SpO2 vs. SaO2 ? - page 2

My instructor's lecture notes state that SpO2 refers to pulse oximetry, and that SaO2 refers to "peripheral capillary saturation". It also says that PaO2 is an invasive procedure used to determine the arterial percentage of... Read More

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    You may already know this but SaO2 and SpO2 do not measure the same thing. SaO2 is an invasive measure of the % of Oxyhemoglobin(oxygen saturated hemoglobin) in the blood using lab tests on arterial blood. SpO2 noninvasive measure the % of saturated hemoglobin in the capillary bed and does not identify what is saturating the hemaglobin. ie: carboxyhemoglobin, methemoglobin, ect. Typically(in a healthy pt) SaO2 and SpO2 measure the same thing, but a difference can be found in patients with conditions such as CO poisoning and rhabdomyolysis.

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    it's important to know that saturation is a measure of the percent of rbcs which are carrying oxygen (saturated). so if your s(aturation)p(peripheral)o2 is 98%, then 98% of the rbcs in your peripheral blood (capillaries) carry oxygen on them.

    it's important to know that p(pressure) a(arterial) o2 is the measure of oxygen dissolved in the blood (on the blood cells and to some extent in the serum), most often measured in an arterial blood sample. this has to do with the amount of oxygen that gets out of the air that's breathed into the alveoli and into the blood on the other side of the alveolar capillary bed; it can be increased by increasing the amount of oxygen breathed in (supplemental oxygen) and decreased by decreased alveolar function (like in a pulmonary disease, pneumonia, drowning, etc). it says nothing about the percentage of rbcs carrying oxygen.

    (there's a way to tell how bad your lungs are by looking at the difference between the pao2 that a given supplemental oxygen level should give you if your lungs were normal, and the actual pao2 your crappy diseased lungs actually allow your blood to carry. quick and dirty, your pao2 should be roughly 4-5x the % of oxygen you're breathing; normal pao2 is 80-100 if room air is around 20% oxygen. if you breathe 100% oxygen, your pao2 should measure around 400-500. if it doesn't, your lungs are doing a lousy job of gas exchange. same with any other supplemental level. this is usually called "fio2," "fraction of inspired oxygen;" breathing 60% o2 is an fio2 of .6 and 100% is 1.0, but i digress.)

    the reason that a lot of people get these confused is because normal saturation is 95-100% on room air, and normal pao2 is 80-100 torr or mmhg (note, this is not a percent, it's a pressure measurement) on room air. as your saturation goes down, your pao2 goes down too. please look at this graph:


    dang, i wanted that to appear here but you'll have to open it in another window and follow along.this is called the oxyhemoglobin dissociation curve-- it shows the relationship between % saturated hemoglobin and blood oxygen.

    it has the % saturation on the up-down axis and the pao2 (po2 in this graph, same thing for purposes of this discussion) on the side-side axis. as you can see, if your sat is around 95-100%, your pao2 is like 80-100 mmhg (or torr), normal. but if your sat is 90%, your pao2 is down around 60-ish, which is pretty bad, and if it's 80%, your pao2 is in the 40s, which is not really compatible with life for very long at all.

    you can see how if somebody said, "oh, his spo2 is 89, that's fine," she is really wrong, he is not fine.
    i hope this helps you see the relationship between the two and why that is so.

    last.... think about hematocrit. someone with a 98% saturation has 98% of his rbcs carrying oxygen. that's great. but someone in bed a with a hematocrit of 12 is carrying one-third the amount of oxygen to his cells than the guy in the next bed b with a hematocrit of 36, because he only has one-third as much hemoglobin to carry oxygen on. both sats of 95%, but bed a has cells that are getting short on o and bed b is fine. so look at both to see how well your patient is oxygenating his body.

    hope this helps.
    NRKB likes this.

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