Brain hypoxia is exacerbated in hypobaria during aeromedical evacuation in swine with traumatic brain injury

Background: There is inadequate information on the physiologic effects of aeromedical evacuation on wounded war fighters with traumatic brain injury (TBI). At altitudes of 8,000 ft, the inspired oxygen is lower than standard sea level values. In troops experiencing TBI, this reduced oxygen may worsen or cause secondary brain injury. We tested the hypothesis that the effects of prolonged aeromedical evacuation on critical neurophysiologic parameters (i.e., brain oxygenation [P_btO₂]) of swine with a fluid percussion injury/TBI would be detrimental compared with ground (normobaric) transport. METHODS: Yorkshire swine underwent fluid percussion injury/TBI with pretransport stabilization before being randomized to a 4-hour aeromedical transport at simulated flight altitude of 8,000 ft (HYPO, n = 8) or normobaric ground transport (NORMO, n = 8). Physiologic measurements (i.e., P_btO₂, cerebral perfusion pressure, intracranial pressure, regional cerebral blood flow, mean arterial blood pressure, and oxygen transport variables) were analyzed. RESULTS: Survival was equivalent between groups. Measurements were similar in both groups at all phases up to and including onset of flight. During the flight, P_btO₂, cerebral perfusion pressure, and mean arterial blood pressure were significantly lower in the HYPO than in the NORMO group. At the end of flight, regional cerebral blood flow was lower in the HYPO than in the NORMO group. Other parameters such as intracranial pressure, cardiac output, and mean pulmonary artery pressure were not significantly different between the two groups. CONCLUSION: A 4-hour aeromedical evacuation at a simulated flight altitude of 8,000 ft caused a notable reduction in neurophysiologic parameters compared with normobaric conditions in this TBI swine model. Results suggest that hypobaric conditions exacerbate cerebral hypoxia and may worsen TBI in casualties already in critical condition.