Decreased antithrombin activity in the early phase of trauma is strongly associated with extravascular leakage, but not with antithrombin consumption: a prospective observational study

Trauma is well known to induce dynamic coagulofibrinolytic changes, which increase bleeding tendency in the initial phase of trauma when the hemostasis becomes uncontrollable [1, 2]. Coagulofibrinolytic disorder has been termed trauma-induced coagulopathy (TIC). The mechanisms of this disorder are still controversial, though they may include disseminated intravascular coagulation (DIC) with a fibrinolytic phenotype or acute traumatic coagulopathy (ATC) [3‐5].

The pathophysiology of DIC in the early phase of trauma consists of coagulation activation, hyperfibrinolysis and consumption coagulopathy. Tissue injury due to trauma leads to systemic coagulation activation and thrombin generation via procoagulants such as damage-associated molecular patterns (DAMPs), microparticles or tissue factors. Trauma also induces impairment in anticoagulant activities, which causes dysregulation of coagulation activation and promotes systemic hypercoagulation. Simultaneously, hyperfibrinolysis occurs through the expression of tissue plasminogen activator (t-PA). In the first hours after trauma, plasminogen activator inhibitor-1 (PAI-1) activity has not increased sufficiently to counteract this, and hyperfibrinolysis consumes α₂-plasmin inhibitor (α₂PI), which accelerates further fibrinolysis. This pattern of hypercoagulation and hyperfibrinolysis causes consumption of platelets and coagulation factors, resulting in DIC with a fibrinolytic phenotype characterized by bleeding tendency when hemostasis becomes dysregulated [3]. On the other hand, ATC is a state of activation of the anticoagulant protein C (PC) pathway caused by shock and tissue hypoperfusion, leading to hypocoagulation and subsequent fibrinolysis. Activated PC pathway also abrogates PAI-1 and increases t-PA, resulting in further hyperfibrinolysis [4, 5].

Antithrombin (AT) plays an important role in anticoagulation against intravascular thrombin formation through its ability to bind and inactivate thrombin by forming a thrombin-antithrombin complex (TAT). A decrease in AT level is well known to occur in the early phase of trauma as one of the factors impairing anticoagulation. It is especially notable that AT depletion occurs immediately after trauma [6‐9]. Some reports have suggested that decreased AT levels are associated with persistent thrombin generation which could be a potential risk of subsequent thromboembolic complications in trauma patients [7, 10, 11]. Thus in the initial phase of trauma we need to manage not only bleeding tendency, but also hypercoagulation due to the impairment of the anticoagulation system in the subsequent phase. Several studies addressing sepsis or septic DIC have suggested that decreased AT levels occur due to extravascular leakage, increased AT consumption, decreased protein synthesis or degradation by enzymes released from neutrophils [12‐15]. Nevertheless, the mechanism of AT depletion in trauma is still unclear. Furthermore, coagulofibrinolytic responses in patients who survive bleeding or organ damage in the early phase of trauma have not been well evaluated. Accordingly, in this study we focused on the impairment in anticoagulation that develops subsequent to the initial coagulopathy in patients who were admitted for longer than seven days as an inclusion criterion. We hypothesized that impaired anticoagulation in trauma would lead to problems even after the initial coagulopathy and organ damage had been overcome. Thus, we carried out a prospective observational study to survey dynamic changes in coagulofibrinolytic responses and to investigate the mechanisms as well as the influence of AT reduction.

It is known that the evaluation of physiological hemostatic responses to trauma through the measurement of coagulofibrinolytic biomarkers can be compromised if the measurement is taken during one of three events: fibrinolytic activation, fibrinolytic shutdown and fibrinolytic reactivation [20]. Just after a trauma insult, fibrinolytic activation occurs simultaneously with coagulatory activation. This is followed by fibrinolytic inhibition (also known as fibrinolytic shutdown) due to increasing tPAI-1, a controller for excessive fibrinolysis; fibrinolytic shutdown lasts from several to 24 h after a trauma insult or even for several days. After the repair of injured vessels and tissues, tPAI-1 decreases, reactivating fibrinolysis to allow the removal of the fibrin attached to the vessels for hemostasis. In this study, each of these three phases of physiological hemostatic response to trauma was actually observed. This shows that trauma- induced hypercoagulation and the triphasic changes of fibrinolytic activation will occur even in mild trauma patients such as those included in this study. Thus we should be aware of these time course changes to ensure the appropriate timing of treatments such as transfusions, antifibrinolytics or anticoagulants.

This study also focused on impairment in anticoagulation and decreased AT activity in the early phase of trauma. On day 0, which is thought to lie within the fibrinolytic activation phase, AT levels altered along with complex changes in coagulation factors and anticoagulants as well as fibrinolytic factors and their inhibitors. In particular, coagulofibrinolytic markers such as TAT and PIC changed simultaneously and in parallel with one another, even shortly after trauma. Nevertheless, AT did not linearly correlate with TAT and PIC on day 0, which means that, even if coagulation and fibrinolysis are activated, as evidenced by elevated TAT and PIC levels, AT levels do not decrease in response to their activation. AT inactivates thrombin’s effects by forming a covalent stable stoichiometric 1:1 complex, TAT. Thus, TAT levels directly reflect the consumption of AT against intravascular thrombin formation. Indeed, TAT value is used as a marker of coagulation activity in the diagnosis of DIC. The normal concentration of AT in human plasma is approximately 125 to 160 μg/mL, which corresponds to 80 to 130% AT activity [18, 19]. Actually, Aibiki et al. have demonstrated a very strong linear correlation between AT activity level and its concentration [15]. Regarding the association between AT and TAT, for example, even the maximum level of TAT in this study was 4385 μg/L (approximately 4.4 μg/mL), an amount much smaller than the AT levels in plasma as mentioned above. Taking these findings together, it is reasonable to assume that decreased AT levels do not result merely from AT consumption even if coagulation is activated.

In this study, changes in plasma Alb, which was one of the predictors of AT on day 0, showed a very strong linear correlation with AT. Alb, a 66-kDa protein synthesized in the liver, is well known as a parameter reflecting an impairment in liver function or extravascular leakage when vascular permeability increases resulting from acute systemic inflammatory responses [21, 22]. Our results suggest that the decrease in AT activity in the initial phase of trauma could be mainly due to systemic responses to trauma strongly associated with Alb values on day 0. Previously, the mechanisms underlying the decrease in AT during trauma have been thought to be related to increased AT consumption, decreased synthesis, extravascular leakage or degradation by enzymes released from neutrophils [3, 8, 9, 23]. However, as presented clearly in this study, AT level does not decrease merely through AT consumption. Furthermore, in this study, hemodilution through fluid administration is not likely to have caused AT concentration changes because one criterion of this study excluded patients who had been administered more than 500 mL of fluids before admission (day 0), and because the blood samples on admission were generally drawn before the initial infusion was started.

Previous studies analyzing septic or obstetrical DIC have demonstrated strong correlations between AT and Alb and revealed that one of the main causes of decreasing AT is leakage from the capillary vessels [14, 15, 24]. Although there is a possibility of impaired synthesis of AT and Alb in the liver, it is highly unlikely if not impossible that liver dysfunction would occur immediately after trauma such that it could simultaneously cause decreased AT and Alb levels just after the insult. Although we were unable to obtain measurements of Alb values in our study subjects before their trauma insults, there were no medical histories of malnutrition or comorbidities that might affect Alb levels as far as we know. On the other hand, it is well known that vascular permeability to plasma contents is restricted to around 70 kDa, so Alb (66 kDa) is mostly retained in the intravascular space under normal conditions. In inflammatory situations, however, vascular permeability increases such that even high molecular weight proteins including Alb become permeable [25]. The molecular weight of AT is 64 kDa, similar to that of Alb, so it is reasonable to presume that AT exhibits similar dynamics in extravascular leakage depending on vascular permeability. Our results clearly show that AT levels decreased immediately after trauma and that this decrease was accompanied by a very strong linear correlation with Alb. Furthermore, these decreases in AT were significantly associated with elevations of IL-6 and TM, markers of systemic inflammatory responses and endothelial injury, respectively. Thus, the present results indicate that AT could decrease due to trauma-induced systemic responses. Also, it is likely that vascular permeability strongly affects AT metabolism. The endothelial glycocalyx layer (EGL) is known as a major player in determining vascular permeability [26, 27]. EGL decreases in response to elevatied IL-6 levels during sepsis and trauma [28, 29]. Di Battista et al. have reported that endotheliopathy, which is associated with glycocalyx breakdown, occurs in the initial phase of brain injury [30]. Furthermore, Rodriguez et al. demonstrated that traumatic endotheliopathy was associated with leakage of Alb even on admission [31]. Although we explored the correlations between AT and Alb, IL-6 or TM, we did not measure any direct markers that reflect vascular permeability in this study. In the future we aim to examine the relationship between EGL and AT activity in trauma. As another plausible explanation for AT depletion after trauma, neutrophil elastase involvement is possible [3, 9], though this awaits further clarification.

Decreased AT could be a potential risk factor for subsequent thrombosis [32]. Although in this study we could not detect an association between decreased AT activity and the development of DVT, decreased AT levels were found to be associated with subsequent thrombin activation indicated by increases in TAT on days 2 and 4. A previous study in patients with AT deficiency showed a persistent elevated thrombin activation [33]. In hereditary AT deficiency, AT levels are typically 40–60% of normal levels and patients have a lifetime risk of venous thromboembolism (VTE) [18]. Furthermore, an increased risk of recurrence of VTE has been reported even in mild AT deficiency (70–80%) [34]. These reports suggest that a reduction in AT levels should be recognized as a cause of thromboembolic complications even when AT levels are not severely decreased. In trauma patients, previous studies have demonstrated systemic increases in thrombin generation in connection with depleted AT levels [7, 10], which supports the present results. Furthermore, low AT levels in trauma patients have been reported to be associated with thromboembolic complications [11]. We need to explore the necessity and the timing of anticoagulant therapy including AT supplementation with regard to vascular permeability for patients with decreased AT activity due to trauma.

Coagulofibrinolytic responses occurred even in mild trauma patients who survived initial coagulopathy and organ injuries. In such situations our results showed an impairment in anticoagulation due to decreased AT activity, which could result in prolonged hypercoagulation. This is the first report examining the mechanisms of decreased AT levels in the early phase of trauma. The observed decrease in AT levels in the initial phase of trauma is unlikely to occur through AT consumption accompanied by coagulofibrinolytic activation. One important cause of decreased AT levels could be trauma-induced systemic responses such as vascular leakage as suggested by Alb depletion along with elevated IL-6 and TM.