Preclinical and clinical investigations strongly indicate that hyperbaric oxygen (HBO2) is physiologically active in improving the destructive processes in severe Traumatic Brain Injury (TBI). However, prior to a definitive efficacy study, important information is required regarding optimizing the HBO2 treatment paradigm instituted in terms of pressure and whether NBH enhances the clinical effectiveness of the HBO2 treatment. Preclinical investigators working with TBI models have used pressures varying from 1.5 to 3.0 atmospheres absolute (ATA). Clinical investigators have used pressure varying from 1.5 to 2.5 ATA. However, the lungs in severe TBI patients have frequently been compromised by direct lung injury and/or acquired ventilator pneumonia and are susceptible to oxygen (O2) toxicity. Working within these constraints, it is essential to determine the most effective HBO2 treatment parameters without producing O2 toxicity and clinical complications. This proposed clinical trial is designed to answer these questions and to provide important data to plan a definitive efficacy trial.