[piper77]

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?

[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.

[piper77]

Thanks, sweetdreams! That was helpful. I should have remembered Henderson-Hasselbach...but it's been a few years since chemistry...... Thanks again.

[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...

[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.

[ticu]

I could not agree more Brenna's Dad!

[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.

[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.

[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...

[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!