This research assesses 2 standard care strategies for monitoring and treating patients with sever traumatic brain injury (TBI). Measuring and reacting to intracranial pressure (ICP) (as the brain may swell after injury) has been standard practice in intensive care. This standard practice will be compared against a different strategy of measuring the oxygen level in a small area of the brain (PbtO2) in addition to measuring ICP. In this strategy, doctors try to prevent rises in ICP as well as trying to prevent or minimise any decreases in the brain oxygen; using tiered interventions of certain drugs and dosages, intravenous fluids and management of the breathing machine (ventilator).