J Trauma Acute Care Surg. 2019 Jan;Epub ahead of print

# Hypobaria during long-range flight resulted in significantly increased histopathological evidence of lung and brain damage in a swine model.

Scultetus A, Jefferson M, Haque A, Ho L, Hazzard B, Saha B, Chun S, Auker C, Moon-Massat P, McCarron R, Malone D

BACKGROUND: Aeromedical evacuation to definitive care is standard in current military conflicts. However, there is minimal knowledge on the effects of hypobaria (HYPO) on either the flight crew or patients. The effects of HYPO were investigated using healthy swine.

METHODS: Anesthetized Yorkshire swine underwent a simulated 4 h "transport" to an altitude of 2,441 m (8,000 feet.; HYPO, N = 6) or at normobaric conditions (NORMO, N = 6). Physiologic and biochemical data were collected. Organ damage was assessed for hemorrhage, inflammation, edema, necrosis, and for lungs only, microatelectasis.

RESULTS: All parameters were similar prior to and after "transport" with no significant effects of HYPO on hemodynamic, neurologic, or oxygen transport parameters, nor on blood gas, chemistry, or complete blood count data. However, the overall Lung Injury Score was significantly worse in the HYPO than the NORMO group (10.78 ± 1.22 vs. 2.31 ± 0.71, respectively) with more edema/fibrin/hemorrhage in the subpleural, interlobular and alveolar space, more congestion in alveolar septa, and evidence of microatelectasis (vs. no microatelectasis in the NORMO group). There was also increased severity of pulmonary neutrophilic (1.69 ± 0.20 vs. 0.19 ± 0.13) and histiocytic inflammation (1.83 ± 0.23 vs. 0.47 ± 0.17) for HYPO versus NORMO, respectively. On the other hand, there was increased renal inflammation in NORMO compared with HYPO (1.00 ± 0.13 vs. 0.33 ± 0.17, respectively). There were no histopathological differences in brain (whole or individual regions), liver, pancreas, or adrenals.

CONCLUSION: Hypobaria, itself, may have an adverse effect on the respiratory system, even in healthy individuals, and this may be superimposed on combat casualties where there may be preexisting lung injury. The additional effects of anesthesia and controlled ventilation on these results are unknown, and further studies are indicated using awake models to better characterize the mechanisms for this pathology and the factors that influence its severity.