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... Read More

  1. by   AliNajaCat
    With regards to your interpretation of the PaO2 being bad at 60, despite an SpO2 of 95%, some clarity is important.

    That's not what he said. He said,

    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.
    I also took altitude physiology and worked in this field for a number of years, and while your description of how to calculate alveolar oxygen is true, when you run the numbers for PaO2 at sea level, it is true that for clinical purposes of useful estimation, PaO2 is approximately 4-5x FIO2. Of course the precision will be affected by changes in pH, temperature, and DPG, and we're not talking about Quechuas at 14,000 feet, LOL where the atmospheric pressure of oxygen is a fraction of what it is at sea level.

    The explanation from nurseprnRN is useful for clinical practice and is certainly clear for students trying to grasp the very real difference between PaO2 and SpO2.
  2. by   DrEffinSteamy
    Oh, I see, I wrote 95% when I intended to write 90% at 60 mm HG. My mistake. As for the rest, I don't believe I disputed the practical application for the calculation based on FIO2. I also commended his explanation for being one of the best I have seen.

    That said, the statement was made that an SpO2 of 90% means a PaO2 of 60, so I think it was appropriate for me to point out that the correlation isn't that straight forward, and critical thinking must take into account other factors affecting affinity, including BPG, CO2, temperature, and blood pH, and that SpO2 can not substitute ABG's.