Hypobaria Exposure Worsens Cardiac Function and Endothelial Injury in AN Animal Model of Polytrauma: Implications for Aeromedical Evacuation

Validation of Preload Assessment Technologies at Altitude in a Porcine Model of Hemorrhage

INTRODUCTION

Dynamic preload assessment measures including pulse pressure variation (PPV), stroke volume variation (SVV), pleth variability index (PVI), and hypotension prediction index (HPI) have been utilized clinically to guide fluid management decisions in critically ill patients. These values aid in the balance of correcting hypotension while avoiding over-resuscitation leading to respiratory failure and increased mortality. However, these measures have not been previously validated at altitude or in those with temporary abdominal closure (TAC).

METHODS

Forty-eight female swine (39 ± 2 kg) were separated into eight groups (n = 6) including all combinations of flight versus ground, hemorrhage versus no hemorrhage, and TAC versus no TAC. Flight animals underwent simulated aeromedical evacuation via an altitude chamber at 8000 ft. Hemorrhagic shock was induced via stepwise hemorrhage removing 10% blood volume in 15-min increments to a total blood loss of 40% or a mean arterial pressure of 35 mmHg. Animals were then stepwise transfused with citrated shed blood with 10% volume every 15 min back to full blood volume. PPV, SVV, PVI, and HPI were monitored every 15 min throughout the simulated aeromedical evacuation or ground control. Blood samples were collected and analyzed for serum levels of serum IL-1β, IL-6, IL-8, and TNF-α.

RESULTS

Hemorrhage groups demonstrated significant increases in PPV, SVV, PVI, and HPI at each step compared to nonhemorrhage groups. Flight increased PPV (P = 0.004) and SVV (P = 0.003) in hemorrhaged animals. TAC at ground level increased PPV (P < 0.0001), SVV (P = 0.0003), and PVI (P < 0.0001). When TAC was present during flight, PPV (P = 0.004), SVV (P = 0.003), and PVI (P < 0.0001) values were decreased suggesting a dependent effect between altitude and TAC. There were no significant differences in serum IL-1β, IL-6, IL-8, or TNF-α concentration between injury groups.

CONCLUSIONS

Based on our study, PPV and SVV are increased during flight and in the presence of TAC. Pleth variability index is slightly increased with TAC at ground level. Hypotension prediction index demonstrated no significant changes regardless of altitude or TAC status, however this measure was less reliable once the resuscitation phase was initiated. Pleth variability index may be the most useful predictor of preload during aeromedical evacuation as it is a noninvasive modality.

A murine multiple-injury model for the study of thromboinflammation

INTRODUCTION

Neutrophil extracellular traps (NETs) contribute to trauma-induced coagulopathy. We aimed to develop a murine multiple-injury model that induces thrombo-inflammatory response, that is, NETosis and accelerated thrombin generation.

METHODS

Wild-type male mice (n = 10, aged 8-12 weeks) underwent multiple injuries (gastrocnemius crush, femur fracture, and laparotomy) and were compared with an uninjured control group (n = 10). Mice were euthanized by cardiac puncture performed 3 hours after injury. Whole blood samples were immediately processed to platelet poor plasma for thrombin generation kinetics (calibrated automated thrombogram), myeloperoxidase (MPO), and von Willebrand factor quantification. Immunohistochemistry of lung tissue was performed to assess for citrullinated histone 3 (CitH3) and MPO. A NETosis cluster was defined as 3+ neutrophils staining for CitH3 at 400× magnification (CitH3 cluster). Data were presented either as mean (SD) or median (interquartile range) with p < 0.05 significant. Sham and trauma treated animals were compared by the two-sample Wilcoxon rank-sum test.

RESULTS

Animals subjected to multiple injuries had accelerated thrombin generation compared with controls with greater peak height (61.3 [41.2-73.2] vs. 28.4 [19.5-37.5] nM, p = 0.035) and shorter time to peak (3.37 [2.81-3.81] vs. 4.5 [4.08-4.75] minutes, p = 0.046). Markers of neutrophil activation were greater following multiple injuries than in controls (MPO, 961.1 [858.1-1116.8] vs. 481.3 [438.0-648.9] ng/mL; p = 0.004). NETosis, as evidenced by the aforementioned defined number of CitH3 clusters in the lung, was greater in multiple-injury animals than in controls (mean [SD], 3 [2.9] vs. 0.2 [0.7]; p = 0.009).

CONCLUSION

This is the first study to demonstrate that NETosis and accelerated thrombin generation can be induced using a murine multiple-injury model, as early as 3 hours following injury.

Early lyophilized cryoprecipitate enhances the ADAMTS13/VWF ratio to reduce systemic endotheliopathy and lessen lung injury in a mouse multiple-trauma hemorrhage model

BACKGROUND

Recent studies in severely injured patients suggest an important role of von Willebrand Factor (VWF) and ADAMTS13 in the endotheliopathy of trauma (EoT). We hypothesized that the early use of cryoprecipitate would be effective as an endothelial protector by supplementing physiologic VWF and ADAMTS13 to reverse the EoT. We tested a pathogen-reduced lyophilized cryoprecipitate (LPRC) that could expedite the early administration of cryoprecipitate in the battlefield.

METHODS

A mouse multiple-trauma model with uncontrolled hemorrhage (UCH) from liver injury was utilized followed by hypotensive resuscitation (mean arterial pressure, 55-60) × 3 hours with lactated Ringer's (LR), fresh frozen plasma (FFP), conventional pathogen-reduced cryoprecipitate (CC), and LPRC. Blood was collected for measurement of syndecan-1, VWF, and ADAMTS13 by ELISA. Lungs were stained for histopathologic injury and syndecan-1 and bronchial alveolar lavage (BAL) fluid harvested for protein as an indicator of permeability. Statistical analysis was by ANOVA followed by Bonferroni correction.

RESULTS

Following multiple trauma and UCH, blood loss was similar across groups. Mean volume of resuscitation was higher in the LR group compared with the other resuscitation groups. Lung histopathologic injury, syndecan-1 immunostaining and BAL protein were higher with LR compared with resuscitation with FFP and CC, while LPRC further reduced BAL compared with FFP and CC. The ADAMTS13/VWF ratio was significantly lower in LR but improved with FFP and CC, comparable to shams while LPRC further increased this ratio.

CONCLUSION

The protective effects of CC and LPRC were comparable to FFP in ameliorating the EoT in our murine multiple trauma and UCH model. Lyophilized cryoprecipitate may also provide additional benefit by enhancing the ADAMTS13/VWF ratio. These data provide evidence of the safety and efficacy of LPRC and warrants further investigation for its potential application in military settings once approved for human administration.

Effects of sequential aeromedical evacuations following traumatic brain injury in swine

INTRODUCTION

Traumatic brain injuries (TBI) represent a significant percentage of critical injuries in military conflicts. Following injury, wounded warfighters are often subjected to multiple aeromedical evacuations (AE) and associated hypobaria, yet the impact in TBI patients remains to be characterized. This study evaluated the impact of two consecutive simulated AEs in a fluid-percussion TBI model in swine to characterize these effects.

METHODS

Following instrumentation, anesthetized Yorkshire swine underwent a frontal TBI via fluid-percussion. A hypobaric chamber was then used to simulate AE at simulated cabin pressure equivalent to 8000ft (hypobaria) in a 6 h initial flight on day 3, followed by a 9 h flight on day 6, and were monitored for 14 days. Animals in the normobaria group were subjected to the same steps at sea level while Sham animals in both groups were instrumented but not injured. Parameters measured included physiologic response, intracranial pressure (ICP), hematology, chemistry, and serum cytokines. Histopathology of brain, lung, intestine, and kidney was performed, as well as fluorojade staining to evaluate neurodegeneration. All animals were divided into sub-groups by block randomization utilizing a 2-way ANOVA to analyze independent variables.

RESULTS

Survival was 100% in all groups. Physiologic parameters were largely similar across groups as well during both 6 and 9 h AE. Animals exposed to hypobaria in both the TBI and Sham groups had elevated heart rate (HR) during the 6 h flight (p<0.05). Three animals in the TBI hypo group demonstrated leukocytosis with histologic evidence of meningeal inflammatory response. Expression of serum cytokines was low across all groups. No significant neuronal degeneration was identified in areas away from the site of injury.

CONCLUSION

Aeromedical evacuation in swine was not associated with significant differences in physiologic measures, cytokine expression or levels of neuronal degeneration. Histological examination revealed higher risk of meningeal inflammatory response and leucocytosis in swine exposed to hypobaria.

Blood product resuscitation mitigates the effects of aeromedical evacuation after polytrauma

BACKGROUND

The combined injury of traumatic brain injury and hemorrhagic shock has been shown to worsen coagulopathy and systemic inflammation, thereby increasing posttraumatic morbidity and mortality. Aeromedical evacuation to definitive care may exacerbate postinjury morbidity because of the inherent hypobaric hypoxic environment. We hypothesized that blood product resuscitation may mitigate the adverse physiologic effects of postinjury flight.

METHODS

An established porcine model of controlled cortical injury was used to induce traumatic brain injury. Intracerebral monitors were placed to record intracranial pressure, brain tissue oxygenation, and cerebral perfusion. Each of the 42 pigs was hemorrhaged to a goal mean arterial pressure of 40 ± 5 mm Hg for 1 hour. Pigs were grouped according to resuscitation strategy used-Lactated Ringer's (LR) or shed whole blood (WB)-then placed in an altitude chamber for 2 hours at ground, 8,000 ft, or 22,000 ft, and then observed for 4 hours. Hourly blood samples were analyzed for proinflammatory cytokines and lactate. Internal jugular vein blood flow was monitored continuously for microbubble formation with altitude changes.

RESULTS

Cerebral perfusion, tissue oxygenation, and intracranial pressure were unchanged among the six study groups. Venous microbubbles were not observed even with differing altitude or resuscitation strategy. Serum lactate levels from hour 2 of flight to the end of observation were significantly elevated in 22,000 + LR compared with 8,000 + LR and 22,000 + WB. Serum IL-6 levels were significantly elevated in 22,000 + LR compared with 22,000 + WB, 8,000 + LR and ground+LR at hour 1 of observation. Serum tumor necrosis factor-α was significantly elevated at hour 2 of flight in 8,000 + LR versus ground+LR, and in 22,000 + LR vs. 22,000 + WB at hour 1 of observation. Serum IL-1β was significantly elevated hour 1 of flight between 8,000 + LR and ground+LR.

CONCLUSION

Crystalloid resuscitation during aeromedical transport may cause a prolonged lactic acidosis and proinflammatory response that can predispose multiple-injury patients to secondary cellular injury. This physiologic insult may be prevented by using blood product resuscitation strategies.