Ischemic brain damage is found in 80% of patients who die from severe head injury and studies have shown that early, transient cerebral hypoperfusion of unknown origin is present in about 40% of these patients. In several early research studies, it is documented that about one-third of severe head injured patients have reduced brain oxygen tension (<25 mm Hg) especially during the first 6 to 12 hours following severe head injury. In this group of patients with low brain oxygen, the clinical prognosis is poor with death being a frequent outcome.