ABG's

  1. I have an ABG question - sorry if it seems simple, but I'm having trouble researching it....

    Any of the hospitals I have worked at always sent back an HCO3 level with the rest of the gas numbers. My new place of employment does not...when you send a gas, you just get the PaO2, PaCO2, and the pH. My question is: is the HCO3 always a calculated number and not actually measured? (I always thought it was measured...ooops..) If thats so, is there a good reference for me to figure out how to calculate the bicarb level (when I asked at work, they just said "we just use the serum CO2 level as an approximation". Just trying to sort it out.......thanks for the point in the right direction....

    PS...... looking in the ICU Book I'm guessing that one can use the formula:

    24 X (PCO2/HCO3) = H+

    (then take the negative log of H+ ....pH); rearranging the formula you can derive the bicarb from the PCO2 and pH.... is that correct?
    •  
  2. 21 Comments

  3. by   sweetdreams
    Mt SinaiMedical Center; link at http://www.mtsinai.org/pulmonary/papers/eq/eqhen.html
    has some good info on the Henderson-Hasselbalch equation used to calculate HC03. Also includes a short/simplified version for approximating the value.

    Hope this helps.
  4. by   piper77
    Thanks, sweetdreams! That was helpful. I should have remembered Henderson-Hasselbach...but it's been a few years since chemistry...... Thanks again.
  5. by   Tenesma
    as far as ABGs go - you should ask yourself what information you are looking for... HCO3 is a calculated number - and therefore is about as useless as Cardiac Index (instead of Cardiac Output) or SVR .... if i were you i would stick with ph/pco2/po2 as that gives you all the information you need for management of oxygenation/ventilation, etc...
  6. by   Brenna's Dad
    I not sure why you think CI is a useless number Tenesma. After all, one can only derive CI from an actual CO measurement.

    I'm also thinking that HCO3 levels are also helpful. It gives you an easy indication of the metabolic acidosis that may be occuring with your patient.
  7. by   ticu
    I could not agree more Brenna's Dad!
  8. by   nilepoc
    I imagine, that there really isn't enough time to do the math once youa re behind the ether screen. Also, a CI is a five to ten minute old number, and should be treated as such. It is calculated over a period of time, to improve its accuracy, an instantanious value is not that indicative of function.

    Like tenesma says, you can treat what you need to treat with three simple numbers, and not resort to algebra.
  9. by   g8rlimey
    The age of CI depends on the type of monitoring unit and Swan that is being used. The newest units update all the time, hence the CI is at the most, 30-60 seconds old, and considering the speed at which most inotropes/pressors act, is ample time to base clinical adjustments.

    As far as HCO3 goes, I have to agree with BDad--the more information available really helps to zero in on specific problems. Of course, I am speaking from an ICU standpoint, with no experience on the ether screen side of things.

    I understand the prejudice to derived measurements, and I always rely first on assessment, but those derived msmt's really help put together the whole picture in complex processes.
  10. by   ticu
    Of course g8rlimey, you are totally right.

    I am an ICU nurse on modern cardio-thoracic unit, I too rely (among many other things) on CI and HCO3 and my patients are better off because of that...
  11. by   Squirrel
    I do not mean to get off of the original question, but this makes me wonder if you can use the CO2 level from the comprehensive metabolic panel (Chem 21) in place of the HCO3 level to calculate the anion gap?

    I think that the CO2 level is 1-2 units more than the HCO3 level from the ABG.

    Does anybody use the anion gap?

    Thanks!
  12. by   g8rlimey
    I have only used the anion gap during DKA and sometimes burns.
  13. by   MTM
    As far as I know, you can substitute the serum CO2 for the bicarb value. At least that was how they did it at Mass General. Here's a little excerpt from the above mentioned link:

    Most hospitalized patients have at least one bicarbonate measurement as part of routine serum electrolytes; this is usually called the 'CO2' or 'total CO2' when measured in venous blood. (Total CO2 includes bicarbonate and the CO2 contributed by dissolved carbon dioxide, the latter 1.2 mEq/L when PaCO2 is 40 mm Hg. For this reason, and because bicarbonate concentration is slightly higher in venous than in arterial blood, total CO2 runs a few mEq/L higher than the bicarbonate value calculated using the H-H equation.)
  14. by   Tenesma
    reasons why CI is useless: it is a calculated with the body surface area... now please tell me how accurate body surface areas are??? now let's say somebody has bilateral BKAs, now what is the body surface area (since you are using height as part of that calculation as well)... here is the example: two people have the exact same heart w/ the same cardiac ouput but one person has no legs or might be missing arms... their CI will be very different!! (if you are using the true body surface area)... but my suspicion is that when you guys do CI you just enter height and weight and let the software do the work for you.... remember these numbers are just numbers and you have to use the numbers AND clinical judgement that are going to be the most useful for you...

    by the way you don't need to know the HCO3 to know if somebody has a metabolic acidosis - HCO3 is an added (calculated) number that people in the 80s thought would be more useful, just like some hospital labs add the Base Excess calculation to their ABGs... how often does the Base Excess assist you???

close