VBG vs ABG in neurosurgery

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ethilo

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I had a case of a ruptured aneurysm clipping on a patient with COPD yesterday with a poorly functioning A-line. We hyperventilated to an ETCO2 of 19 that correlated to a pCO2 of 32 by A-line. Shortly thereafter we lost the A-line because it just wouldn't draw back. We thought we'd just go off of the EtCO2 at that point and would be fine.

On closing, the surgery team noted the brain was too swollen to close the dura. They demanded a pCO2 which we couldn't get off of the A-line. We tried a VBG thinking it would correlate roughly with a ABG. The VBG demonstrated pCO2 of 41 with EtCO2 at 21 which made us scratch our heads. Since the brain was swollen without another clear reason, we hyperventilated to EtCO2 of 16, repeated, and pCO2 off of VBG was 41 again. We hyperventilated even more down to EtCO2 of 12 and eventually got an A-line sample with much effort which demonstrated a pCO2 of 22.

Needless to say, we backed off hyperventilating. to the original approximate goal pCO2 30-35. We gave another dose of mannitol which shrunk the brain enough for them to close.

I have been under the impression a pCO2 from a VBG should be generally in the ballpark of the ABG. I know that with higher pCO2s it starts to not hold true anymore.

Can someone tell me why ABG and VBG values should get different at higher and lower than normal pCO2s?

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The only VBG that is in the same ballpark as an ABG is the one obtained from the dorsum of the hand, and that's in well-perfused patients.
 
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I had a case of a ruptured aneurysm clipping on a patient with COPD yesterday with a poorly functioning A-line. We hyperventilated to an ETCO2 of 19 that correlated to a pCO2 of 32 by A-line. Shortly thereafter we lost the A-line because it just wouldn't draw back. We thought we'd just go off of the EtCO2 at that point and would be fine.

On closing, the surgery team noted the brain was too swollen to close the dura. They demanded a pCO2 which we couldn't get off of the A-line. We tried a VBG thinking it would correlate roughly with a ABG. The VBG demonstrated pCO2 of 41 with EtCO2 at 21 which made us scratch our heads. Since the brain was swollen without another clear reason, we hyperventilated to EtCO2 of 16, repeated, and pCO2 off of VBG was 41 again. We hyperventilated even more down to EtCO2 of 12 and eventually got an A-line sample with much effort which demonstrated a pCO2 of 22.

Needless to say, we backed off hyperventilating. to the original approximate goal pCO2 30-35. We gave another dose of mannitol which shrunk the brain enough for them to close.

I have been under the impression a pCO2 from a VBG should be generally in the ballpark of the ABG. I know that with higher pCO2s it starts to not hold true anymore.

Can someone tell me why ABG and VBG values should get different at higher and lower than normal pCO2s?

I wouldve replaced that arterial line. but anway, it seems like you had a PaCO2 of 32 when ETCO2 is 19... then later on the in case you had EtCo2 of 12 and ABG of 22. SO ABG seems to be about 10-13 higher, which isn't a huge difference.
With the VGB studies have shown about +5.7 difference in CO2 in normocapnia but doesn't correlate well when not in normocapnia.. In some studies they actually found vbg had a lower co2 than abg..

I dont think there is an accepted explanation, i imagine there are many variables.. did ventilation change between the ABG and VBG sample? where were the samples taken? how long did you tourniquet??? how long did you wait before running them? were their any air bubbles (air has pco2 of 0) .etc etc
 
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I wouldve replaced that arterial line. but anway, it seems like you had a PaCO2 of 32 when ETCO2 is 19... then later on the in case you had EtCo2 of 12 and ABG of 22. SO ABG seems to be about 10-13 higher, which isn't a huge difference.
With the VGB studies have shown about +5.7 difference in CO2 in normocapnia but doesn't correlate well when not in normocapnia.. In some studies they actually found vbg had a lower co2 than abg..

I dont think there is an accepted explanation, i imagine there are many variables.. did ventilation change between the ABG and VBG sample? where were the samples taken? how long did you tourniquet??? how long did you wait before running them? were their any air bubbles (air has pco2 of 0) .etc etc

A-line was in L forearm. I agree would have been nice to place a fresh A-line but they were in dura with microscope so any touching of patient caused an earthquake in their field. I thought about insisting the surgeons pause but at that point we didn't really think the A-line was really totally necessary.

First VBG was off of a 16g saphenous without tourniquet used. 2nd VBG I placed a fresh 18g PIV in L hand without a tourniquet used, drew a sample of blood off of it and sent it to the gas analyzer right away.
 
To get a 20 mmHg a-v pCO2 difference in the dorsal arch, that guy must have had some crappy peripheral perfusion (chronic or acute).

The lesson for all of us is: the only thing you can say about a VBG is that the ABG won't be worse than it.
 
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Gradient of 13 between ETCO2 and arterial CO2.

Gradient of 20 between ETCO2 and venous CO2.

That doesn't seem too surprising to me.

25. Etco2 was 16 and vbg 41.

My guess is his perfusion sucked. Maybe his BP wasn't super high or the low pco2 was causing some constriction thus worsening perfusion.
 
A tight brain with an ETCO2 of less than 20 is attributed to hypercarbia? Especially after correlating in previously with an ABG? Different discussion, but that's a reach.
 
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If you were able to get a second (unnecessary) IV in the hand, why not just a radial arterial stick instead.

Edit: I guess other hand had to be accessible.
 
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The one thing i like about neurosurgery is a lot of the times we get teh feet in front of us so easy access to IVss or arterial line replacement if needed
 
The one thing i like about neurosurgery is a lot of the times we get teh feet in front of us so easy access to IVss or arterial line replacement if needed
That and pedi are good places to develop those skills.
 
Yea, if you have that much concern about teh VBG results, then just untuck one of the arms and either place another a-line or get jut stick once for gas sample. Or liek others have said, the patient's feet are right in front of you and you should have lots of room to work to get either a stick or new a-line. Get an ultrasound if needed.
 
Yea, if you have that much concern about teh VBG results, then just untuck one of the arms and either place another a-line or get jut stick once for gas sample. Or liek others have said, the patient's feet are right in front of you and you should have lots of room to work to get either a stick or new a-line. Get an ultrasound if needed.

I think this is much easier said than done. What about the big instrument table that covers the patient?

The feet may be easy except when the patient is obese, diabetic, vasculopath, etc.
 
Not saying it's the easiest thing in the world just seems like a good option. Didn't sound like any was attempted. Ultrasound helps with these types of patients.
 
In the first place, I question the safety of aggressive hyperventilation to reduce intracranial arterial blood volume in the context of a swollen, injured brain. You have lots of maneuvers to reduce intracranial venous blood volume and interstitial fluid volume.

If hyperventilation based on paCO2 is truly that important, then they can wait to close dura while you replace the arterial line. Most likely, the art line is being semi-kinked while the patient's arms are tucked. Often this is because a radial line is too distal. I know you said this one was in the forearm.
 
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