Friday, 8 August 2008

How do you calculate the oxygen saturation percent?

Well, you usually just measure it. Measuring oxygen saturation (SaO2) by means of pulse oximetry is non-invasive, dirt cheap, quick, and easily available. Also, almost any arterial blood gas analysis will provide the saturation percent for you.

But sometimes (actually, mostly in exams!) you do need to calculate the figure. To get a rough idea, you need only two things - the arterial partial pressure of oxygen (PaO2) in the patient, and the oxygen-haemoglobin dissociation curve. You need the latter because there is no easy way to know how saturated the haemoglobin will be at a given PaO2, since the curve looks like this:
If you do have that curve handy (which is unlikely), or you have some automated way of converting from PaO2 to SaO2, you'll quickly get the rough answer. There's an online version that'll do the work for you here. Alternatively, if you want your own list of figures to print out or consult, it can be found here.

However, you may recall that the haemoglobin's affinity for oxygen is modified by several factors - the pH, the temperature, and the concentration of the red blood cell metabolite 2,3 DPG. Usually, the effect of this is small, but if you really want to take these effects into account, there's an online applet that accounts for the first two variables available here.

6 comments:

  1. Jeremy, (saw you at Stephen's blog)
    what do you know about Hashimoto's Thyroiditis?

    More Specifically:
    1.) management by TSH only. Should you not also monitor T3 and T4? I here that patients benefit greatly from T3 replacement.
    - I here that many people with Hashi's, even though they may be within normal range (TSH), are still nonetheless symptomatic.
    2.) Is there a benefit between sythetic hormone vs. natural?
    3.) Because hashi's is chronic, why waste the time with a lifetime of monitoring and dose changes, why not simply remove the thyroid all together?
    4.) I here there are advances on the way to cure auto immune disease, do you know anything about this?

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  2. One more thing,
    what about adrenals and cortisol as they relate.

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  3. Hi Andrew,

    Thanks for the good questions (it really helps that you gave specifics, 'cause this topic is BIG!)

    I've tried to answer them in my next post. If you're still unsure, leave a comment there and I'll get back to you.

    Jeremy

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  4. calculate the arterial and venous O2 content for an anemia patient who have a hemocrit=20% and PaO2=100mmhg.

    calculate the arterial and venous O2 content if the patient PaO2=500mmhg and hemocrit=40%

    Does anyone know how to do this

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  5. Hmm anonymous, I'm not sure that you have enough data to calculate oxygen content in total, let alone the arterial vs venous components.

    My post on blood oxygen content (as opposed to haemoglobin saturation) is available here.

    As it states, you need to know 3 variables for the calculation: Hb, % saturation, and PaO2. Of these, the question above only gives the PaO2.

    You could assume certain things though to make an estimate of the blood's oxygen content:

    (1) If the oxygen-haemoglobin dissociation curve (see above) is normal in the above patients, then you can assume that a PaO2 of 100 saturates around 98% of the haemoglobin, and a PaO2 of 500 saturates 100% of the haemoglobin.

    (2) If the mean corpuscular haemoglobin concentration (MCHC) is further assumed to be normal, you can estimate the haemoglobin concentration from the haematocrit:

    Hb = Hct * MCHC

    Assuming a normal MCHC of 32-36 g/dl (I've chosen 35 for convenience), then the first patient's Hb is about 7 and the second patient's Hb is about 14.

    Whew! What a lot of assumptions!! Anyway, if all the above can be used, then you just have to plug the figures into the equation that I linked to above.

    My results are roughly:
    (a) Patient 1 - 9.6 ml O2/100 ml
    (b) Patient 2 - 20.6 ml O2/100 ml

    These figures are the ARTERIAL oxygen contents. To know the venous content, you'd have to also know the body's oxygen utilisation, which is impossible from the above figures.

    Hope that helped!

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  6. (HbO2)
    ------------------------ x 100% = oxygen saturation
    (HbO2)+(HO2)

    ReplyDelete