A systematic review of pre-hospital intubation for head injured patients failed to show evidence of benefit of tracheal intubation or invasive ventilation. The authors acknowledge the lack of methodological quality in the studies reviewed and the predominance of US paramedic-delivered intubations without the use of anaesthetic drugs.

Pre-hospital tracheal intubation in patients with traumatic brain injury: systematic review of current evidence
Br J Anaesth. 2009 Sep;103(3):371-86