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When SpO2 values dont' match PaO2

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Can someone please help me understand how an SpO2 value can be 95 while the PaO2 reads as low as 40 to 60. I know SpO2 is Hgb saturation% and SaO2 is O2 pressure in plasma. How can the values in this patient be so off in relation? Does this mean a perfusion/ventilation problem. I am a new grad nurse working in PCCU cardiac.

CoolDork

Specializes in ED, SICU. Has 5 years experience.

The physiology of SaO2 and SpO2 is totally different. SaO2 is directly related Hgb and how saturated the blood is with O2 molecules. SpO2 is the measurement of dissolved O2 molecules in the blood/plasma and solely dependent on lung functioning. Therefore, a patient can have an adequate SaO2 (saturation of O2 on Hgb) but in general the lung functions can be dec. causing a low PaO2. The patient needs oxygen.

In conclusion, PaO2 is dependent on lung functioning and SaO2 is dependent on SpO2(dissolved O2 in the blood that eventually binds to Hgb).

Thank you! I did find Dr. Martin's information last night after sending the message and he does explain it so well. Thank you in addition for your extra articles. Much appreciated.

Thankyou so much for the information. Helpful.

You also must clarify if the SPO2 on the blood gas is calculated or done on a co-oximeter. Standard blood gases will not include the actual measured SaO2. Many places will not run or report the co-oximeter values unless specifically ordered due to the charge. But, some places will include the co-oximeter value or you can ask the RT for it. If lab ran it, you probably won't get that value without an order.

GrannyRRT said:
You also must clarify if the SO2 on the blood gas is calculated or done on a co-oximeter. Standard blood gases will not include the actual measured SaO2. Many places will not run or report the co-oximeter values unless specifically ordered due to the charge. But, some places will include the co-oximeter value or you can ask the RT for it. If lab ran it, you probably won't get that value without an order.

I quoted my previous post with some corrections. I really must disable autocorrect.

If the ABG is done without a Co-oximeter measurement, you cannot see what else attached to the hemoglobin.

A smoker could have an SpO2 of 98% and be complaining of shortness of breath. The PaO2 may also come back low. A co-oximeter might measure the SaO2 at 85% and 10 - 13% COHb. The patient could also have a low hemoglobin in combination with the COHb. I prefer to print out various calculations associated with the blood gas such as content and A-a. But, some places feel that is a waste or too confusing which I find that insulting to those working in critical care.

Another example would be a young woman who had taken to much OTC Pyridium for a UTI. This would lead to Methemoglobinemia. A standard blood gas may actually look fairly normal but a co-oximeter will tell a different story.

The typical blood gas interpretation class is only an introduction. If you work in critical care you need much more detail such as what is discussed in Diana ' s link. Correlation with the anion gap and other lab values is very important also. The standard ABG by itself only provides a small amount of info and really does not give a broad enough clinical picture necessary for appropriate tx.

Thank you. Very ineresting and helpful information. Especially interesting with the anion gap. I was looking into that while studying up after my post. The next thing I run into is that CO2 in serum is really HC03. I was discussing this at work last night with my preceptor but she wasn't aware, rather looking at serum C02 and assuming it was acidic rather than the other way around. I wonder how many people get that wrong. That point confused me in nursing school because C02 serum had the same normal values as HC03 now I see why. Please can you remind me of the significance of the anion gap. Isn't it Na+K - something = anion gap.

Edited by wannadowell
left out information.

Interpretation of Arterial Blood Gases (ABGs)

The above link is much of what you need to know about the anion gap and MUDPILES.

Below is a very important quote from that website.

Remember an acidosis or alkalosis may be present even if the pH is in the normal range (7.35 – 7.45)You will need to check the PaCO2, HCO3- and anion gap.