Question about the "CO2" level on a BMP/ Chem 6

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Specializes in Critical Care- Medical ICU.

I hope this isn't a silly question, but I'm a new grad on orientation in an ICU and I am having trouble understanding this concept.

So the CO2 level that is given on a BMP isn't actually a measure of CO2, its an indirect measure of bicarb? Why is this? why aren't the CO2 on an ABG and the CO2 on a BMP the same thing?

I tried looking this up but I haven't found anything so I hope someone here can explain to me.

Thanks so much :)

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This will be a short answer. This question comes up often...

Put on your chemistry hats...

1. Carbon dioxide enters the RBC and does two things:

a. combines with hemoglobin CO2 + Hgb => HgbCO2

b. combines with water CO2 + H2O to form carbonic acid (H2CO3) this process is then enhanced by an enzyme within the RBC called carbonic anhydrase (CA) . CO2 + H2O + CA => H2CO3

2. The carbonic acid from step b disassociates into a bicarbonate ion and a hydrogen ion H2CO3 HCO3- + H+

3. The bicarbonate ion leaves the RBC and enters the plasma, allowing the chloride ion to enter the RBC.

4. The hydrogen ion from step 2 binds with hemoglobin H + Hgb => HHgb

To summarize. CO2 is a cellular waste product. When it's dissolved in water and not directly taken care of by the lungs (which 70% of the CO2 is) it forms carbonic acid. This process is quickened by carbonic anyhydrase. Once carbonic acid is formed it immediately breaks down into a bicarbonate ion that floats around extracellularly and a hydrogen ion. Using the measure of bicarbonate ion we can deduce a CO2 "value".

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A great deal of your CO2 in your blood is carried as bicarb, HCO3, because CO2 is readily converted to bicarb. Something on the order of 80% or so if I remember correctly. A BMP measures the bound CO2, which is really bicarb.

ABG's measure unbound CO2 as well as bicarb.

The BMP measure of bicarb is quite useful. There have been a handful of times I've drawn a BMP on someone and noted it was high and assumed some acidosis, and when I drew an ABG I was spot on. You can also look at it for trends in your CO2 retainer populations (COPD, PF, sarcoidosis).

On another note, do you also understand why etCO2 is lower than CO2 from an ABG?

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Specializes in Critical Care- Medical ICU.

No :( Could you please explain to me? I thought that the etCO2 was supposed to be the same or very close to the PAO2.

I really appreciate everyones answers and help!

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No :( Could you please explain to me? I thought that the etCO2 was supposed to be the same or very close to the PAO2.

I really appreciate everyones answers and help!

Yes the etCO2 and PaCO2 are very close. But the etCO2 is usually 3-5mmHg less than PaCO2. Therefore norm PaCO2 35-45, norm etCO2 30-40 or so.

This happens because the total level of CO2 within the body measured by a PaCO2 takes into consideration the CO2 bound up as bicarbonate ions. The etCO2 looks only at exhaled CO2 levels and does not measure bicarbonate ions so you get a lower but still a very useful value.

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I was going to go with the fact exhaled CO2 is mixed with other gases in anatomical dead space (oro/hypopharynx, trachea), and in alveolar dead space not all alveoli are constantly perfused so they can hang on to more CO2, therefore less is exhaled.

It's what I've always been taught by CRNA's I've shadowed and my own research seems to agree with it.

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Specializes in Critical Care- Medical ICU.

Thanks!

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