Serum bicarb - CO2?

Cowboyardee · Sep 14, 2016

Because serum CO2 is essentially a measurement of bicarb.

Bicarb is indeed HCO3, but it's not transported in the body in that form. It is readily changed back and forth in the body from bicarb (HCO3) to carbonic acid (H2CO3, picking up a hydrogen ion) to carbon dioxide and water (H2O + CO2).

In the arterial system, CO2 partial pressure tells you information about the respiratory system's ability to ventilate (expel CO2) - arterial blood has recently been to the lungs and has not yet supplied the rest of a body's tissues. Venous CO2 is indicative of metabolic processes - venous blood has already interacted with a body's tissues and is heading back toward the lungs.

NurseyB · Sep 16, 2016

Because serum CO2 is essentially a measurement of bicarb.
Bicarb is indeed HCO3, but it's not transported in the body in that form. It is readily changed back and forth in the body from bicarb (HCO3) to carbonic acid (H2CO3, picking up a hydrogen ion) to carbon dioxide and water (H2O + CO2).
In the arterial system, CO2 partial pressure tells you information about the respiratory system's ability to ventilate (expel CO2) - arterial blood has recently been to the lungs and has not yet supplied the rest of a body's tissues. Venous CO2 is indicative of metabolic processes - venous blood has already interacted with a body's tissues and is heading back toward the lungs.

thank you for explaining this so simply. It's one of those things I "knew" but could never articulate. ðŸ‘ðŸ»

offlabel · Sep 27, 2016

Total CO2 and calculated -HCO3 are not the same thing and are not interchangeable, especially in the presence of a metabolic acidosis. It get's confusing when some abg results report both.

Cowboyardee · Sep 28, 2016

Total CO2 and calculated -HCO3 are not the same thing and are not interchangeable, especially in the presence of a metabolic acidosis. It get's confusing when some abg results report both.

I believe the question is whether a serum bicarbonate (listed as CO2 on a BMP) is the same thing as an arterial bicarbonate value (listed as HCO3 on an ABG). I understand that the two are not, strictly speaking, measuring the exact same thing. A serum bicarb is also sometimes called either a measured bicarb or, as you wrote, a total CO2 and does contain some information about the total CO2 dissolved in venous blood, the great majority of which is actually bicarb, but not quite the entirety. An arterial bicarb, meanwhile is sometimes referred to as a calculated bicarbonate value, since it is calculated using the pH and paCO2 values rather than measured directly.

With all that said, the two are so closely related that you might as well think of them as the same value. As I mentioned above, the vast majority of serum CO2 really is in fact bicarbonate. While there can be differences between the two values in some patients, these patients are quite rare. Here is a summary of one study in which over 17,000 sets of lab values are analyzed, and serum CO2 and aterial bicarbonate were found to be significantly different only 0.65% of the time:

https://www.researchgate.net/file.PostFileLoader.html?id=56feeb753d7f4b8e2744d070&assetKey=AS%3A346179352186885%401459546997618

In practice, a serum CO2 is often one of the first signs I see that someone is experiencing acid-base irregularities. It doesn't tell me everything that an ABG does, but along with the anion gap it is a good early alert (and somewhat less painful to collect than an ABG).

offlabel · Sep 28, 2016

I believe the question is whether a serum bicarbonate (listed as CO2 on a BMP) is the same thing as an arterial bicarbonate value (listed as HCO3 on an ABG). I understand that the two are not, strictly speaking, measuring the exact same thing. A serum bicarb is also sometimes called either a measured bicarb or, as you wrote, a total CO2 and does contain some information about the total CO2 dissolved in venous blood, the great majority of which is actually bicarb, but not quite the entirety. An arterial bicarb, meanwhile is sometimes referred to as a calculated bicarbonate value, since it is calculated using the pH and paCO2 values rather than measured directly.
With all that said, the two are so closely related that you might as well think of them as the same value. As I mentioned above, the vast majority of serum CO2 really is in fact bicarbonate. While there can be differences between the two values in some patients, these patients are quite rare. Here is a summary of one study in which over 17,000 sets of lab values are analyzed, and serum CO2 and aterial bicarbonate were found to be significantly different only 0.65% of the time:
https://www.researchgate.net/file.PostFileLoader.html?id=56feeb753d7f4b8e2744d070&assetKey=AS%3A346179352186885%401459546997618
In practice, a serum CO2 is often one of the first signs I see that someone is experiencing acid-base irregularities. It doesn't tell me everything that an ABG does, but along with the anion gap it is a good early alert (and somewhat less painful to collect than an ABG).

Oh, right...my assumption was ABG from the get go, not the BMP...I will say that in the patients that I serve a difference is quite common. The massive blood loss and profound tissue ischemia that is routine in major vascular surgery makes the separation of these two values very evident.