We present the first evidence that the development of acute traumatic coagulopathy correlates with the activation of protein C pathway. We further report that after this initial activation of protein C, there is in some patients a rapid depletion of the plasma levels of the protein C zymogen that is associated with a significantly increased risk for developing nosocomial lung infection. These results suggest that the maintenance of adequate plasma levels of the protein C after severe trauma may protect against the development of later lung infection and represent a putative biological link between coagulopathic bleeding after injury and later infectious complications in the same trauma patients.
Coagulopathy after trauma is a common, and some degree of coagulopathic perturbation affects most severely injured patients at some point during their surgical and resuscitative course. For decades, it was commonly believed that any hypocoagulable state after trauma was due to iatrogenic mechanisms. Indeed, the accepted concept within the trauma community had long been that any coagulopathy after injury was secondary to hypothermia, dilution and metabolic acidosis that resulted from resuscitation and surgical care. Anecdotally however, several investigators had observed coagulopathy that took place before significant resuscitation, dilution and hypothermia. These observations were finally systematically studied in 2003 by Brohi and Macleod in two separate reports who documented acute traumatic coagulopathy 1 hour after injury in approximately 30% of patients[10
]. To test potential mechanisms to explain this coagulopathy, we performed a prospective study of 209 severely injured patients admitted to San Francisco General Hospital and reported that this acute traumatic coagulopathy was associated with a depletion of plasma levels of protein C zymogen at the admission to the hospital[12
]. Subsequent work showed that this effect was also present in patients with isolated traumatic brain injury[13
]. However, we did not measure in that study plasma levels of activated protein C. Furthermore, we did not collect longitudinal sampling and we therefore were unable to examine the relationship between early activation, later depletion of the protein C pathway and subsequent propensity to organ failure or infection. Our new data presented here shows that patients with a combination of severe tissue injury (elevated ISS) and shock (elevated BD) are coagulopathic nearly immediately after their injury. This coagulopathy is strongly associated with the activation of protein C pathway. Further supporting protein C activation are the strong inverse correlation between plasma levels of aPC and factor Va and VIIIa inactivation and the derepression of fibrinolysis. As expected, this activation of protein C and resultant coagulopathy is associated with increased fluid and blood product resuscitation and poorer outcome (MOF, VAP and Death).
The second significant finding our study is the relationship between early coagulopathy, subsequent depletion of the protein C system and propensity toward infection. Indeed, our new data indicates that, as early as 6 hours after trauma (and initial coagulopathy), patients begin to segregate into those who are ‘depleters’ defined by depletion of protein C stores and those who maintain physiologic plasma levels of protein C zymogen. Those who deplete and do not recover their protein C plasma levels have a significant propensity to later infectious complications.
Along with its well-described anticoagulant functions, activated protein C also has newly described cytoprotective effects[27
]. Recombinant aPC has been shown to protect baboons and mice from sepsis and to attenuate LPS-mediated inflammatory signaling in monocytes [14
]. This non-anticoagulant effect of protein C is mediated through PAR-1 and EPCR, and multiple downstream signaling pathways including Rac-1 and NF-kappaB. Evidence for a link between protein C depletion and sepsis in humans has been described. Indeed, a well-established correlation between diminished plasma levels of protein C and worsened outcome exists in patients with septic shock.[29
] The understanding that a dysfunction of the protein C pathway is a likely mechanism for the end-organ damage observed after sepsis led to several groups testing the efficacy protein C, sTM, EPCR and activated protein C as therapy in septic animals and humans.[30
]. Finally, recent work from our laboratory has shown that the administration of murine activated protein C attenuated lung injury induced by P. aeruginosa
in a mouse model of pneumonia. [18
Along with abundant data that septic shock is associated with protein C depletion, there is also evidence linking other shock states with protein C activation and depletion. Adrie et al studied survivors of cardiac arrest and showed that patients in shock from cardiac arrest had increased plasma levels of aPC followed by later depletion to undetectable levels. [32
] Along with increased plasma levels of aPC, these patients also were hypocoagulable early after cardiac arrest. Because perturbations in the activity of the protein C pathway are present in both septic and cardiogenic shock, it is not surprising that trauma and hemorrhagic shock would be associated with similar abnormalities in the protein C pathway.
There are several limitations of and questions raised by this study. First, because we cannot obtain a baseline (pre injury) sample, it is impossible to know the true level of protein C activation for an individual patient. We attempted to control for this by using the aPC /PC ratio; however, we are aware that this is a best case approximation of true activation of the protein C pathway for each patient. Secondly, it is difficult to know how severe the depletion of activated protein C has to be to affect its ability to protect the vascular endothelium after severe trauma. It is possible that the propensity towards higher rate of lung infection in the protein C depletion group results from the inability to activate protein C zymogen in response to colonization of the distal airways by nosocomial bacteria. Indeed, a recently published work from our laboratory has demonstrated that the administration of activated protein C significantly attenuates P. aeruginosa
-mediated damage to the alveolar-capillary barrier via its non-anticoagulant cytoprotective domain.[18
] Of additional interest is the relative contribution of non-iatrogenic acute biological mechanisms (aPC) and iatrogenic (dilution, hypothermia) coagulopathy to the overall coagulopathy of trauma. Our data here support a larger role for non-iatrogenic coagulopathy early after major trauma. Indeed, our relative short pre hospital transport times and predisposition toward minimal pre hospital resuscitation make our first ED sample not confounded by excessive dilution or by the well-described effects of chloride containing resuscitation fluids. Interestingly our non-coagulopathic group received nearly a double prehospital fluid volume compared to the coagulopathic group potentially strengthening the argument for an acute biological mechanism, although the fluid resuscitation volumes in both groups were minimal and not statistically distinct. Also understood is that fact that many of the correlations between coagulation mediators while statistically significant are not statistically strong. While we strive for and achieve the highest level of standardization in our sampling and measurement, these patients are extremely heterogenous in their demographics, injuries, and physiology. This is layered on sampling in a very chaotic environment. Despite this difficulty, we have shown that we can achieve reliable sampling to characterize acute traumatic coagulopathy. To truly mechanistically test these correlations require laboratory models in which the plasma levels of activated protein C can be manipulated. Indeed, we have previously reported that activated protein C drives acute traumatic coagulopathy a mouse model of injury and hemorrhagic shock [26
]. Lastly, The effect of blood transfusion on the outcomes studied also deserves discussion. While others have published direct effects of blood transfusion on outcome after severe trauma[33
], our data did not demonstrate a direct association between blood transfusion and outcome in the present study. There are several putative reasons for this negative finding. First, our study was not designed to study the effect of blood transfusion on outcome parameters after trauma and may thus have been underpowered to detect such an association. Second, over the past several years several investigators have called into question the deleterious effects of blood transfusion and have proposed that earlier blood product transfusion in ratios closer to whole blood are in fact beneficial to the survival of trauma patients[36
]. During the completion of our study, blood transfusion was not standardized at our institution, although high ratio of FFP to packed red cells are usually administered to our trauma patients. In addition we do not have the data necessary to determine the reasons or granular timing for transfusion of blood products. Therefore, we were unable to determine the transfusion ratios and unable to discern whether the lack of association between the amount of blood products and outcome was in fact due to the lack of biological effect or rather due to the fact that our study was not designed to test this association. A multicenter study is currently underway to test the relationship between acute traumatic coagulopathy, blood product transfusion and outcome.
In conclusion, we present here the first evidence for an early activation of the protein C pathway that is associated with the acute coagulopathy observed in severely traumatized patients. In addition, we found an association between this early coagulopathy, later depletion of protein C stores and propensity to develop nosocomial lung infection, a common complication in severely injured patients who survive their initial injury. The, combination of the present clinical data with our previously published data on the mechanistic role of the protein C pathway in the development of coagulopathy associated with trauma-hemorrhage in mice [26
] indicate that the anticoagulant function of activated protein C represents an unfortunate side-effect of a profound anti-inflammatory response being released by the body in response to severe trauma and shock. In our previous mouse work, the cytoprotective effect of aPC was necessary for survival through the acute phase while the anticoagulant function when blocked had no effect on survival. Taken together with these data it seems that acute traumatic coagulopathy represents a maladaptive response to severe injury. Indeed, after severe trauma and tissue hypoperfusion, the body is likely releasing a large amount of an antiinflammaroy and cytoprotective molecule (aPC) in an attempt to prevent the development of a lethal microvascular thrombosis and endothelial and epithelial destruction. In a perfect example of maladaptive response, activation of the protein C pathway after severe injury has a profound anticoagulant effect which results in the development of a clinically significant coagulopathy. Shortly after this ‘to much of a good thing’ response which is complicated by a bleeding diathesis, there is n certain trauma patients, a depletion of the response (‘to little of a good thing later’) and a propensity to orgain injury and infection. The implications of this connection between early acute traumatic coagulopathy and later organ dysfunction and propensity to infection are significant. Indeed, future studies are warranted to identify drivers of both the early coagulopathy and later depletion of the protein C system. From this knowledge, putative future clinical intervention could involve blocking of the anticoagulant domain of aPC early after trauma, which would correct the early posttraumatic coagulopathy, while maintaining the cytoprotective effect of that protein that is critical for the homeostasis of the vascular endothelium. Later, augmentation of the depleted protein C response by the administration of a protein C mutant that does not have the anticoagulant effect of the wild-type protein could be considered. There is thus a need for additional experimental and clinical studies to fully understand the role the protein C pathway after severe trauma.