Irregardless of herniation, a head injured patient should have their EtCO2 maintained at the low end of normal (35-37mmHg).
I had a patient a few weeks ago... was a younger male pt. kicked in the head during a fight and initially refused EMS care. EMS was called back a little while later when pt. was unresponsive and seizing. Pt. had a subarachnoid bleed and was intubated in the ED and my unit was called to transfer this patient to a trauma center.
Long story short... transfer was about two hours with morning, rush hour traffic, and sending facility did not provide a vent. So we had no choice but to bag the patient for two hours. Using waveform capnography, we were able to maintain an EtCO2 of 35-37mmHg throughout care.
When we got to the trauma center, I overheard one of the trauma docs comment, "his ABG's really aren't that bad for being bagged for several hours". I just wanted to use this experience as an example of how useful EtCO2 monitoring is and how important it is to pay close attention to assisted ventilation especially in a head injured patient.