Platelet activation and dysfunction in a large-animal model of traumatic brain injury and hemorrhage:

Identification of injury and shock driven effects on ex vivo platelet aggregometry: A cautionary tale of phenotyping

BACKGROUND

Platelet behavior in trauma-induced coagulopathy is poorly understood. Injured patients have impaired platelet aggregation (dysfunction) in ex vivo agonist-stimulated platelet aggregometry (PA). However, PA assumes that platelets are inactivated before ex vivo stimulated aggregation, which may be altered by injury. We hypothesized that following trauma, platelet aggregation (area under the curve) is decreased regardless of injury burden, but that (1) minor injury is associated with an increased baseline electrical impedance, characteristic of a functional platelet phenotype (platelets that activate in response to injury), and that (2) severe injury is not associated with an increased baseline electrical impedance, characteristic of a dysfunctional phenotype (platelets that do not activate well in response to injury) compared with healthy controls.

METHODS

Blood from 458 trauma patients and 30 healthy donors was collected for PA. Baseline electrical impedance (Ω); platelet aggregation stimulated by adenosine diphosphate, collagen, thrombin, and arachidonic acid; and rotational thromboelastometry were measured. Multivariate regression was performed to identify associations of PA measures with blood transfusion.

RESULTS

Compared with healthy controls, injured patients had impaired platelet aggregation in response to ex vivo stimulation, regardless of injury burden. However, minorly injured patients had increased endogenous platelet activation (baseline electrical impedance, Ω: with shock, p = 0.012; without shock, p = 0.084), but severely injured patients did not have significant increases in endogenous platelet activation (baseline electrical impedance, Ω: with shock, p = 0.86; without shock, p = 0.37). For every 10 Ω increase in baseline electrical impedance, there was an 8% decrease in units of blood transfused in the first 24 h (-0.08; confidence interval, -0.14 to -0.02; p = 0.015).

CONCLUSION

Injury and shock confer differential patterns of platelet aggregation in PA. Minor injury overestimates the presence of platelet dysfunction, while severe injury induces a truly dysfunctional phenotype-platelets that do not activate nor aggregate appropriately after injury. This is consequential in improving accurate phenotyping of postinjury platelet behavior for platelet-based therapeutics.

LEVEL OF EVIDENCE

Prognostic, level IV.

Microvesicles generated following traumatic brain injury induce platelet dysfunction via adenosine diphosphate receptor

BACKGROUND

Traumatic brain injury (TBI) can result in an acute coagulopathy including platelet dysfunction that can contribute to ongoing intracranial hemorrhage. Previous studies have shown adenosine diphosphate (ADP)-induced platelet aggregation to be reduced after TBI. In addition, circulating microvesicles (MVs) are increased following TBI and have been shown to play a role in post-TBI coagulopathy and platelet function. We hypothesized that post-TBI MVs would affect platelet aggregation in a murine head injury model.

METHODS

Moderate TBI was performed using a weight-drop method in male C57BL6 mice. Whole blood, plasma, MVs, and MV-poor plasma were isolated from blood collected 10 minutes following TBI and were mixed separately with whole blood from uninjured mice. Platelet aggregation was measured with Multiplate impedance platelet aggregometry in response to ADP. The ADP P2Y12 receptor inhibitor, R-138727, was incubated with plasma and MVs from TBI mice, and platelet inhibition was again measured.

RESULTS

Whole blood taken from 10-minute post-TBI mice demonstrated diminished ADP-induced platelet aggregation compared with sham mice. When mixed with normal donor blood, post-TBI plasma and MVs induced diminished ADP-induced platelet aggregation compared with sham plasma and sham MVs. By contrast, the addition of post-TBI MV-poor plasma to normal blood did not change ADP-induced platelet aggregation. The observed dysfunction in post-TBI ADP platelet aggregation was prevented by the pretreatment of post-TBI plasma with R-138727. Treatment of post-TBI MVs with R-138727 resulted in similar findings of improved ADP-induced platelet aggregation compared with nontreated post-TBI MVs.

CONCLUSION

Adenosine diphosphate-induced platelet aggregation is inhibited acutely following TBI in a murine model. This platelet inhibition is reproduced in normal blood by the introduction of post-TBI plasma and MVs. Furthermore, observed platelet dysfunction is prevented when post-TBI plasma and MVs are treated with an inhibitor of the P2Y12 ADP receptor. Clinically observed post-TBI platelet dysfunction may therefore be partially explained by the presence of the ADP P2Y12 receptor within post-TBI MVs.

LEVEL OF EVIDENCE

Level III.

A systematic review of large animal models of combined traumatic brain injury and hemorrhagic shock

Traumatic brain injury (TBI) and severe blood loss (SBL) frequently co-occur in human trauma, resulting in high levels of mortality and morbidity. Importantly, each of the individual post-injury cascades is characterized by complex and potentially opposing pathophysiological responses, complicating optimal resuscitation and therapeutic approaches. Large animal models of poly-neurotrauma closely mimic human physiology, but a systematic literature review of published models has been lacking. The current review suggests a relative paucity of large animal poly-neurotrauma studies (N = 52), with meta-statistics revealing trends for animal species (exclusively swine), characteristics (use of single biological sex, use of juveniles) and TBI models. Although most studies have targeted blood loss volumes of 35-45%, the associated mortality rates are much lower relative to Class III/IV human trauma. This discrepancy may result from potentially mitigating experimental factors (e.g., mechanical ventilation prior to or during injury, pausing/resuming blood loss based on physiological parameters, administration of small volume fluid resuscitation) that are rarely associated with human trauma, highlighting the need for additional work in this area.

Valproic acid modulates platelet and coagulation function ex vivo

: Trauma-induced coagulopathy is associated with adverse patient outcome. Animal models demonstrate that histone deacetylase inhibitors, such as valproic acid (VPA), improve survival following injury. While in-vivo data suggest that improved survival may in part be because of an attenuation of coagulopathy, it remains unknown whether this is a direct effect of the drug, or the establishment of an overall prosurvival phenotype. We thus conducted an ex-vivo experiment to determine if VPA has an effect on coagulation and platelet function. Ten swine were subjected to traumatic brain injury (TBI) and hemorrhagic shock (HS). Blood samples were drawn prior to TBI+HS insult (Healthy group) and 2 h following TBI+HS (Shock group). Samples were incubated with VPA or vehicle controls for 1 h. Platelet aggregation was analyzed via impedance aggregometry and coagulation was measured using thromboelastography. Addition of VPA to the healthy blood did not affect platelet aggregation or coagulation parameters. In shock blood, incubation with VPA significantly reduced collagen-(P = 0.050), arachidonic acid-(P = 0.005), and adenosine diphosphate-(P = 0.023) induced platelet aggregation. VPA also significantly increased the clot strength (P = 0.002) and clot formation rate (P = 0.011). This is the first study to investigate the effect of VPA on platelet function ex vivo. Our results suggest that VPA has no effect on normal blood, but it decreases platelet activation and improves clot dynamics (strength and rate of formation) in blood from shocked animals. This suggests that VPA is capable of exerting a selective platelet sparing effect while enhancing the clot integrity.

Coagulopathy of Trauma

Coagulopathy is common after injury and develops independently from iatrogenic, hypothermic, and dilutional causes. Despite considerable research on the topic over the past decade, trauma-induced coagulopathy (TIC) continues to portend poor outcomes, including decreased survival. We review the current evidence regarding the diagnosis and mechanisms underlying trauma induced coagulopathy and summarize the debates regarding optimal management strategy including product resuscitation, potential pharmacologic adjuncts, and targeted approaches to hemostasis. Throughout, we will identify areas of continued investigation and controversy in the understanding and management of TIC.

Traumatic brain injury causes platelet adenosine diphosphate and arachidonic acid receptor inhibition independent of hemorrhagic shock in humans and rats

BACKGROUND

Coagulopathy in traumatic brain injury (CTBI) is a well-established phenomenon, but its mechanism is poorly understood. Various studies implicate protein C activation related to the global insult of hemorrhagic shock or brain tissue factor release with resultant platelet dysfunction and depletion of coagulation factors. We hypothesized that the platelet dysfunction of CTBI is a distinct phenomenon from the coagulopathy following hemorrhagic shock.

METHODS

We used thrombelastography with platelet mapping as a measure of platelet function, assessing the degree of inhibition of the adenosine diphosphate (ADP) and arachidonic acid (AA) receptor pathways. First, we studied the early effect of TBI on platelet inhibition by performing thrombelastography with platelet mapping on rats. We then conducted an analysis of admission blood samples from trauma patients with isolated head injury (n = 70). Patients in shock or on clopidogrel or aspirin were excluded.

RESULTS

In rats, ADP receptor inhibition at 15 minutes after injury was 77.6% ± 6.7% versus 39.0% ± 5.3% for controls (p < 0.0001). Humans with severe TBI (Glasgow Coma Scale [GCS] score ≤ 8) showed an increase in ADP receptor inhibition at 93.1% (interquartile range [IQR], 44.8-98.3%; n = 29) compared with 56.5% (IQR, 35-79.1%; n = 41) in milder TBI and 15.5% (IQR, 13.2-29.1%) in controls (p = 0.0014 and p < 0.0001, respectively). No patient had significant hypotension or acidosis. Parallel trends were noted in AA receptor inhibition.

CONCLUSION

Platelet ADP and AA receptor inhibition is a prominent early feature of CTBI in humans and rats and is linked to the severity of brain injury in patients with isolated head trauma. This phenomenon is observed in the absence of hemorrhagic shock or multisystem injury. Thus, TBI alone is shown to be sufficient to induce a profound platelet dysfunction.

Fresh frozen plasma resuscitation attenuates platelet dysfunction compared with normal saline in a large animal model of multisystem trauma

BACKGROUND

Platelet dysfunction following trauma has been identified as an independent predictor of mortality. We hypothesized that fresh frozen plasma (FFP) resuscitation would attenuate platelet dysfunction compared with 0.9% normal saline (NS).

METHODS

Twelve swine were subjected to multisystem trauma (traumatic brain injury, liver injury, rib fracture, and soft tissue injury) with hemorrhagic shock (40% of estimated blood volume). Animals were left in shock (mean arterial pressure, 30-35 mm Hg) for 2 hours followed by resuscitation with three times shed volume NS (n = 6) or one times volume FFP (n = 6) and monitored for 6 hours. Platelet function was assessed by adenosine diphosphate (ADP)-induced platelet aggregation at baseline, after 2 hours of shock following resuscitation, and 6 hours after resuscitation. Fibrinogen levels and markers of platelet activation (transforming growth factor β [TGF-β], sP-Selectin, and CD40L) as well as endothelial injury (intercellular adhesion molecule 1 [ICAM-1], vascular cell adhesion molecule 1 [VCAM-1]) were also assayed. Thromboelastography was used to measure clotting activity.

RESULTS

ADP-induced platelet aggregation was significantly higher in the FFP group (46.3 U vs. 25.5 U, p < 0.01) following resuscitation. This was associated with higher fibrinogen levels (202 mg/dL vs. 80 mg/dL, p < 0.01) but lower endothelial activation (VCAM-1, 1.25 ng/mL vs. 3.87 ng/mL, p = 0.05). Other markers did not differ.After 6 hours of observation, ADP-induced platelet aggregation remained higher in the FFP group (53.8 U vs. 37.0 U, p = 0.03) as was fibrinogen levels (229 mg/dL vs. 153 mg/dL, p < 0.01). Endothelial activation was lower (ICAM-1, 21.0 ng/mL vs. 24.4 ng/mL, p = 0.05), whereas TGF-β levels were higher (2,138 pg/mL vs. 1,802 pg/mL, p = 0.03) in the FFP group. Other markers did not differ. Thromboelastography revealed increased clot strength in the FFP group at both postresuscitation time points.

CONCLUSION

Resuscitation with FFP resulted in an immediate and sustained improvement in platelet function and clot strength compared with high-volume NS resuscitation. This was associated with an increase in fibrinogen levels and an attenuation of endothelial activation.

Assessment of coagulopathy, endothelial injury, and inflammation after traumatic brain injury and hemorrhage in a porcine model

BACKGROUND

Traumatic brain injury (TBI) and hemorrhagic shock (HS) can be associated with coagulopathy and inflammation, but the mechanisms are poorly understood. We hypothesized that a combination of TBI and HS would disturb coagulation, damage the endothelium, and activate inflammatory and complement systems.

METHODS

A total of 33 swine were allocated to either TBI + HS (n = 27, TBI and volume-controlled 40% blood loss) or controls (n = 6, anesthesia and instrumentation). TBI + HS animals were left hypotensive (mean arterial pressure, 30-35 mm Hg) for 2 hours. Blood samples were drawn at baseline, 3 minutes and 15 minutes after injury, as well as following 2 hours of hypotension. Markers of coagulation, anticoagulation, endothelial activation/glycocalyx shedding, inflammation, complement, and sympathoadrenal function were measured.

RESULTS

The TBI + HS group demonstrated an immediate (3 minutes after injury) activation of coagulation (prothrombin fragment 1 + 2, 289 ng/mL vs. 232 ng/mL, p = 0.03) and complement (C5a, 2.83 ng/mL vs. 2.05 ng/mL, p = 0.05). Shedding of the endothelial glycocalyx (syndecan 1) was evident 15 minutes after injury (851.0 ng/ml vs. 715.5 ng/ml, p = 0.03) while inflammation (tumor necrosis factor α [TNF-α], 81.1 pg/mL vs. 50.8 pg/mL, p = 0.03) and activation of the protein C system (activated protein C, 56.7 ng/mL vs. 26.1 ng/mL, p = 0.01) were evident following the 2-hour hypotension phase.

CONCLUSION

The combination of TBI and shock results in an immediate activation of coagulation and complement systems with subsequent endothelial shedding, protein C activation, and inflammation.

A prospective evaluation of platelet function in patients on antiplatelet therapy with traumatic intracranial hemorrhage

BACKGROUND

Platelet transfusion is increasingly used in patients with traumatic intracranial hemorrhage (ICH) on aspirin therapy to minimize the progression of ICH. We hypothesized (null) that platelet transfusion in this cohort of patients does not improve platelet function.

METHODS

We performed a prospective interventional trail on patients with traumatic ICH on daily high-dose (325 mg) aspirin therapy. All patients received one pack of apheresis platelets. Blood samples were collected before and 1 hour after platelet transfusion. Platelet function was assessed using Verify Now Platelet Function Assay, and a cutoff of greater than 550 aspirin reaction units was used to define functioning platelets (FP).

RESULTS

Twenty-eight patients were enrolled in the study. On presentation, 79% (22 of 28) of the patients had nonfunctioning platelets (NFPs), and transfusion of platelets did not improve platelet function as 81% (18 of 22) still had NFP. Of the 22 patients, 4 converted from NFP to FP after transfusion. There was no difference in the progression of ICH (37.5% vs. 30%, p = 0.7) or neurosurgical intervention (12.5% vs. 15%, p = 0.86) between patients with FP and NFP after platelet transfusion.

CONCLUSION

Administration of one pack of apheresis platelet did not improve platelet function. In our study, progression of ICH and the need for neurosurgical intervention were independent of platelet function. Further randomized clinical trials are required to assess both the dose dependence effect and role of platelet transfusion in patients on antiplatelet therapy with traumatic ICH.

LEVEL OF EVIDENCE

Therapeutic study, level III.