Low saO2, normal sO2?

  1. 0
    Background: ventilated burn patient. No real medical history per family. (So don't know if this is reliable or not.)

    saO2 is reading 86%-88%. It's reliable -- same in every conceivable extremity, good waveform, heart rate is matching his leads. However, this guy's ABGs are all within normal limits with his sO2 between 98%-100%. His CBC was also normal.

    Any thoughts on what could cause this?
  2. 4,048 Visits
    Find Similar Topics
  3. 7 Comments so far...

  4. 0
    Was a carboxyhemoglobin run?
  5. 0
    Quote from ckh23
    Was a carboxyhemoglobin run?
    Yes, and it was very low.

    But a +carboxy level leads to artificially *high* saO2 readings.

    I'm wondering if maybe he had a shunt? Talked to my father who is a doc and he said that could be a possibility, but the resulting heart murmer would probably be heard across the room.

    I imagine that the mystery will be solved next time I go into work so the question is purely academic.
  6. 0
    Pt has a condition called Methemoglobinemia.

    Question answered.
  7. 0
    Interesting, never heard of that. When in doubt though, an ABG is always more reliable than the SpO2.
  8. 0
    Methemoglobinemia was the first thing I though of when reading this case, I'm glad to know I was right; though I had a similar case not to long ago.

    I thought I would add resource for people who would like to learn more on this:
    http://emedicine.medscape.com/article/815613-overview


    the reason your SpO2 was low was because your patient probably had low-level Methemoglobinemia which results in low readings. Whereas, high-level Methemoglobinemia would have resulted in a normal false SpO2 reading.

    Important Note**** this is a great example of how the SaO2 we get from our ABGs are not always accurate. SaO2 from ABGs are usually calculated not measured from our measured PaO2 ( a measured SaO2 would come out low). However, a PaO2 will come out normal with Methemoglobinemia, beause it is normal.

    The issue with Methemoglobinemia is not the amount of O2 but the change in our hemoglobins affinity for O2. However, the problem for the patient is the same. No O2 to tissues = hypoxia and death.

    Great case !! thanks for sharing.
  9. 0
    If the link does not work its probably because you need an account (which is free). Go to medscape and search "Methemoglobinemia in emergency medicine."
  10. 0
    Quote from dirtyhippiegirl
    Background: ventilated burn patient. No real medical history per family. (So don't know if this is reliable or not.)

    saO2 is reading 86%-88%. It's reliable -- same in every conceivable extremity, good waveform, heart rate is matching his leads. However, this guy's ABGs are all within normal limits with his sO2 between 98%-100%. His CBC was also normal.

    Any thoughts on what could cause this?
    SaO2 is the measured value and the "a" stands for arterial. If you are moving a pulse ox probe around and looking at a waveform you are measuring the SpO2.

    If you are looking at SO2 on the ABG then you are looking at a calculated value. It is calculated from the pH and PaO2 with an algorithm which assumes normal O2 affinity, normal 2,3-diphosphoglycerate concentratons and no dyshomglobins or hemoglobinsopathies.

    If it is a true SaO2 reading from a co-oximeter machine you will have the most accurate number.

    Someone with severe carboxyhemoglobin or MethHb may have a very normal ABG if it is not ran with a co-oximeter.

    Many labs will run the co-oximeter values with the ABG but will only report what is ordered. The exception might be a Respiratory Therapist ran lab but then they will have to get an order to place it in the chart. Usually they have no problem writing a standing protocol order for that.








Top