Patients admitted to a level 1 trauma centre with traumatic brain injury whose end-tidal CO2 was kept with the Brain Trauma Foundation recommended limits of 30-35 mmHg (3.9-4.6 kPa) had a lower mortality than those whose CO2 was outside this range. The group in which the target was not achieved had a greater injury severity, which may have contribute to the difficulty in optimising ETCO2.

Prehospital Hypocapnia and Poor Outcome After Severe Traumatic Brain Injury
J Trauma. 2009 Jun;66(6):1577-82