Prehospital tranexamic acid is associated with a dose-dependent decrease in syndecan-1 after trauma: A secondary analysis of a prospective randomized trial

Prehospital TXA was associated with a dose-dependent decrease in circulating syndecan-1 concentrations suggesting a pro-endothelial therapeutic effect.

Abstract

BACKGROUND

In the Study of Tranexamic Acid During Air and Ground Prehospital Transport (STAAMP) Trial, prehospital tranexamic acid (TXA) was associated with lower mortality in specific patient subgroups. The underlying mechanisms responsible for a TXA benefit remain incompletely characterized. We hypothesized that TXA may mitigate endothelial injury and sought to assess whether TXA was associated with decreased endothelial or tissue damage markers among all patients enrolled in the STAAMP Trial.

METHODS

We collected blood samples from STAAMP Trial patients and measured markers of endothelial function and tissue damage including syndecan-1, soluble thrombomodulin (sTM), and platelet endothelial cell adhesion molecule-1 at hospital admission (0 hours) and 12 hours, 24 hours, and 72 hours after admission. We compared these marker values for patients in each treatment group during the first 72 hours, and modeled the relationship between TXA and marker concentration using regression analysis to control for potential confounding factors.

RESULTS

We analyzed samples from 766 patients: 383 placebo, 130 abbreviated dosing, 119 standard dosing, and 130 repeat dosing. Lower levels of syndecan-1, TM, and platelet endothelial cell adhesion molecule measured within the first 72 hours of hospital admission were associated with survival at 30 days (p < 0.001). At hospital admission, syndecan-1 was lower in the TXA group (28.30 [20.05, 42.75] vs. 33.50 [23.00, 54.00] p = 0.001) even after controlling for patient, injury, and prehospital factors (p = 0.001). For every 1 g increase in TXA administered over the first 8 hours of prehospital transport and hospital admission, there was a 4-ng/mL decrease in syndecan-1 at 12 hours controlling for patient, injury, and treatment factors (p = 0.03).

CONCLUSION

Prehospital TXA was associated with decreased syndecan-1 at hospital admission. Syndecan-1 measured 12 hours after admission was inversely related to the dose of TXA received. Early prehospital and in-hospital TXA may decrease endothelial glycocalyx damage or upregulate vascular repair mechanisms in a dose-dependent fashion.

LEVEL OF EVIDENCE

Therapeutic/Care Management; Level III.

Traumatic injury remains an important cause of morbidity and mortality. Rapid hemorrhage control is crucial, and early, targeted, damage-control strategies administered prior to hospital arrival demonstrate survival benefits following trauma.^1–3 In addition to providing prompt hemostasis for severely injured and bleeding patients, prehospital interventions may also mitigate downstream endothelial and immune system damage and dysregulation.⁴

Tranexamic acid (TXA) in particular has emerged as a useful therapeutic agent to promote hemostasis and to prevent blood loss in injured patients. Prior to its application to trauma, TXA was routinely used in orthopedic, obstetrics and gynecology, and cardiothoracic patients in hospital and surgical settings.^5–7 Most recently, in the Study of Tranexamic Acid During Air and Ground Prehospital Transport (STAAMP) Trial, prehospital TXA was associated with lower mortality in specific subgroups of injured trauma patients who received TXA close to the time of their injuries.^3,8,9 However, the underlying mechanisms responsible for the benefits of prehospital TXA remain unknown.

There may be a beneficial relationship between TXA and the ensuing coagulopathy,^10,11 endotheliopathy,¹² and dysregulated immune responses^13,14 induced by shock and traumatic hemorrhage.^9,15–17 In vitro and animal experiments have shown that TXA reduces syndecan-1 shedding¹⁸ and protects the endothelial glycocalyx.^19,20 Importantly, these results may be sensitive to the dose and timing of TXA administration.^15,21 Tranexamic acid has also been shown to have beneficial effects on vascular permeability, inflammation, and glycocalyx degradation beyond antifibronolysis, highlighting several gaps in our mechanistic understanding of this drug.⁷

While there is compelling evidence for a link between TXA and endothelial function, there remains a paucity of human studies examining the relationship between early TXA administration and endothelial damage in injured trauma patients.⁷ Our objective was to address this gap by characterizing key circulating markers of endothelial damage and function in injured patients who received prehospital TXA and were at risk for hemorrhage. We hypothesized that TXA mitigates endothelial injury and sought to assess whether TXA is associated with lower concentrations of circulating endothelial markers among STAAMP Trial patients (Fig. 1).

Figure 1.

Hypothesized mechanism of action of TXA in relation to hemostasis. Tissue damage activates the coagulation cascade. Primary and secondary hemostasis begin with platelet plug formation. Plasmin activates the degradation of fibrin mesh. These degradation products are hypothesized to promote inflammation and endothelial damage. TXA acts to reduce the conversion of plasminogen to plasmin, thereby reducing fibrinolysis. The dashed lines represent hypothesized relationships. Illustration adapted from previously published work.^4,13

METHODS

Study Design and Patient Population

We conducted a secondary analysis of the STAAMP Trial, a prospective prehospital Phase 3 multicenter double-blind randomized placebo-controlled trial. We enrolled patients at four level 1 trauma centers in the United States. We included patients at risk for hemorrhage based on prehospital hypotension (systolic blood pressure <90 mm Hg) or tachycardia (heart rate >110) before arrival to the hospital within an estimated 2 hours of injury. We randomized patients to receive 1 g of prehospital TXA or a placebo (saline) infused for 10 minutes followed by an in-hospital phase in which patients were randomized to an abbreviated (1 g total TXA), standard (2 g total TXA), or repeat bolus (3 g total TXA) dose at trauma center arrival (Supplemental Digital Content, Figure e1, http://links.lww.com/TA/C997). All in-hospital doses of TXA were administered within the first 8 hours of hospital admission prior to the 12-hour sampling time.³ Our study adheres to the CONSORT reporting guidelines and a detailed checklist is available in the Supplemental Digital Content, http://links.lww.com/TA/C998.

Blood Sample Collection and Analysis

We collected blood samples from STAAMP Trial patients at hospital admission (0 hours) and 12 hours, 24 hours, and 72 hours after admission. We analyzed three key mediators of endotheliopathy: syndecan-1, soluble thrombomodulin (sTM), and platelet endothelial cell adhesion molecule-1 (PECAM-1). Together, these markers represent injury associated with endothelial glycocalyx shedding (syndecan-1), endothelial cell injury (sTM), and tight junction disruption (PECAM-1).²² All three of these markers have been associated with mortality in hospitalized patients,²² and we chose these specific markers for their integral role in the host response to injury. Syndecan-1, sTM, and PECAM-1 were assayed by commercially available immunoassays in ethylenediaminetetraacetic acid plasma according to the manufacturer’s recommendations.^12,23 We analyzed soluble biomarkers representing damage to the glycocalyx (sCD138; Nordic Biosite, Copenhagen, Denmark), endothelium (sTM: Nordic Biosite, Copenhagen, Denmark), and tight junctions (PECAM-1: CD31; R&D Systems, Abingdon, UK). All marker concentrations are reported in ng/mL.

Statistical Analysis

We first compared marker concentrations for patients who received TXA vs. those of patients who received the placebo at each sampled time point over the first 72 hours of hospital admission. We used the 0 hour time point to compare all patients who received prehospital TXA vs. those patients who received the placebo (i.e., the hospital admission time point was used to compare all patients who had received 1 g of TXA to those who had received the placebo). We used the 12-hour time point as the first time point to compare patients who received variable doses of TXA over the first 8 hours. We also compared marker levels for survivors and nonsurvivors at 30 days (the primary STAAMP Trial endpoint).

We built two generalized linear models to understand the effect of (1) prehospital TXA on marker concentration at hospital admission (0 h) and (2) TXA dose administered in the prehospital and early in-hospital stay (12 hours). We modeled the relationship between TXA and marker concentration using regression analysis to control for known confounders in this trauma population. We analyzed the endothelial marker concentration at admission (0 hours) as a function of prehospital TXA controlling for age, traumatic brain injury (TBI), Abbreviated Injury Score (AIS) head, AIS chest, AIS abdomen, prehospital shock, prehospital crystalloid, prehospital packed red blood cells (PRBCs), and transfer status. Considering the TXA dose model, we analyzed the marker concentration at 12 hours as a function of TXA dose received over the first 8 hours and adjusted for factors important at this later time point: age, TBI, AIS head, AIS chest, AIS abdomen, prehospital shock, and transfer status. We analyzed resulting model coefficients using robust standard errors with a sandwich estimator. Both models were informed by previous biomarker analyses of injured trauma patients.⁴

We analyzed our data using R Version 4.1.2.²⁴ Categorical variables are presented as frequencies and percentages and tested using the Pearson's χ² test. Continuous variables are expressed as medians and interquartile ranges (IQRs) and tested using Mann-Whitney or Kruskal-Wallis tests, as appropriate. Statistical significance was determined at the probability (P) <0.05 level.

RESULTS

The STAAMP Trial assessed 30-day mortality for 927 patients who received prehospital TXA vs. those who received a placebo. After exclusion, the intention-to-treat study cohort comprised 903 patients: 447 patients received prehospital TXA and 456 patients received the placebo. This cohort included mostly male patients (686 [74%]) with a mean [SD] age of 42 years [18 years], a median [IQR] Injury Severity Score (ISS) of 12 [5, 22], and 30-day mortality of 9.1%. Patients sustained primarily blunt injuries and most were transferred directly from the scene of their injuries. The number of adverse events (e.g., venous thromboembolism) was similar across trial arms. Patients who received TXA compared with placebo in the prehospital setting did not have a lower rate of 30-day mortality (8.1% vs. 9.9%). In a post hoc secondary analysis, a lower 30-day mortality was found when TXA was administered to patients within 1 hour of injury (4.6% vs. 7.6% p = 0.002) and in patients with severe prehospital shock (SBP ≤ 70) (18.5% vs. 35.5% p = 0.003).

Of the STAAMP Trial patients, we analyzed blood samples from 766 patients at hospital admission: 383 placebo (0 g TXA), 130 abbreviated dosing (1 g TXA in the prehospital phase), 119 standard dosing (1 g in the prehospital phase and 1 g in the hospital phase, 2 g total), and 134 repeat dosing (1 g in the prehospital phase and 2 g in the hospital phase, 3 g total) (Supplemental Digital Content, Figure e1, http://links.lww.com/TA/C997).

The subset of STAAMP Trial patients for which samples were obtained was similar to those patients included in the primary STAAMP trial analysis. These patients were mostly male (75%) patients with median ages of 39 years [26, 55 years] who sustained blunt injuries (84%) with a median ISS of 12 [5, 21]. This cohort had a 30-day mortality rate of 6.4%, which was slightly lower than that of the overall cohort. In a comparison of the sampled and unsampled patients, patients who received blood samples were similar in terms of patient and injury factors but were more injured representative of a spectrum bias inherent to this type of study (i.e., patients who died prior to sample collection or who were too injured to receive blood sampling were not included in biomarker analyses). More specifically, patients who did not receive blood samples had higher ISS (14 [5, 29] vs. 12 [5, 21], p = 0.03), higher rates of Glasgow Coma Scale ≤ 8 (32% vs. 20%, p = 0.003), and greater mortality at 30 days (22.4% vs. 6.4%, p < 0.001).

Of the patients in this sampled cohort, patients who received the placebo were similar to those who received prehospital TXA in terms of patient demographics, injury characteristics, prehospital interventions, and outcomes, suggesting that similar to the original study these were well-randomized populations even after subsampling the population for biomarker analysis. Specifically, patients were similar ages (placebo 39 [26, 56] vs. TXA 38 [26, 51], p = 0.20), similarly injured (placebo ISS 11 [4, 21] vs. TXA ISS 12 [56, 21], p = 0.3). Patients who received TXA survived to 30 days at similar rates as those who received the placebo (94% vs. 91%, p = 0.4). There was also no difference in laboratory values including lactate (2.90 [1.90, 3.80] vs. 2.60 [1.80, 3.90], p = 0.6) and international normalized ratio (1.10 [1.00, 1.20] vs. 1.10 [1.00, 1.20], p = 0.8), nor was there any difference in adverse events across groups for multiple organ failure (7% vs. 8%, p = 0.6) or venous thromboembolism (VTE) (5% vs. 2%, p = 0.06) (Table 1).

TABLE 1.

Demographic and Clinical Characteristics of STAAMP Cohort With Blood Samples

Variables	Placebo (n = 381)	TXA (n = 379)	p
Age	39 [26–56]	38 [26–51]	0.20
Male sex	290 (76.1)	281 (74.1)	0.59
Blunt injury	325 (85.3)	313 (82.6)	0.36
TBI	86 (22.6)	88 (23.2)	0.92
ISS	11 [4–21]	12 [5–21]	0.32
GCS	14 [11–15]	15 [12–15]	0.21
Transfer	53 (14.0)	56 (15.0)	0.77
PH intubation	95 (24.9)	84 (22.2)	0.42
PH shock	102 (27.9)	106 (29.1)	0.77
PH crystalloid	500 [100–1,000]	500 [125–1,000]	0.86
PH blood	0 [0–0]	0 [0–0]	0.48
INR	1.10 [1.00–1.20]	1.10 [1.00–1.20]	0.80
Lactate	2.60 [1.80–3.90]	2.90 [1.90–3.80]	0.61
Alive at 30 d	348 (91.3)	355 (93.7)	0.42
MOF	31 (8.1)	26 (6.9)	0.60
VTE	9 (2.4)	20 (5.3)	0.06

Values are represented by numbers (percentages) and means [IQR]. TXA refers to patients who received prehospital TXA. Marker values for patients stratified by the primary intervention (prehospital TXA). The hours are represented by 0, 12, 24, and 72.

GCS, Glasgow Coma Scale; transfer, transferred from another hospital; PH, prehospital; MOF, multiple organ failure.

We first sought to assess whether there were differences in marker concentrations stratified by survival. Lower levels of syndecan-1, sTM, and PECAM-1 measured within the first 72 hours of hospital admission were associated with survival at 30 days (p < 0.001) illustrating the relationship of these markers with damage and death even as late as 30 days (Fig. 2). The only notable exception to this observation was that PECAM-1 did not differ among 30-day survivors and nonsurvivors at the 0-hour time point, suggesting that the 0-hour measurement of PECAM-1 was not associated with poor outcomes. Concentrations of syndecan-1 appeared to peak for nonsurvivors at 12 hours, while concentrations of sTM and PECAM-1 appeared to peak slightly later at approximately 24 hours following hospital admission.

Figure 2.

Mean concentration and standard error of the mean vs. biomarker sample time stratified by survival at 30 days (813 survivors and 81 nonsurvivors) for markers syndecan-1, sTM, and PECAM-1. All concentrations are in ng/mL. Survivors and nonsurvivors are statistically different for all survival group comparisons except for PECAM-1 at hospital admission (0 hours).

We next compared marker concentrations of syndecan-1, sTM, and PECAM-1 over the first 72 hours of hospital admission stratified by prehospital TXA administration without adjusting for potential confounding factors. At hospital admission, syndecan-1 was lower in the TXA group (28.30 [20.05, 42.75] vs. 33.50 [23.00, 54.00] p = 0.001) (Table 2). Importantly, this difference persisted even after controlling for patient, injury, and prehospital factors (p = 0.001) (Table 3). Platelet endothelial cell adhesion molecule-1 was slightly elevated for the TXA group at hospital admission (9.26 [7.52, 10.78] vs. 8.68 [7.32, 10.50], p = 0.043) (Table 2). However, after controlling for potential confounding variables, this difference did not persist (Table 3), consistent with the observation that PECAM-1 did not differ at the 0-hour time point when stratified by 30-day survival. There were no significant differences in marker concentrations at later time points (12–72 hours) (Table 2).

TABLE 2.

Marker Values for Patients Stratified by the Primary Intervention

Markers	Hours	Placebo	TXA	p
		(n = 381)	(n = 379)
PECAM-1	0	8.68 [7.32–10.50]	9.26 [7.52–10.78]	0.04
PECAM-1	12	9.01 [7.36–10.88]	8.96 [7.33–10.98]	0.70
PECAM-1	24	8.84 [7.04–10.84]	8.86 [7.45–10.47]	0.90
PECAM-1	72	8.22 [6.82–9.80]	8.16 [6.90–10.13]	0.69
Syndecan-1	0	33.50 [23.00–54.00]	28.30 [20.05–42.75]	0.001
Syndecan-1	12	29.25 [21.45–50.50]	29.90 [21.15–51.75]	0.64
Syndecan-1	24	27.90 [20.90–43.65]	29.20 [20.80–46.90]	0.58
Syndecan-1	72	30.90 [24.10–46.60]	32.00 [23.12–48.10]	0.67
sTM	0	4.25 [2.80–6.32]	4.15 [2.60–6.38]	0.55
sTM	12	4.10 [2.92–6.00]	3.90 [2.70–6.20]	0.37
sTM	24	4.20 [2.90–5.90]	4.30 [2.55–6.65]	0.79
sTM	72	3.30 [2.10–5.68]	3.50 [2.20–5.60]	0.92

The hours are represented by 0, 12, 24, and 72. TXA refers to patients who received prehospital TXA. Values are represented by means [IQR] and all values are in ng/mL.

TABLE 3.

Model Results at Hospital Admission (0 Hours) and 12 Hours

Marker	Hour	Estimate	p
PECAM-1	0	0.073	0.39
Syndecan-1	0	−5.2	0.001
sTM	0	−0.13	0.12
PECAM-1	12	−0.081	0.39
Syndecan-1	12	−4.2	0.03
sTM	12	−0.2	0.09

The hospital admission model accounts for prehospital TXA and the 12-hour model accounts for TXA dose (1, 2, or 3 g) prior to marker sampling time. Estimates are in ng/mL.

We then modeled the relationship between marker concentration at hospital admission and prehospital TXA treatment controlling for factors known to be important in this trauma population. Notably, syndecan-1 was lower for patients who received prehospital TXA even after for controlling for potential differences among patients (−5.2 ng/mL, p = 0.001). There was no difference in concentrations of PECAM-1 or sTM at hospital admission (0 hour) model (Table 3). We further explored whether these results were robust by conducting a sensitivity analysis for the syndecan-1 model. We found the difference in syndecan-1 to persist despite model adjustment.

Finally, we modeled the relationship between marker concentrations immediately following prehospital and in-hospital TXA doses over the first 8 hours of hospital admission (12-hour time point). For every 1 g increase in TXA administered over the first 8 hours of prehospital transport and hospital admission, there was a 4 ng/mL decrease in syndecan-1 at 12 hours controlling for patient, injury, and treatment factors (p = 0.03), further suggesting a dose-dependent relationship between early TXA administration and circulating levels of syndecan-1. There was no difference in marker concentrations of PECAM-1 or sTM at 12 hours, emphasizing the apparent importance of syndecan-1 over other circulating markers of endothelial damage and function (Table 3 and Fig. 3).

Figure 3.

(A) Mean syndecan-1 concentration (and standard error of the mean) at 0 hours grouped by prehospital TXA intervention. (B) Mean syndecan-1 concentration at 12 hours versus TXA dose prior to the sampled time point. Gray shaded area represents the model-estimated error. Concentrations are in ng/mL.

We conducted the same analyses for the 24-hour and 72-hour time points. However, there was no difference in marker values at these time points when we compared patients by prehospital TXA administration. In addition, there was no relationship between TXA and marker values when we analyzed TXA or the TXA dose response in these later (24- and 72-hour) models.

DISCUSSION

Through our evaluation of plasma endothelial markers among trauma patients at risk of hemorrhage, we demonstrated that treatment with prehospital TXA was associated with lower levels of syndecan-1 at hospital admission. In addition, circulating levels of syndecan-1 were related to TXA administration in a dose-dependent fashion. For every additional 1 g dose of TXA administered in the first 8 hours of hospital admission, there was a 4 ng/mL decrease in syndecan-1. We hypothesize that TXA may reduce glycocalyx shedding and serve as a pro-endothelial therapeutic agent. We did not find differences in circulating levels of syndecan-1 at subsequent time points (24 or 72 hours) or persistent changes in sTM or PECAM-1, which may reflect a temporal relationship between TXA and endothelial cell protection. Our results add further weight to the notion that it is important to not only assess clinical outcomes, but also to understand the mechanisms by which these interventions alter trauma biology at the cellular and molecular level.^25–28

Shock-induced endotheliopathy is a life-threatening complication of trauma²⁹ which may be precipitated within minutes following injury.³⁰ The vascular barrier serves not only as structural protection and support, but as an active mediator of molecular and chemical signaling.^13,31,32 The endothelial glycocalyx in particular is an important mediator of endothelial damage and immune function following injury.^{4,10,12,13,32,33} Advancements in trauma care increasingly recognize the importance of targeting the dynamic and complex nature of this host response to injury.^4,34,35 In in vitro and animal models of trauma, TXA has been shown to reduce glycocalyx degradation²⁰ and protect the endothelium.^7,21 Prehospital interventions impact the host response to injury and may specifically mitigate dysregulated endothelial and inflammatory processes.^4,32

We demonstrated a strong association between 30-day survival and circulating endothelial marker concentrations measured in this study. Syndecans are transmembrane glycoproteins that govern cytokines, chemokines, and adhesion molecule binding and signaling.¹⁸ Both syndecan-1 and sTM are endothelial damage markers^12,36 used to assess the degree of endothelial glycocalyx damage and predict mortality following trauma.^36–38 Platelet endothelial cell adhesion molecule-1 (PECAM-1) is expressed on endothelial cells and regulates cell adhesion, barrier function, and molecular signaling.³¹ Platelet endothelial cell adhesion molecule-1 aids in the ability of tissues to respond to injury and trauma³⁹ and is an important regulator of vascular permeability and immune mediator signaling in the setting of acute respiratory distress syndrome (ARDS).⁴⁰ After tissues are damaged, inflammatory mediators upregulate cell adhesion molecules such as PECAM-1 to promote leukocyte migration.³⁹ In our study, there was a significant difference in circulating syndecan-1, sTM, and PECAM-1 at 0-hour, 12-hour, 24-hour, and 72-hour time points stratified by 30-day survival, supporting the hypothesis that these markers are prognostic of long-term survival in injured trauma patients.

In part due to the time-sensitive nature and resource-limitations of trauma care, several studies have aimed to identify the patients who may benefit most from TXA.^6,41 It has been shown that TXA improves survival in patients who receive it as early as possible following injury,^3,42 are more severely injured or experience severe shock,³ concurrently receive blood products,⁹ and have mild to moderate TBI.⁴³ These findings may be related to the potential mechanisms of action of TXA. The only significant marker differences between the TXA and placebo groups occurred at 0 hour and 12 hours. These two time points represent the closest blood samples relative to TXA administration following the prehospital TXA administration and the standard and repeat TXA doses, respectively. Tranexamic acid effects may be both dose- and time-dependent^33,44 and the timing of biomarker sampling may also be important to properly understand the endothelial response. In an exploratory analysis of normotensive (SBP ≥ 90 mm Hg) moderate-to-severe TBI patients, patients who received TXA within 2 hours of their injuries were noted to have lower levels of syndecan-1, consistent with our results.⁴⁵ In contrast, a prospective study of 91 injured trauma patients without evidence of isolated TBI showed no difference in circulating concentrations of syndecan-1 or sTM stratified by TXA, but did reveal a strong temporal relationship between these markers and traumatic injury as well as multiple organ dysfunction syndrome.³⁰ Participants in the latter study differed in several ways, including in that they had blood samples collected between 4 hours and 12 hours after injury (in contrast to arrival at the hospital or closely following additional in-hospital TXA doses as measured in this analysis). Our results suggest that the timing of sample collection may be important; that the response to TXA may be as quick as the response to the injury itself. Our analysis is also limited by the difficulties inherent to studying time-sensitive interventions in trauma and the availability of pre-intervention sampling and collection time data. However, in future studies it would be important to obtain high-resolution blood sampling as close to the time of injury and intervention as possible to better understand the kinetics of endothelial biomarkers and to capture the host response to injury and interventions.

Tranexamic acid prevents the conversion of plasminogen to plasmin thereby decreasing the degradation of fibrin and reducing bleeding, though it is uncertain whether the beneficial effects of TXA reported here and in the literature are entirely plasmin-mediated, proendothelial, and/or anti-inflammatory.^7,44 We did not measure markers of inflammation or hyperfibrinolysis in this analysis. However, future studies should target these mechanistic effects by building plasmin-free models and analyzing additional d-dimer and thromboelastography data to distinguish hyperfibrinolytic-independent effects. We performed a sensitivity analysis of our models. Removing prehospital blood and fluid administration from our hospital admission model did not change our results, suggesting that the beneficial effect of TXA may extend beyond reducing the transfusion requirements.

The results we report in this study represent an important step toward understanding the relationship between trauma and potential mitigators of endotheliopathy such as TXA treatment. However, there are several limitations inherent to this secondary analysis of a prospective randomized trial. First, there are differences among individual prehospital and in-hospital providers, though this bias may be partially mitigated by the randomization scheme and our statistical analyses. Second, although our models were robust to several sensitivity analyses, there may be unknown and unaccounted for confounding factors missing in our models. Third, the markers measured in this study are known to exist in other tissues of the body.²² Circulating markers measured in this study may also represent contributions from non-endothelial sources. Fourth, this analysis is limited by the times at which we were able to sample blood from injured trauma patients, which may mask important differences. Some of the sickest patients were also unable to be sampled due to time critical interventions or early death, representative of spectrum bias, however this would likely bias our results to the null rather than increasing the observed differences reported here. Similar to the primary trial analysis, rates of VTE were slightly higher in the TXA group, though there was no significant difference in VTE across trial arms in our study population. This may reflect survival bias, as patients must survive their initial injuries in order to develop adverse events. It is nonetheless important to understand these outcomes and to try to optimize the patients for whom TXA would benefit in future work. Finally, it is unclear whether any potential beneficial effects of TXA are entirely plasmin mediated and whether the effects are predominantly anti-inflammatory versus proendothelial. Future studies should aim to assess these mechanistic effects further by testing plasmin-free models and concomitantly interrogating d-dimer and thromboelastography data to distinguish hyperfibrinolytic-independent effects. Importantly, although the absolute reduction in syndecan-1 in the TXA group is statistically significant, it is unknown if this represents a meaningful difference that equates to a healthier endothelium. The absolute value of syndecan-1 indicative of endothelial health is unknown.

In conclusion, prehospital TXA was associated with lower levels of syndecan-1 at hospital admission. Tranexamic acid administered during the first 8 hours of hospital admission was also associated with lower levels of syndecan-1 in a dose dependent fashion. Based on the results of our study, we hypothesize that TXA may decrease endothelial damage after injury. Further translational and basic science research will be important in order to investigate these proposed mechanisms of TXA in human trauma.

Supplementary Material

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