Early Fibrinogen Replacement to Treat the Endotheliopathy of Trauma: Novel Resuscitation Strategies in Severe Trauma

The endothelial glycocalyx is a gel-like, protective layer of proteoglycans that lines the luminal surface of vascular endothelial cells.¹³  This protective layer is 0.2 to 3.0 µm thick and negatively charged.³  Proteoglycans within the glycocalyx are bound with sialic acid, glycosaminoglycans, and plasma proteins. These proteoglycans are anchored to the apical membrane of endothelial cells and form the structural backbone of the glycocalyx.^13,14  Of the multiple types of proteoglycans, syndecan-1 has been identified as a reproducible marker of clinical glycocalyx injury with high sensitivity.^15,16  Glycosaminoglycans form chains that covalently bind to proteoglycans, including heparan sulfate, chondroitin sulfates, and dermatan sulfates.¹⁷  Other proteins such as albumin, fibrinogen, fibronectin, thrombomodulin, antithrombin III, and cell-adhesion molecules interact with glycosaminoglycans.

Typically, the endothelial glycocalyx serves as a barrier to vascular permeability. An intact glycocalyx maintains an albumin gradient across the endothelial cell membrane that regulates transvascular flux¹⁸  and prevents the adhesion of leukocytes and platelets to endothelial cells.¹⁹  Positively charged and large (more than 70 kDa) molecules such as enzymes, growth factors, cytokines, and amino acids are trapped in the negatively charged meshwork of the glycocalyx.²⁰  In addition, the glycocalyx can induce the endothelial initiation of nitric oxide–mediated vasorelaxation in response to shear forces, provide anticoagulant effects, and shield endothelial cells from oxidative stress.¹³ 

The process known as endotheliopathy of trauma is characterized by impaired blood flow, vascular barrier integrity, and coagulation.¹¹  Endotheliopathy of trauma-mediated destruction of endothelial integrity occurs within 5 to 8 min of injury²¹  and precipitates increased vascular permeability and capillary leak.³  The endothelial glycocalyx responds to severe trauma (and other shock states) by shedding its various components.^11,22  The resulting exposure of endothelial cells to circulating platelets and leukocytes triggers an overwhelming inflammatory response^3,22  and a breakdown of the endothelial glycocalyx mediated by matrix metalloproteases, heparanases, and proteases.²³  Syndecan-1 is considered the prototypical marker of glycocalyx shedding after traumatic injury.³  The presence of circulating syndecan-1, an endothelial glycocalyx constituent, merely indicates the loss of glycocalyx integrity and associated endothelial injury, although the reason for this shedding is unclear.²⁴  Therefore, measurements of plasma concentrations of syndecan-1 fragments reflect the degree of endothelial glycocalyx injury after major trauma. Indeed, an increased plasma syndecan-1 concentration at hospital admission is independently associated with increased trauma mortality, even after adjusting for injury severity.^9,11  Emerging preclinical studies suggest that stabilization of syndecan-1 may mitigate endothelial degradation and stability of the glycocalyx.^3,10,15  As a surrogate for endothelial dysfunction, syndecan-1 concentration in plasma can be used as both a diagnostic and prognostic tool in critically ill trauma patients.

One prevailing theory for the etiology of endotheliopathy of trauma relates to the release of high plasma concentrations of catecholamines after a traumatic injury. Shock-induced endotheliopathy may represent a unifying pathophysiologic mechanism responsible for poor outcomes among severely injured patients.^25,26  Previous observational studies demonstrated an association between trauma-induced catecholamine surges and biomarkers of endothelial damage (including syndecan-1), which are both predictive of mortality.¹²  Shock-induced endotheliopathy may therefore be a critical driver of endothelial glycocalyx damage across a variety of critically ill populations.²⁶  Regardless of the pathophysiologic mechanism of endothelial injury after traumatic injury, correction of endotheliopathy and restoration of endothelial barrier integrity remain important targets for future resuscitative therapies.

Endotheliopathy of trauma plays a role in the development of acute respiratory distress syndrome (ARDS) in critically injured patients. Elevated plasma concentrations of syndecan-1 are a surrogate for endothelial damage and contribute to the development of ARDS in critically ill patients.²⁷  The pulmonary system is the most susceptible organ system, among others, to trauma-induced endothelial glycocalyx damage. The incidence of ARDS after trauma appears to be directly correlated with the volume of resuscitative fluid given, and the volume of fluid administered is associated with the degree of endothelial glycocalyx damage.²⁸  In patients with severe ARDS, mortality remains in excess of 40%.²⁹  Indeed, ARDS is one of the key downstream contributors to late mortality after severe trauma.⁷  Given the emerging role of the endothelial glycocalyx as a central part of trauma pathophysiology, developing therapeutic strategies to address glycocalyx degradation after trauma may mitigate late pulmonary injury.

Endotheliopathy of trauma is associated with multiple organ dysfunction syndrome.²¹  Acute kidney injury (AKI) is also common after traumatic injury, with up to half of critically injured patients experiencing some degree of renal dysfunction.³⁰  While the pathogenesis of post-traumatic AKI remains poorly understood and is likely multifactorial, endothelial dysfunction may play a role. A recent prospective cohort study observed an association between elevated syndecan-1 and soluble thrombomodulin—markers of endothelial breakdown—and increase incidence, severity, and duration of AKI in severely injured patients.³¹  Trauma-induced secondary (indirect) cardiac injury is often undetected, but it has been observed in more than 10% of patients after trauma and is associated with worse clinical outcomes, including mortality.^32,33  Plasma concentrations of syndecan-1 have also been identified as markers of trauma-related myocardial stress.³⁴  Therefore, therapies targeting the endotheliopathy of trauma have the potential to reduce the trauma-related kidney, cardiac, and multiorgan dysfunction.

Two large, well designed studies recently altered our fundamental understanding of transfusion in bleeding patients. The Prospective Observational, Multicenter, Major Trauma Transfusion (PROMMTT) study⁴  observed that higher plasma and platelet ratios early in trauma resuscitation were associated with decreased mortality in patients receiving at least three units of blood products. The Transfusion of Plasma, Platelets, and Red Blood Cells (PROPPR) randomized trial⁵  demonstrated that a balanced initial resuscitation of 1:1:1 (plasma:platelets:erythrocytes) decreased mortality due to bleeding in the first 6 h compared to ratios emphasizing greater erythrocyte proportions. Both studies were large, well designed, and enrolled trauma patients at North American level 1 trauma centers. Their findings suggested that a balanced transfusion approach to optimize bleeding patients’ resuscitation more closely mimics whole blood composition. Despite these compelling findings, recent survey data suggest that many major, level 1 trauma centers in the United States do not promote a balanced resuscitation strategy in their massive transfusion protocols (MTPs).³⁵  Providers should consider emphasizing early plasma transfusion for trauma patients whenever possible.

Three additional studies investigated the role of very early (prehospital) plasma transfusion for trauma patients with conflicting results. The Prehospital Plasma during Air Medical Transport in Trauma Patients at Risk for Hemorrhagic Shock (PAMPer) randomized trial⁷  found that prehospital plasma administration to trauma patients significantly decreased 30-day mortality. Conversely, the Control of Major Bleeding after Trauma (COMBAT)³⁶  trial did not observe a mortality benefit from prehospital plasma administration. The Resuscitation with Blood Products in Patients with Trauma-related Hemorrhagic Shock Receiving Prehospital Care (RePHILL) trial³⁷  also did not show a mortality benefit for prehospital plasma transfusion. These conflicting results require further investigation of the prehospital administration of plasma for early replacement of fibrinogen, but they do not diminish the mortality benefit observed with a balanced transfusion ratio once patients arrive at the hospital.

Although early, plasma-based resuscitation seems to decrease early hemorrhagic death after trauma, the physiologic basis for these findings has only recently been elucidated. Recent findings have shown that the transfusion of plasma inhibits vascular permeability, attenuates tissue edema, and decreases endothelial inflammation.³⁸  Plasma also plays a crucial role in repairing and restoring the endothelial glycocalyx. However, the role of plasma itself in a balanced resuscitation strategy may be overstated. A secondary analysis of the PROPPR trial concluded that early platelet administration (not plasma) may have contributed the most to the observed improvement in hemostasis and mortality.³⁹  Therefore, while the survival benefit of early plasma transfusion in trauma may be related, at least partly, to mitigation of endotheliopathy of trauma, more human data are needed to support these claims.

Fibrinogen depletion is the first abnormal routine coagulation parameter to be detected after traumatic hemorrhage.⁴⁰  It plays an essential role in hemostasis as the precursor of fibrin, which binds platelet glycoprotein IIb/IIIa receptors on the activated platelet surface to create a stabilized, contractile mesh over the site of injury.³  Rapidly decreasing plasma fibrinogen concentrations after major traumatic hemorrhage are associated with poor patient outcomes. For example, hypofibrinogenemia (plasma fibrinogen less than 200 ng/mL) at admission is an independent predictor of mortality in trauma patients.⁴¹  The degree of hypofibrinogenemia is also correlated with increased injury severity.⁴²  Ensuring an adequate plasma fibrinogen concentration should be considered during the initial management of trauma patients.

Fibrinogen appears to be the key component of plasma that contributes to improved mortality by stabilizing the endothelial glycocalyx.^10,43  Human studies first demonstrated a decrease in syndecan-1 shedding via transfusion of plasma.²²  Later, in vitro studies identified fibrinogen as the component in plasma interacting with syndecan-1.⁴⁴  In these studies, pulmonary endothelial cells were incubated in lactated ringers, plasma, fibrinogen, or fibrinogen-deficient plasma. Pulmonary endothelial cells treated with fibrinogen-depleted plasma demonstrated increased endothelial permeability, comparable to lactated Ringer’s solution–treated cells, while cells incubated with plasma and fibrinogen exhibited reduced permeability. In a murine trauma model, cryoprecipitate administration attenuated endotheliopathy of trauma similarly to plasma.⁴⁵  The underlying mechanism appears to be the interaction between fibrinogen and syndecan-1.¹⁰  Fibrinogen stabilizes the syndecan-1 proteoglycan, therefore “rebuilding” the endothelial glycocalyx, restoring microcirculatory barrier integrity, and mitigating the endotheliopathy of trauma. Stabilization of syndecan-1 is also associated with a decreased risk of pulmonary and multiorgan dysfunction after trauma (fig. 1).³  Unfortunately, human studies are still needed to confirm the role of fibrinogen-based stabilization of the endothelial glycocalyx on reduced mortality with early administration of plasma after trauma. Another component of plasma’s mortality benefit may be related to its correction of coagulopathy, which predicts mortality in a dose-dependent fashion as injury severity increases.⁴⁶  Indeed, the fibrinogen concentration in plasma is low, and early plasma transfusion is insufficient to treat endotheliopathy of trauma by itself.

In the United States, cryoprecipitate is often administered to provide targeted fibrinogen replacement. Plasma contains approximately 270 mg/dL of fibrinogen,⁴⁷  and cryoprecipitate contains about 125 mg/dL⁴⁸  or 1.25 g/5-unit pool. Cryoprecipitate is prepared from plasma and has high concentrations of von Willebrand factor, factor VIII, and factor XIII.⁴⁵  However, current MTP strategies include cryoprecipitate only late in the protocol, in response to low plasma fibrinogen concentrations confirmed or suggested by viscoelastic coagulation assays such as rotational thromboelastometry or as an adjunct to uncontrolled bleeding or ongoing coagulopathy. The typically 20 to 30 min required for cryoprecipitate to thaw and other practical challenges of early fibrinogen replacement may hinder the inclusion of early cryoprecipitate in MTPs.^49,50  Not surprisingly, cryoprecipitate use varies widely by institution, ranging from 7 to 82%.⁴⁹  In patients who do receive cryoprecipitate, the median time from admission to administration is 2.7 h.⁴⁹  Timely administration of cryoprecipitate remains an ongoing challenge.

Fibrinogen concentrates are an appealing alternative to cryoprecipitate for fibrinogen replacement. They allow for rapid delivery of a standardized quantity of fibrinogen (1 g in 50 mL, or 2,000 mg/dL) without the risks of hemodilution, volume overload, immune incompatibility reactions, or transmissible diseases inherent to blood products.⁵¹  In hemorrhagic shock, early administration of fibrinogen concentrate may improve survival and does not increase thromboembolic events.⁵²  In other populations, including cardiac surgery,⁵³  postpartum hemorrhage,⁵⁴  and major abdominal surgery,⁵⁵  fibrinogen concentrate has also not been associated with thromboembolic or other serious adverse events. Retrospective and preclinical data demonstrate improved outcomes after the administration of fibrinogen concentrate versus plasma, suggesting that early fibrinogen concentrate administration in trauma patients may be advantageous.^45,56  Nonetheless, transfusion with balanced blood components remains the standard of care for patients with traumatic hemorrhagic shock,^5,6  and fibrinogen concentrate administration remains an uncommon method of fibrinogen replacement, perhaps due to increased cost compared to blood products or insufficient evidence to support its use. Clinicians may also prefer plasma or cryoprecipitate transfusion in the acute setting due to their superior volume expansion properties compared to fibrinogen concentrate. More clinical studies are needed to elucidate whether early transfusion of greater fibrinogen concentrations via cryoprecipitate or fibrinogen concentrate accelerates endothelial recovery and reduces trauma-related organ dysfunction and mortality. Most endotheliopathy of trauma-focused studies showing the interplay between endotheliopathy of trauma, fibrinogen, and organ dysfunction have been limited to in vitro and animal models,^{10,14,15,43,44,57}  and evidence in humans still remains under investigation.

Four small (fewer than 100 patients/study) randomized trials focused on coagulation endpoints have investigated fibrinogen concentrate as replacement therapy for fibrinogen in bleeding patients. The Fibrinogen in the Initial Resuscitation of Severe Trauma (FiiRST) trial⁵⁸  demonstrated the feasibility of administering fibrinogen concentrate within 1 h of hospital arrival for hypotensive bleeding patients. This Canadian trial compared 6 g of fibrinogen concentrate versus placebo in trauma patients who received at least one blood transfusion. They found that 96% of patients received their assigned intervention within 1 h of hospital arrival. Plasma fibrinogen concentrations were greater in the fibrinogen concentrate group (2.9 vs. 1.8 mg/dL) and remained greater 12 h later. Mortality and thromboembolic complications were similar in both groups. The European Fibrinogen concentrate in trauma patients bleeding or presumed to bleed (FIinTIC) trial⁵⁰  also compared early fibrinogen concentrate administration to placebo in a prehospital setting. As expected, median plasma fibrinogen concentrations were higher and the measurements on rotational thromboelastometry fibrinogen function test (FIBTEM) were greater in the fibrinogen concentrate group than patients receiving placebo.

Conversely, the Early Fibrinogen in Trauma 1 (E-FIT 1) trial⁵⁹  investigators in the United Kingdom (again comparing fibrinogen concentrate to placebo) were only able to administer fibrinogen concentrate to 69% of their cohort within 45 min of hospital arrival. The FiiRST, FIinTIC, and E-FIT 1 trials demonstrate the challenges inherent to the timely delivery of fibrinogen concentrate. However, these early clinical results suggest that fibrinogen concentrate administration improves clot strength and reduces blood loss.⁶⁰  The single-center Reversal of Trauma Induced Coagulopathy (RETIC) trial⁶¹  found a decreased need for massive transfusion or rescue therapies among bleeding trauma patients who received coagulation factor concentrates (primarily fibrinogen concentrate) versus plasma. This trial was terminated early for futility and safety reasons because patients receiving plasma demonstrated a higher incidence of requiring rescue coagulopathy therapy and massive transfusion than patients in the fibrinogen concentrate group. Finally, the multicenter Viscoelastic hemostatic assay augmented protocols for major trauma hemorrhage (ITACTIC) trial⁶²  found no difference in outcomes for trauma patients when either viscoelastic coagulation assays or conventional coagulation tests were used to guide the initial resuscitation. Notably, patients in the viscoelastic arm receive a median of 4 g of fibrinogen supplementation, compared with none in the conventional coagulation arm and yet experienced no benefit with respect to avoiding MTP therapy or death.

More recently, the Fibrinogen Early in Severe Trauma studY (FEISTY)⁶³  exploratory multicenter trial randomized adult trauma patients with clinically significant bleeding in Australia to receive either fibrinogen concentrate or cryoprecipitate, focusing on time to fibrinogen supplementation. The median time to administration was significantly faster for fibrinogen concentrate than for cryoprecipitate (29 vs. 60 min). Both methods reliably increased plasma fibrinogen concentrations. A follow-up study, the FEISTY-2 trial (NCT05449834), evaluating the efficacy of fibrinogen concentrate versus cryoprecipitate for improving days alive and out of the hospital at 90 days (primary outcome), is currently enrolling in Australia. Trial completion is anticipated in 2026. Another anticipated study, the CRYOSTAT-2 (ISRCTN 14998314) randomized clinical trial, is currently enrolling patients in the United Kingdom. This trial will be the first to provide large-scale, randomized evidence on the effect of early administration of high-dose cryoprecipitate versus standard therapy on mortality in trauma patients receiving massive transfusion. Finally, the Pilot Randomized trial of Fibrinogen in Trauma Hemorrhage (PRoof-iTH),⁶⁴  comparing fibrinogen concentrate to placebo, has completed recruitment, with results awaited.

Based on preclinical evidence, fibrinogen replacement may mitigate the endotheliopathy of trauma. Most studies of endotheliopathy of trauma are in vitro or rodent models. Very few clinical studies (and no clinical trials) exist to guide endotheliopathy-focused resuscitation in humans. Several clinical studies have demonstrated that early fibrinogen replacement is safe and feasible in trauma patients—and may improve patient outcomes. However, studies linking fibrinogen replacement to endothelial stability in humans are still lacking.

Future clinical trials should prioritize comparing endothelial glycocalyx breakdown products between critically injured patients in hemorrhagic shock who receive early fibrinogen repletion versus those who do not. The mechanistic underpinnings of a novel or repurposed therapy should complement the investigation of clinical outcomes and are essential to further our understanding of transfusion science. If fibrinogen-based stabilization of the endothelial glycocalyx is, in fact, a mechanism for improved clinical outcomes among trauma patients, perhaps such therapies could be applied to other endotheliopathy-inducing disease processes. Sepsis,⁶⁵  cognitive impairment,⁶⁶  and COVID-19⁶⁷  are just a few of the numerous diseases that could be treated in this manner. Unlocking the key to endothelial stabilization could have ramifications across the medical spectrum.

Endotheliopathy of trauma is gaining recognition as an essential component of trauma pathophysiology. Increased plasma concentrations of syndecan-1, a surrogate for endothelial dysfunction, may serve as a diagnostic and prognostic biomarker in trauma. Recent preclinical studies have demonstrated the efficacy of early fibrinogen replacement in mitigating endothelial injury via stabilization of syndecan-1. Further, early fibrinogen replacement after trauma may provide benefits beyond the correction of coagulopathy. The relationship between fibrinogen-based stabilization of the endothelial glycocalyx and improved patient-centered outcomes could offer novel therapeutic targets after traumatic injury. Therefore, further human studies need to establish the benefit of early fibrinogen replacement after trauma regarding endotheliopathy correction and relevant clinical outcomes.

Supported by National Institutes of Health (Bethesda, Maryland) grant No. T32GM135169 (to Dr. Douin).

Dr. Fernandez-Bustamante reports research funding from the National Institutes of Health, the Department of Defense (Washington, D.C.), the Merck Investigator-initiated Studies Program (Rahway, New Jersey), and the Institute for Healthcare Quality, Safety and Efficiency (Aurora, Colorado) for projects unrelated to the discussed work. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing official policies, either expressed or implied, of the National Institutes of Health or the Department of Defense. Dr. Douin declares no competing interests.