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Aeromedical evacuation-relevant hypobaria worsens axonal and neurologic injury in rats after underbody blast-induced hyperacceleration

Proctor, Julie L.; Mello, Kaitlin T.; Fang, Raymond MD; Puche, Adam C. PhD; Rosenthal, Robert E. MD; Fourney, William L. PhD; Leiste, Ulrich H. PhD; Fiskum, Gary PhD

Journal of Trauma and Acute Care Surgery: July 2017 - Volume 83 - Issue 1 - p S35–S42
doi: 10.1097/TA.0000000000001478
Applied and Translational Study

BACKGROUND Occupants of military vehicles targeted by explosive devices often suffer from traumatic brain injury (TBI) and are typically transported by the aeromedical evacuation (AE) system to a military medical center within a few days. This study tested the hypothesis that exposure of rats to AE-relevant hypobaria worsens cerebral axonal injury and neurologic impairment caused by underbody blasts.

METHODS Anesthetized adult male rats were secured within cylinders attached to a metal plate, simulating the hull of an armored vehicle. An explosive located under the plate was detonated, resulting in a peak vertical acceleration force on the plate and occupant rats of 100G. Rats remained under normobaria or were exposed to hypobaria equal to 8,000 feet in an altitude chamber for 6 hours, starting at 6 hours to 6 days after blast. At 7 days, rats were tested for vestibulomotor function using the balance beam walking task and euthanized by perfusion. The brains were then analyzed for axonal fiber injury.

RESULTS The number of internal capsule silver-stained axonal fibers was greater in animals exposed to 100G blast than in shams. Animals exposed to hypobaria starting at 6 hours to 6 days after blast exhibited more silver-stained fibers than those not exposed to hypobaria. Rats exposed to 100% oxygen (O2) during hypobaria at 24 hours postblast displayed greater silver staining and more balance beam foot-faults, in comparison with rats exposed to hypobaria under 21% O2.

CONCLUSION Exposure of rats to blast-induced acceleration of 100G increases cerebral axonal injury, which is significantly exacerbated by exposure to hypobaria as early as 6 hours and as late as 6 days postblast. Rats exposed to underbody blasts and then to hypobaria under 100% O2 exhibit increased axonal damage and impaired motor function compared to those subjected to blast and hypobaria under 21% O2. These findings raise concern about the effects of AE-related hypobaria on TBI victims, the timing of AE after TBI, and whether these effects can be mitigated by supplemental oxygen.

Supplemental digital content is available in the text.

From the Department of Anesthesiology (J.L.P., K.T.M., GF.), University of Maryland School of Medicine, Program in Trauma (R.F., R.E.R.), R. Adams Cowley Shock Trauma Center, University of Maryland Medical Center; U.S. Air Force Center for the Sustainment of Trauma and Readiness Skills (R.F.); Department of Anatomy and Neurobiology (A.C.P.), Department of Emergency Medicine (R.E.R.), University of Maryland School of Medicine, Baltimore; and School of Engineering (W.L.F., U.L.H.), University of Maryland College Park, College Park, Maryland.

Submitted: January 4, 2017, Revised: February 27, 2017, Accepted: March 2, 2017. Published online: April 27, 2017.

This study was presented at the 2016 annual meeting of the Military Health Sciences Research Symposium, August 15–17, 2016, in Kissimmee, Florida.

Supplemental digital content is available for this article. Direct URL citations appear in the printed text, and links to the digital files are provided in the HTML text of this article on the journal’s Web site (

Address for reprints: Gary Fiskum, PhD, Department of Anesthesiology, University of Maryland School of Medicine, 685W. Baltimore St., 5.34 MSTF Baltimore, MD 21201; email:

© 2017 Lippincott Williams & Wilkins, Inc.