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?
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?