44 - Flight Relevant Hypobaria Worsens Neuropathologic and Behavioral Outcomes in Rats Following the Combination of Under Vehicle Blast and Controlled Cortical Impact

Abstract Text:

Introduction: US warfighters exhibit a very high incidence of traumatic brain injury (TBI) that often results in long term neurologic impairment. Most injuries are caused by exposures to blasts, either as an unmounted soldier or present within a vehicle targeted by a land mine. The current study utilized a new rat polytrauma small-scale model of “under-vehicle” blast (B) combined with controlled cortical impact (CCI). This BCCI model extends our characterization of how exposure to aeromedical evacuation-relevant hypobaria can worsen neurologic outcomes (1-5). Hypothesis 1: Exposure to hypobaria (=8000 ft) one day following BCCI worsens cerebral cortex inflammation compared to no hypobaria or to 4000 ft hypobaria. Hypothesis 2: Exposure to moderate hyperoxia (50% O2) during hypobaria worsens outcomes compared to rats maintained under normoxic conditions. Hypothesis 3: Exposure to BCCI results in damage to the cerebellum which is exacerbated by exposure to hypobaria.

Methods: The protocol was reviewed and approved by the University of Maryland, Baltimore Institutional Animal Care and Use Committee (0618003) and the U.S. Air Force Surgeon General’s Office of Research Oversight and Compliance as protocol number FWR-2018-0002A.  These studies were conducted in a facility accredited by AAALAC, in accordance with the Guide for the Care and Use of Laboratory Animals (NRC, 2011) and were performed in compliance with DODI 3216.1. Adult male rats were secured within restraints secured to a metal plate, simulating the hull of a vehicle. An explosive under the plate was detonated, causing acceleration G-force on the rats of 1800G. Rats were then anesthetized and subjected to CCI24 hr after sham surgery or BCCI, rats were placed inside an altitude chamber under normobaric (sea level) or hypobaric (4000 or 8000 ft) pressures in the presence of either normoxic (21-28% O2) or hyperoxic (50% O2) conditions for 6 hr. Anxiety-like behavior was tested using time spent in the open zone of the elevated plus maze at 2, 7, and 14 days post- injury (dpi). Rats were euthanized by perfusion with paraformaldehyde at 2 or 14 dpi. Cortical inflammation was stereologically quantified using immunostained sections for activated macrophages (ED-1) or myeloperoxidase positive neutrophils and astrocytes. Cerebellar injury was assessed in calbindin immunostained sections where Purkinje neurons were morphologically classified as healthy or injured.

Results: A single under vehicle blast followed immediately by moderate CCI induced acute inflammation (MPO) which was not further exacerbated by post injury exposure to hypobaria. Blast CCI injury resulted in a two-fold increase in MPO positive neutrophil populations at 2 days which returned to sham levels by 14 dpi. Peri-lesional astrocyte volumes increased acutely by 7-fold and are not significantly different from shams by 14 dpi. Exposure to hypobaria of 8,000 ft compared to normobaria conditions following BCCI increased both anxiety-like behavior and inflammation in the potentially salvageable penumbra near the site of cortical impact at 14 dpi. Increased anxiety behavior and cortical inflammation were not further exacerbated by hyperoxia, or alleviated by a reduction in ambient pressure to 4,000 ft. Additionally, exposure to blast with CCI increased the number of injured Purkinje neurons in the cerebellum which was not further exacerbated by exposure to hypobaria. BCCI rats exposed to hypobaria under 50% oxygen had fewer injured and dark Purkinje cells compared to when hypobaria was performed under 28% oxygen.

Conclusion: 1. Hypobaria exposure after BCCI increases long term cortical inflammation and anxiety behavior in rats. 2. Blast polytrauma induces an acute increase in cortical MPO staining and long term cerebellar injury which are not further exacerbated by hypobaria. 3. Hypobaria under 50% (compared to 28%) O2 results in less cerebellar injury, no changes to behavior, and greater total cortical lesion volumes. These results raise further concern that exposure of trauma patients to hypobaria during aeromedical transport can worsen neurologic outcomes. The views expressed are those of the authors and do not reflect the official guidance or position of the United States Government, the Department of Defense or of the United States Air Force. Supported by US Air Force FA8650-18-2-6H17.

Keywords: aeromedical evacuation, brain injury, polytrauma, altitude, hyperoxia, inflammation, cerebellar injury