen. I agree with reminding them what the orders are. If the physician wanted ABGs followed, they could just as easily write to maintain PaO2 greater than 'X.'
Mar 25, '16
PaO2 in an ABG is a sensitive indicator of gas exchange at the alveolar level and is very useful when looking at the PaO2/FiO2 ratio. This is assuming you mean the PaO2 level from an ABG and not not PAO2 which is partial pressure of oxygen in the alveoli. For instance, if your O2 sat is 100% and PaO2 is 115 while your patient is only on 40% FiO2, you can obviously come down on FiO2 because your patient is being over-oxygenated. In contrast, if you're PaO2 is 55% on 100% FiO2, and the O2 sat is only 93%, your patient's gas exchange is impaired and your patient isn't being adequately oxygenated. This is when we look to see if we need to increase the PEEP (although we would probably have tried this much earlier at 100% FiO2), put the patient on a high frequency oscillator, or rotoprone bed, etc.
We also look at the PaO2/FiO2 ratio in non-intubated patients. if a patient's PaO2 is < 55 and they're on 15L NRB, and are in obvious distress, then they're getting tubed.
IMO, you can't really wean a patient off a vent without knowing what the PaO2 is.
Mar 28, '16
I recently had a patient on VV ECMO at 100%, on the vent at 100%, Nitric at 40. The spO2 was 92% but the PO2 was 52. So the spO2 is not the end all be all, it definitely helps to have both to guide and direct your team's plan of care.
Not only that, but the spO2 measures can read 100% in a patient with carbon monoxide poisoning because it is reading the carboxyhemoglobin as opposed to the oxyemoglobin that we normally expect it to be. In that case the PO2 can be horrendously low with an spO2 of 100%.
Mar 28, '16
SpO2 doesn't always correlate well with PaO2. Just had one satting in the high 90s last night who came back on the gas with a PaO2 in the 50s on 100%. SpO2 is a piece of the puzzle, but not the only one that needs to be take into account for sure.
Mar 28, '16
If you look at this curve, you'll notice that there can be a wide range of SaO2 for a more narrow range of PaO2. Look at the steep part, pick a PaO2 below 60 (on the x axis) and draw a straight line over to the y axis to see your SaO2.
Oxygen-Haemoglobin Dissociation Curve
You can see that for relative little change in PaO2, you have broad change in SaO2. At about 90% SaO2 and about 60-70 PaO2, the PaO2 is more predictable based on the O2 saturation. Lower than that and you probably need an ABG.
Some things to remember:
1. If the patient has a low H&H, SPO2 at best only tells you the saturation of available hemoglobin saturated with oxygen. A SPO2 of 99% won't do a patient much good if they don't have the vehicle necessary to facilitate transfer and perfusion.
2. SaO2 ≠ SPO2. SPO2 is taken via a pulse oximeter. (The formulas mentioned above need lab values, not estimates, because ultimately, the pulse oximeter is an estimate with a known deviation of error.
3. PaO2 is the heavy weight when talking about arterial content of oxygen. Period. It literally stands for partial pressure of oxygen in the artery.
4. While your patient could have done well with titration of FiO2, your rationale was flawed. It's always good to know why. RT school through the Army was much harder than Nursing school was (personally). We spend the equivalent of a semester on hemodynamics. Some RTs don't work in a function where they use this information daily, but some of us do. Plus, I cannot speak for RTs trained outside of the military.
5. Using the oxygen dissociation curve shows us a major flaw if the physiology isn't fully understood. Notice how there is little change when the SPO2 AND PaO2 are above 95%.
Last edit by RT->CRNA on Mar 26
Slavish adherence to the minutia of the physiology without taking into account the circumstances of the particular patient is naïve. Tens of thousands of people get extubated in ORs and ICUs every day without ABG's. A COPDer that is a retainer is not the elective surgical patient with OSA that got too much narcotic or muscle relaxant or got too cold to extubate.
Sometimes you need a gas, a lot of times you don't.
True, but that isn't what I did. I stated that your patient could have done well, mostly because I don't know the patient, history, or situation.
I was merely explained the thought process that the RT mostly likely was using. We hardly use gasses in my ICU as well. I was addressing your rationale regarding the equations.
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