Discussion
In austere and resource-limited PFC environments, access to WB and blood products may be scarce, and although not ideal, crystalloids may be the last resort for resuscitation and bridging the time to optimal resuscitation and transfusion in casualties. The principle finding of this study is that in a simulated porcine model of PFC after decompensated hemorrhagic shock, large amounts of crystalloid solution can be used to maintain adequate perfusion during 6 hours without increased mortality in spite of dilutional coagulopathy relative to the negative control and hypotensive control groups. Additionally, in absence of tissue trauma, no inflammatory responses were seen to suggest prolonged hypotension or crystalloid volumes led to worsening endotheliopathy.
Marked reductions in hematocrit and hemoglobin revealed that hemodilution did occur during the course of the PFC phase. Although hematocrit reached 20% at the end of PFC, hemoglobin dropped below the critical threshold (80 g/L for patients not considered hemodynamically stable) 4 hours into PFC and continued to fall, indicating the immediate need for a transfusion. Despite these drops, sO2 remained above acceptable levels, indicating there were no issues with the oxygen-carrying capacity of the blood in the presence of the hemodilution. Additionally, the hemodynamic status was improved after WB transfusion at the simulated hospital care and was completely restored to BSLN levels by the end of the protocol, indicating that any deleterious effects during a 6-hour resuscitation period are still reversible with optimal resuscitation (WB) even if delayed.
At the end of the PFC, dilutional coagulopathy was not present when compared with clinical values used for humans. However, it is widely accepted that swine are more resilient to coagulopathy due to higher BSLN levels of platelets and FIB.14 15 This results in differences in coagulation measures to include PTT, CFT, and A10. Therefore, further analysis was used to better characterize changes seen in the porcine model and what those changes would look like based on human reference values16 17 that would be indicative of coagulopathy and the need for transfusion.18 After 6 hours of PFC, platelet counts were approximately 70% of BSLN values and fibrinogen dropped to 66%. If a human patient who presented BSLN values at the bottom of the normal range followed the same trend, platelet counts would still be above the critical threshold; however, FIB would be slightly below,19 20 suggesting increased need for FIB, either in the form of a concentrate, cryoprecipitate (plasma rich in FIB, vWF, factor VIII, and factor XIII), or WB.21
These same trends carried over into other ROTEM and STAGO measures. In the porcine model, none of the measures fell below critical thresholds. PT and ExTEM CT, both of which mimic normal human values, were only increased 15% and 14%, respectively, well below the threshold of 1.5 times normal to be considered ‘coagulopathic’.22–24 Interestingly, PTT was decreased to 69% of its BSLN value. This is opposite of what would be expected in dilutional coagulopathy, as the time for activated partial thromboplastin should increase as hemostasis is impaired.24 25 However, this decrease occurred during the first hour of PFC and remained stable until the end of PFC indicating there was no dilutional effect.
Likely as a result of the decreased FIB levels, FibTEM A10 and MCF saw 40% to 50% reductions in their measures. With the accepted clinical standards of A10 greater than 5 mm and MCF greater than 10 mm, the animals in the protocol remained well above these thresholds. Nevertheless, these marked reductions would leave human values at or below the critical level.
It is well established that neutrophils are increased in response to hemorrhagic shock and contribute to subsequent organ damage and failure.26 27 The Second Hit Theory postulates that hemorrhagic shock-induced ischemia and reperfusion via resuscitation activate the innate immune response before transitioning to an anti-inflammatory phenotype, thereby suppressing adaptive immune responders, and ultimately multiple organ dysfunction. The immune response observed here follows this pattern, as evidenced by the increased neutrophil counts through PFC. However, this increase is attenuated upon hospital resuscitation, and perhaps surprisingly, remains so 24 hours later. NLR was calculated due to its reported correlations with outcomes in patients with severe hemorrhage requiring massive transfusion protocol.28 The NLR of the Decomp/PH group at PFC1 of 1.6 entered the range of 1.1 to 5.2 reported for patients with hemorrhage at admission, and continued to climb to 3.1 by END PFC, surpassing the reported mean of 2.3 but still within the admission range. The study in human patients demonstrated that at day 3 and day 10 post-admission, NLR values greater than 8.82 and 13.69, respectively, were associated with increased in-hospital mortality. Comparatively, after resuscitation, the NLR of animals presented here had decreased by EHC to 2.6, reaching BSLN levels by the final time point. As such, after severe hemorrhage and 6 hours of prolonged hypotension with dilutional coagulopathy, NLR values here did not exceed the predictive cut-off thresholds for anticipated mortality.
The cytokine profile throughout PFC demonstrated no significant systemic change in the neutrophil activator IL-6 or the neutrophil chemotactic marker IL-8. Taken together with the unchanged IL-10 and syndecan-1 levels, and the previously published insignificant systemic organ damage,13 it can be concluded that 6 hours of prolonged hypotension and high volumes of crystalloid resuscitation do not generate the level of endotheliopathy or exacerbate neutrophil tissue invasion enough to create subsequent damage. However, changes in the nature of endothelial integrity, and certainly the dilutional coagulopathy, may be more detrimental in a model that uses concomitant tissue or vascular injury to induce a trauma-induced coagulopathy. The compounding effect of both dilutional and trauma-induced coagulopathies and endothelial disruption may result in more severe outcomes which WB resuscitation alone may not be able to overcome.
Limitations
The data presented here wield multiple limitations. The original study, designed to evaluate prolonged hypotension after hypovolemia, required animals to be recovered from anesthesia for neurological and behavioral monitoring. As such, neither severe traumatic injury nor uncontrolled hemorrhage could be safely incorporated into the protocol. Therefore, we used an established porcine model of pressure-targeted cardiovascular decompensation.29 30 The comparative differences in uncontrolled, pressure-targeted, and volume-targeted porcine hemorrhage modeling are discussed by Sondeen et al, who demonstrate pressure-targeted animals required more blood loss (21.5 mL/kg) to match the shock profiles of those undergoing uncontrolled hemorrhage (17.6 mL/kg).31 In our pressure-targeted model, our Decomp/PH group lost 34.4 mL/kg.13 These volumes per weight are higher than either group and additionally, for the Decomp/PH group, which spent between 60 and 90 min in shock before decompensating, levels of lactate (7.8±2.9 mmol/L) were higher here than those of Sondeen et al’s uncontrolled (6.0±0.8 mmol/L), pressure (6.7±1.1 mmoL/L), or volume (6.0±0.8 mmol/L) targeted modalities for equivalent time in shock. This controlled decompensated hemorrhage effectively mimics a clean penetrating trauma with hemorrhage, with severe hypovolemia representing the primary mechanism of physiologic burden. Hypovolemia itself has been associated with coagulopathy in clinical patients,26 and began to develop in this model as well, as demonstrated by significant declines in ATIII, Factor II, and Factor VII for the Decomp/PH group by EOS. As this injury pattern has been associated with coagulopathy due to the activation of fibrinolysis,32 combined with the anticipated dilutional coagulopathy expected from the resuscitation, rebleeding during PFC was not evaluated due to the risk of unnecessary and expected mortality. Additionally, as stated above, the coagulation profile of swine is different from humans, as they are less likely to become coagulopathic. To determine clinical significance compared with human values, percentages of BSLN values were used in the interpretation of the results as porcine blood has been shown to have similar responses to human blood.15