The results of the present in vitro study suggest that clotting factor derangement leading to impaired thrombin generation is fundamental to the development of the ACOT rather than the dynamics of clot formation, fibrin cross-linking, clot strength/platelet function, or fibrinolysis. This animal model of ACOT employing TEG provides similar results as those recently reported in severely injured patients.14
In this model, significant tissue injury and hemorrhage of approximately 50% of the total blood volume was achieved to generate a base deficit with a mean difference of 13.95 mEq/L from baseline. In addition, resuscitation with twice the volume of shed blood with NS did cause hemodilution; however, care was maintained to avoid critical anemia (Hb < 7.0 g/dL), making this model clinically relevant.
As in all animal models, there are inherent limitations which do not entirely reflect the human condition, and as with this study, there are pronounced species differences in thrombelastography coagulation profiles between humans and rats.18
At baseline, rats form clots significantly faster and stronger, and have less lysis compared to humans. In spite of these differences, the highly conserved pathways of coagulation have analogous functions throughout species, and likely respond to trauma/hemorrhagic shock similarly. Thus, animal models utilizing TEG are still instrumental in identifying the basic components of post-injury coagulopathy. Currently, there are two primary mechanisms proposed for the ACOT, which continue to be debated. The first proposed mechanism suggests DIC with consumption of coagulation factors as the primary etiology.3,19
The current consensus of DIC is that it has multiple etiologies, but is defined by two stages.20
The first stage involves a widespread and unregulated generation of clotting factors and thrombin, which induces platelet activation and aggregation in addition to fibrin deposition in the microvasculature. In addition, there are increased levels of plasminogen activator inhibitor-1 (PAI-1), which prevents fibrinogen breakdown, leading to a hypercoagulable state. Consequently, this surge in thrombin allows excess substrate to bind to receptors on endothelial cells, which stimulates the release of TPA, as well as activation of thrombin-activatable fibrinolysis inhibitor, neutrophil elastase, and plasmin. These anti-thrombotics, along with the excessive activation and consumption of coagulation factors and platelets, lead to the second stage of secondary fibrinolysis. This is evident by decreased levels of circulating clotting factors, fibrinogen, and platelets.
The alternative mechanism involves the thrombomodulin-protein C pathway.4
In the setting of both tissue hypoperfusion and injury, thrombomodulin is expressed on the endothelium, and ultimately forms a complex with thrombin. This complex then activates protein C, which inhibits factors V and VIII as well as PAI-1. Consequently, less fibrin is formed and the fibrin already in existence is degraded by plasmin due to unopposed tissue plasminogen activator (TPA) activity.
This study is limited in discerning between these two proposed mechanisms since no direct measurements of clotting factors, platelets, d-dimer, or fibrinogen were obtained. However, the activity or presence of these factors can be quantified using thrombelastography. Furthermore, TEG tracings are pathognomonic for specific coagulopathies including coagulation factor dysfunction and DIC (). If the common pathway of DIC is excessive thrombin generation and increased PAI-1 activity leading to a hypercoagulable state, TEG should reveal shortened R and delta values along with a stable or increased LY30 value from baseline to shock. The opposite of this was seen in this study, in which R and delta were prolonged and the LY30 did not significantly change at the end of shock. Even following resuscitation, no hypercoagulable state was detected, making this hypothesis less likely.
Figure III Characteristic tracings of a normal thrombelastogram and specific coagulopathies. A: Represents a normal thrombelastogram. B: Thrombelastogram reflecting impaired protease activity as represented by a prolonged R and K-time. F: Represents Stage I of DIC (more ...)
These data show that even though thrombin generation was impaired, overall clot integrity was preserved, which favors the proposed mechanism of activated protein C inhibiting Factor V and VIII activity. Although, to fully support this mechanism, factor levels, as well as protein C levels, would need to be quantified in this model. A further limitation of this study is that only three time points were used to evaluate TEG parameters, and therefore, the course of this post-injury coagulopathy cannot be determined. One might speculate that an early hypercoagulable state is possible within the shock period, but other factors are implicated rather than DIC. The sympathetic response to injury, simulated by infusion of stress hormones in animal and clinical experiments, is sufficient to cause a hypercoagulable state.21–23
Even under adequate anesthesia, significant stress is likely following femoral vessel cannulation, tracheotomy, and laparotomy. For this reason, the baseline TEG blood sample was acquired through cardiac puncture. It is unlikely that an earlier, hypercoagulable state due to DIC is missed by these time points since hemostatic potential persists to the end of the experiment, suggesting no consumption of clotting factors.
Furthermore, additional time points following resuscitation were not measured. The proponents of the DIC hypothesis state that clinically, patients develop a hypercoagulable state (DIC with a prothrombotic phenotype) 3–5 days following the initial trauma secondary to increased levels of PAI-1.3
Extending time points over several days, to determine if a compensatory hypercoagulable state is evident, would require frequent blood draws from a small animal already anemic from hemorrhage, and thus, making this experiment prohibitive in this model.
It is also important to note that clinically, the mechanisms and severity of trauma vary greatly between blunt, penetrating, and crush injuries. This model focuses on one standardized type of injury (penetrating trauma associated with hemorrhagic shock) and cannot be generalized to all trauma patients. However, the purpose of this study was to evaluate the basic components of post-injury coagulopathy utilizing TEG in this specific model. Therefore, it is difficult to speculate how TEG parameters would change in different models of trauma, but this warrants further investigation. The proponents of DIC would likely hypothesize that tissue ischemia is all that is needed to incite DIC, and therefore, there would be no change in the TEG parameters. Alternatively, the proponents of the thrombomodulin-activated protein C pathway would propose that the mechanism of trauma matters since both tissue factor and tissue ischemia are required to develop the ACOT. Therefore, with worse tissue injury (increased tissue factor), a more pronounced ACOT would develop.
In summary, clotting factor derangement leading to impaired thrombin generation is fundamental to the development of the ACOT. Due to the limitations of this study, it is difficult to discern between the DIC or the thrombomodulin-protein C mechanism as the fundamental pathway of the ACOT. Although the thrombomodulin-protein C pathway appears likely, additional pathways must be considered due to the complexities of the coagulation, anti-thrombotic, and inflammatory systems. These data warrant further investigation to further elucidate a mechanism in this animal model.