Hemolysate-mediated platelet aggregation: an additional risk mechanism contributing to thrombosis of continuous flow ventricular assist devices

Understanding the role of red blood cells in venous thromboembolism: A comprehensive review

Traditionally, red blood cells (RBCs) have been perceived as passive entities within the fibrin network, without any significant role in the pathophysiology of venous thromboembolism (VTE). This review explores the involvement of RBCs in the VTE process, summarizing previous study findings and providing a comprehensive review of the latest theories. At first, it explores the influence of abnormal RBC counts (as seen in polycythemia vera and with erythropoietin use) and the exposure of RBCs to phosphatidylserine (Ptd-L-Ser) in the pathophysiology of VTE. The mechanisms of endothelial injury induced by RBCs and their adhesion to the endothelium under different disease models are then demonstrated. We explore the role of physical and chemical interactions between RBCs and platelets, as well as the interactions between RBCs and neutrophils - particularly the neutrophil extracellular traps (NETs) released by neutrophils - in the process of VTE. Additionally, we investigate the effect of RBCs on thrombin activation through two pathways, namely, the FXIIa-FXI-FIX pathway and the prekallikrein-dependent pathway. Lastly, we discuss the impact of RBCs on clot volume. In conclusion, we propose several potential methods aimed at unraveling the role of RBCs and their interaction with other components in the vascular system in the pathogenesis of VTE.

Cryopreserved red blood cells maintain allosteric control of oxygen binding when utilizing trehalose as a cryoprotectant

One of the most common life-saving medical procedures is a red blood cell (RBC) transfusion. Unfortunately, RBCs for transfusion have a limited shelf life after donation due to detrimental storage effects on their morphological and biochemical properties. Inspired by nature, a biomimetics approach was developed to preserve RBCs for long-term storage using compounds found in animals with a natural propensity to survive in a frozen or desiccated state for decades. Trehalose was employed as a cryoprotective agent and added to the extracellular freezing solution of porcine RBCs. Slow cooling (-1 °C min-1) resulted in almost complete hemolysis (1 ± 1 % RBC recovery), and rapid cooling rates had to be used to achieve satisfactory cryopreservation outcomes. After rapid cooling, the highest percentage of RBC recovery was obtained by plunging in liquid nitrogen and thawing at 55 °C, using a cryopreservation solution containing 300 mM trehalose. Under these conditions, 88 ± 8 % of processed RBCs were recovered and retained hemoglobin (14 ± 2 % hemolysis). Hemoglobin's oxygen-binding properties of cryopreserved RBCs were not significantly different to unfrozen controls and was allosterically regulated by 2,3-bisphosphoglycerate. These data indicate the feasibility of using trehalose instead of glycerol as a cryoprotective compound for RBCs. In contrast to glycerol, trehalose-preserved RBCs can potentially be transfused without time-consuming washing steps, which significantly facilitates the usage of cryopreserved transfusible units in trauma situations when time is of the essence.

Microembolic signal monitoring in patients with HeartMate 3 and HeartWare left ventricular assist devices: Association with antithrombotic treatment and cerebrovascular events

BACKGROUND

In patients with left ventricular assist devices (LVADs), ischemic and hemorrhagic stroke are dreaded complications. Predictive markers for these events are lacking. This study aimed to investigate the prevalence and predictive value of microembolic signals (MES) for stroke, detected by Transcranial Doppler sonography (TCD) in patients with HeartMate 3 (HM 3) or HeartWare (HW).

METHODS

A thirty-minute bilateral TCD monitoring of the middle cerebral artery (MCA) was performed in 62 outpatients with LVAD (HM 3 N = 31, HW N = 31) and 31 healthy controls. Prevalence and quantity of MES were investigated regarding clinical and laboratory parameters. Cerebrovascular events (CVE) were recorded on follow-up at 90 and 180 days.

RESULTS

MES were detected in six patients with HM 3, three patients with HW, and three controls. Within the LVAD groups, patients on monotherapy with vitamin-K-antagonist (VKA) without antiplatelet therapy were at risk for a higher count of MES (negative binomial regression: VKA: 1; VKA + ASA: Exp(B) = 0.005, 95%CI 0.001-0.044; VKA + clopidogrel: Exp(B) = 0.012, 95%CI 0.002-0.056). There was no association between the presence of MES and CVE or death on follow-up (p > 0.05).

CONCLUSION

For the first time, the prevalence of MES was prospectively investigated in a notable outpatient cohort of patients with HM 3 and HW. Despite optimized properties of the latest LVAD, MES remain detectable depending on antithrombotic therapy. No association between MES and CVE could be detected.

Effects of the interactions between platelets with other cells in tumor growth and progression

It has been confirmed that platelets play a key role in tumorigenesis. Tumor-activated platelets can recruit blood cells and immune cells to migrate, establish an inflammatory tumor microenvironment at the sites of primary and metastatic tumors. On the other hand, they can also promote the differentiation of mesenchymal cells, which can accelerate the proliferation, genesis and migration of blood vessels. The role of platelets in tumors has been well studied. However, a growing number of studies suggest that interactions between platelets and immune cells (e.g., dendritic cells, natural killer cells, monocytes, and red blood cells) also play an important role in tumorigenesis and tumor development. In this review, we summarize the major cells that are closely associated with platelets and discuss the essential role of the interaction between platelets with these cells in tumorigenesis and tumor development.

Risk Factors of Thrombocytopenia After Cardiac Surgery with Cardiopulmonary Bypass

INTRODUCTION

Postoperative thrombocytopenia is common in cardiac surgery with cardiopulmonary bypass, and its risk factors are unclear.

METHODS

This retrospective study enrolled 3,175 adult patients undergoing valve surgeries with cardiopulmonary bypass from January 1, 2017 to December 30, 2018 in our institute. Postoperative thrombocytopenia was defined as the first postoperative platelet count below the 10th quantile in all the enrolled patients. Outcomes between patients with and without postoperative thrombocytopenia were compared. The primary outcome was in-hospital mortality. Risk factors of postoperative thrombocytopenia were assessed by logistic regression analysis.

RESULTS

The 10th quantile of all enrolled patients (75×109/L) was defined as the threshold for postoperative thrombocytopenia. In-hospital mortality was comparable between thrombocytopenia and non-thrombocytopenia groups (0.9% vs. 0.6%, P=0.434). Patients in the thrombocytopenia group had higher rate of postoperative blood transfusion (5.9% vs. 3.2%, P=0.014), more chest drainage volume (735 [550-1080] vs. 560 [430-730] ml, P<0.001), and higher incidence of acute kidney injury (12.3% vs. 4.2%, P<0.001). Age > 60 years (odds ratio [OR] 2.25, 95% confidence interval [CI] 1.345-3.765, P=0.002], preoperative thrombocytopenia (OR 18.671, 95% CI 13.649-25.542, P<0.001), and cardiopulmonary bypass time (OR 1.088, 95% CI 1.059-1.117, P<0.001) were positively independently associated with postoperative thrombocytopenia. Body surface area (BSA) (OR 0.247, 95% CI 0.114-0.538, P<0.001) and isolated mitral valve surgery (OR 0.475, 95% CI 0.294-0.77) were negatively independently associated with postoperative thrombocytopenia.

CONCLUSION

Positive predictors for thrombocytopenia after valve surgery included age > 60 years, small BSA, preoperative thrombocytopenia, and cardiopulmonary bypass time. BSA and isolated mitral valve surgery were negative predictors.

The Role of RBC Oxidative Stress in Sickle Cell Disease: From the Molecular Basis to Pathologic Implications

Sickle cell disease (SCD) is an inherited monogenic disorder and the most common severe hemoglobinopathy in the world. SCD is characterized by a point mutation in the β-globin gene, which results in hemoglobin (Hb) S production, leading to a variety of mechanistic and phenotypic changes within the sickle red blood cell (RBC). In SCD, the sickle RBCs are the root cause of the disease and they are a primary source of oxidative stress since sickle RBC redox state is compromised due to an imbalance between prooxidants and antioxidants. This imbalance in redox state is a result of a continuous production of reactive oxygen species (ROS) within the sickle RBC caused by the constant endogenous Hb autoxidation and NADPH oxidase activation, as well as by a deficiency in the antioxidant defense system. Accumulation of non-neutralized ROS within the sickle RBCs affects RBC membrane structure and function, leading to membrane integrity deficiency, low deformability, phosphatidylserine exposure, and release of micro-vesicles. These oxidative stress-associated RBC phenotypic modifications consequently evoke a myriad of physiological changes involved in multi-system manifestations. Thus, RBC oxidative stress in SCD can ultimately instigate major processes involved in organ damage. The critical role of the sickle RBC ROS production and its regulation in SCD pathophysiology are discussed here.

Oxidative Stress and Thrombosis during Aging: The Roles of Oxidative Stress in RBCs in Venous Thrombosis

Mid-life stage adults are at higher risk of developing venous thrombosis (VT)/thromboembolism (VT/E). Aging is characterized by an overproduction of reactive oxygen species (ROS), which could evoke a series of physiological changes involved in thrombosis. Here, we focus on the critical role of ROS within the red blood cell (RBC) in initiating venous thrombosis during aging. Growing evidence has shifted our interest in the role of unjustifiably unvalued RBCs in blood coagulation. RBCs can be a major source of oxidative stress during aging, since RBC redox homeostasis is generally compromised due to the discrepancy between prooxidants and antioxidants. As a result, ROS accumulate within the RBC due to the constant endogenous hemoglobin (Hb) autoxidation and NADPH oxidase activation, and the uptake of extracellular ROS released by other cells in the circulation. The elevated RBC ROS level affects the RBC membrane structure and function, causing loss of membrane integrity, and decreased deformability. These changes impair RBC function in hemostasis and thrombosis, favoring a hypercoagulable state through enhanced RBC aggregation, RBC binding to endothelial cells affecting nitric oxide availability, RBC-induced platelet activation consequently modulating their activity, RBC interaction with and activation of coagulation factors, increased RBC phosphatidylserine exposure and release of microvesicles, accelerated aging and hemolysis. Thus, RBC oxidative stress during aging typifies an ultimate mechanism in system failure, which can affect major processes involved in the development of venous thrombosis in a variety of ways. The reevaluated concept of the critical role of RBC ROS in the activation of thrombotic events during aging will help identify potential targets for novel strategies to prevent/reduce the risk for VT/E or VT/E recurrences in mid-life stage adults.

Hemolysis and Nonhemorrhagic Stroke During Venoarterial Extracorporeal Membrane Oxygenation

BACKGROUND

Hemolysis, even at low levels, activates platelets to create a prothrombotic state and is common during mechanical circulatory support. We examined the association of low-level hemolysis (LLH) and nonhemorrhagic stroke during venoarterial extracorporeal membrane oxygenation (VA ECMO) support.

METHODS

A single-center retrospective review of all adult patients placed on VA ECMO from January 2012 to September 2017 was conducted. To determine the association between LLH and nonhemorrhagic stroke, patients were categorized as those with and without LLH. LLH was defined by 48-hour plasma free hemoglobin (PFHb) of 11 to 50 mg/dL after VA ECMO implantation.

RESULTS

Of 201 patients who underwent VA ECMO placement, 150 (75%) met inclusion criteria and comprised the study population. They were 55 ± 14 years of age and 50 (33%) were women. Sixty-two (41%) patients had LLH. Patients with LLH had a higher likelihood of incident nonhemorrhagic stroke during VA ECMO support (20 [32%] versus 4 [5%]; adjusted hazard ratio [HR], 7.6; 95% confidence interval [CI], 2.2 to 25.9; p = 0.001). The severity of LLH was associated with an incrementally higher likelihood of a nonhemorrhagic stroke (PFHb 26 to 50 mg/dL: HR, 11.3; 95% CI, 3.6 to 35.1; p = 0.001; PFHb 11 to 25 mg/dL: HR, 4.4; 95% CI, 1.36 to 14.85; p = 0.014) in comparison with no LLH. Those with LLH had a 2-fold greater increase in mean platelet volume after VA ECMO placement (0.98 ± 1.1 fL versus 0.49 ± 0.96 fL; p = 0.03). Patients with a nonhemorrhagic stroke had a higher operative mortality (20 [83%] versus 57 [45%]; adjusted HR, 3.1; 95% CI, 1.8 to 5.3; p < 0.001).

CONCLUSIONS

Hemolysis at low levels during VA ECMO support is associated with subsequent nonhemorrhagic stroke.

Red blood cells: the forgotten player in hemostasis and thrombosis

New evidence has stirred up a long-standing but undeservedly forgotten interest in the role of erythrocytes, or red blood cells (RBCs), in blood clotting and its disorders. This review summarizes the most recent research that describes the involvement of RBCs in hemostasis and thrombosis. There are both quantitative and qualitative changes in RBCs that affect bleeding and thrombosis, as well as interactions of RBCs with cellular and molecular components of the hemostatic system. The changes in RBCs that affect hemostasis and thrombosis include RBC counts or hematocrit (modulating blood rheology through viscosity) and qualitative changes, such as deformability, aggregation, expression of adhesive proteins and phosphatidylserine, release of extracellular microvesicles, and hemolysis. The pathogenic mechanisms implicated in thrombotic and hemorrhagic risk include variable adherence of RBCs to the vessel wall, which depends on the functional state of RBCs and/or endothelium, modulation of platelet reactivity and platelet margination, alterations of fibrin structure and reduced susceptibility to fibrinolysis, modulation of nitric oxide availability, and the levels of von Willebrand factor and factor VIII in blood related to the ABO blood group system. RBCs are involved in platelet-driven contraction of clots and thrombi that results in formation of a tightly packed array of polyhedral erythrocytes, or polyhedrocytes, which comprises a nearly impermeable barrier that is important for hemostasis and wound healing. The revisited notion of the importance of RBCs is largely based on clinical and experimental associations between RBCs and thrombosis or bleeding, implying that RBCs are a prospective therapeutic target in hemostatic and thrombotic disorders.

A Novel Toroidal-Flow Left Ventricular Assist Device Minimizes Blood Trauma: Implications of Improved Ventricular Assist Device Hemocompatibility

BACKGROUND

Continuous-flow left ventricular assist devices (LVADs) cause blood trauma that includes von Willebrand factor degradation, platelet activation, and subclinical hemolysis. Blood trauma contributes to bleeding, thrombosis, and stroke, which cause significant morbidity and mortality. The TORVAD (Windmill Cardiovascular Systems, Inc, Austin, TX) is a first-of-its kind, toroidal-flow LVAD designed to minimize blood trauma. We tested the hypothesis that the TORVAD causes less blood trauma than the HeartMate II (Abbott Laboratories, Pleasanton, CA) LVAD.

METHODS

Whole human blood was circulated for 6 hours in ex vivo circulatory loops with a HeartMate II (n = 8; 10,000 rpm, 70 ± 6 mm Hg, 4.0 ± 0.1 L/min) or TORVAD (n = 6; 144 rpm, 72 ± 0.0 mm Hg, 4.3 ± 0.0 L/min). von Willebrand factor degradation was quantified with electrophoresis and immunoblotting. Platelet activation was quantified by cluster of differentiation (CD) 41/61 enzyme-linked immunosorbent assay (ELISA). Hemolysis was quantified by plasma free hemoglobin ELISA.

RESULTS

The TORVAD caused significantly less degradation of high-molecular-weight von Willebrand factor multimers (-10% ± 1% vs -21% ± 1%, p < 0.0001), accumulation of low-molecular-weight von Willebrand factor multimers (22% ± 2% vs 45% ± 2%, p < 0.0001), and accumulation of von Willebrand factor degradation fragments (7% ± 1% vs 25% ± 6%, p < 0.05) than the HeartMate II. The TORVAD did not activate platelets, whereas the HeartMate II caused significant platelet activation (CD 41/61: 645 ± 20 ng/mL vs 1,581 ± 150 ng/mL, p < 0.001; normal human CD 41/61, 593 ng/mL; range, 400 to 800 ng/mL). Similarly, the TORVAD caused minimal hemolysis, whereas the HeartMate II caused significant hemolysis (plasma free hemoglobin: 11 ± 2 vs 109 ± 10 mg/dL, p < 0.0001; normal human plasma free hemoglobin <4 mg/dL).

CONCLUSIONS

The TORVAD design, with markedly lower shear stress and pulsatile flow, caused significantly less blood trauma than the HeartMate II. LVADs with reduced blood trauma are likely to improve clinical outcomes and expand LVAD therapy into patients with less advanced heart failure.

Comprehensive Management Considerations of Select Noncardiac Organ Systems in the Cardiac Intensive Care Unit

As the acuity and complexity of pediatric patients with congenital cardiac disease have increased, there are many noncardiac issues that may be present in these patients. These noncardiac problems may affect clinical outcomes in the cardiac intensive care unit and must be recognized and managed. The Pediatric Cardiac Intensive Care Society sought to provide an expert review of some of the most common challenges of the respiratory, gastrointestinal, hematological, renal, and endocrine systems in pediatric cardiac patients. This review provides a brief overview of literature available and common practices.

Routine clinical anti-platelet agents have limited efficacy in modulating hypershear-mediated platelet activation associated with mechanical circulatory support

INTRODUCTION

Continuous flow ventricular assist devices (cfVADs) continue to be limited by thrombotic complications associated with disruptive flow patterns and supraphysiologic shear stresses. Patients are prescribed complex antiplatelet therapies, which do not fully prevent recurrent thromboembolic events. This is partially due to limited data on antiplatelet efficacy under cfVAD-associated shear conditions.

MATERIALS AND METHODS

We investigated the efficacy of antiplatelet drugs directly acting on three pathways: (1) cyclooxygenase (aspirin), (2) phosphodiesterase (dipyridamole, pentoxifylline, cilostazol), and (3) glycoprotein IIb-IIIa (eptifibatide). Gel-filtered platelets treated with these drugs were exposed for 10min to either constant shear stresses (30dyne/cm² and 70dyne/cm²) or dynamic shear stress profiles extracted from simulated platelet trajectories through a cfVAD (Micromed DeBakey). Platelet activation state (PAS) was measured using a modified prothrombinase-based assay, with drug efficacy quantified based on PAS reduction compared to untreated controls.

RESULTS AND CONCLUSIONS

Significant PAS reduction was observed for all drugs after exposure to 30dyne/cm² constant shear stress, and all drugs but dipyridamole after exposure to the 30th percentile shear stress waveform of the cfVAD. However, only cilostazol was significantly effective after 70dyne/cm² constant shear stress exposure, though no significant reduction was observed upon exposure to median shear stress conditions in the cfVAD. These results, coupled with the persistence of reported clinical thrombotic complication, suggest the need for the development of new classes of drugs that are especially designed to mitigate thrombosis in cfVAD patients, while reducing or eliminating the risk of bleeding.

Sildenafil Is Associated With Reduced Device Thrombosis and Ischemic Stroke Despite Low-Level Hemolysis on Heart Mate II Support

Background:

Persistent low-level hemolysis (LLH) during continuous-flow mechanical circulatory support is associated with subsequent thrombosis. Free hemoglobin from ongoing hemolysis scavenges nitric oxide (NO) to create an NO deficiency which can augment platelet function leading to a prothrombotic state. The phosphodiesterase-5 inhibitor, sildenafil, potentiates NO signaling to inhibit platelet function. Accordingly, we investigated the association of sildenafil administration and thrombotic events in patients with LLH during Heart Mate II support.

Methods and Results:

A single-center review of all patients implanted with a Heart Mate II who survived to discharge (n=144). LLH was defined by a discharge lactate dehydrogenase level of 400 to 700 U/L. Patients were categorized as (1) LLH not on sildenafil, (2) LLH on sildenafil, (3) no LLH not on sildenafil, and (4) no LLH on sildenafil. Age, sex, platelet count, and mean platelet volume were similar between groups. Seventeen patients had either device thrombosis or ischemic stroke. Presence of LLH was associated with a greater risk of thrombosis (adjusted hazard ratio, 15; 95% confidence interval, 4.5–50; P <0.001 versus no LLH, not on sildenafil). This risk was reduced in patients with LLH on sildenafil (adjusted hazard ratio, 1.7; 95% confidence interval, 0.2–16.1; P =0.61). Device thrombosis and ischemic stroke were associated with an increase in mean platelet volume (9.6±0.5 to 10.9±0.8 fL, P <0.001). Patients with LLH not on sildenafil had a greater increase in mean platelet volume in comparison to those with LLH on sildenafil ( P <0.001).

Conclusions:

Sildenafil is associated with reduced device thrombosis and ischemic stroke during ongoing LLH on Heart Mate II support.

Clinical and In Vitro Evidence That Subclinical Hemolysis Contributes to LVAD Thrombosis

BACKGROUND

Recent data suggest that hemolysis contributes to left ventricular assist device (LVAD) thrombosis, but the mechanism is unknown. In a clinical study, we measured plasma free hemoglobin (pfHgb) and the incidence of LVAD thrombosis. In an in vitro study, we examined biophysical relationships between shear stress, pfHgb and von Willebrand factor (vWF) metabolism toward understanding mechanisms of LVAD thrombosis.

METHODS

In the clinical study, blood samples were obtained from continuous-flow LVAD patients (n = 30). Plasma free hemoglobin was measured via enzyme-linked immunosorbent assay. Plasma lactate dehydrogenase (LDH) was measured with a fluorimetric assay. In the in vitro study, to investigate mechanism, human plasma (n = 10) was exposed to LVAD-like shear stress (175 dyne/cm²) with and without free hemoglobin (30 mg/dL). ADAMTS-13 (the vWF protease) activity was quantified with Förster resonance energy transfer. vWF size was quantified with immunoblotting. vWF clotting function was quantified with an enzyme-linked immunosorbent assay.

RESULTS

In the clinical study, LVAD support caused subclinical hemolysis. In all patients, LDH increased significantly from 213 ± 9 U/L to 366 ± 31 U/L at 10 days of support (p < 0.0001) and remained significantly elevated at 280 ± 18 U/L at 1 month of support (p < 0.01). In 21 patients that did not develop LVAD thrombosis, pfHgb increased early but decreased over time (pre-LVAD: 5.2 ± 0.8 mg/dL; 1 week: 19.8 ± 4.4 mg/dL, p < 0.01; 3 months: 9.3 ± 2.2 mg/dL, p = 0.07). In 9 patients that developed LVAD thrombosis, pfHgb was significantly elevated versus patients without thrombosis before (p < 0.001) and after 3 months (p < 0.05) of support (pre-LVAD: 20.2 ± 6.3 mg/dL; 1 week: 17.3 ± 3.7 mg/dL; 3 months: 21.5 ± 7.8 mg/dL). Similarly, after 3 months, patients that did not develop LVAD thrombosis had an LDH of 271 ± 28 U/L, whereas patients that later developed LVAD thrombosis had a significantly higher LDH of 625 ± 210 U/L (p = 0.02). In the in vitro study, shear stress degraded vWF similarly to an LVAD. Free hemoglobin inhibited ADAMTS-13 activity during shear stress (633 ± 27 ng/mL to 565 ± 24 ng/mL; p < 0.001). vWF was thereby protected from degradation, 4 vWF fragments decreased significantly (p ≤ 0.05), and vWF clotting function increased (1.15 ± 0.09 U/mL to 1.29 ± 0.09 U/mL, p = 0.06).

CONCLUSIONS

These are the first data to demonstrate mechanistic relationships between subclinical hemolysis and a procoagulant state during continuous-flow LVAD support. Patients with high pfHgb and LDH were more likely to develop LVAD thrombosis. In vitro experiments demonstrated that free hemoglobin inhibited ADAMTS-13, protected vWF from degradation, increased vWF clotting function, and created a procoagulant state. As such, pfHgb may be a clinical target to prevent LVAD thrombosis.

Role of red blood cells in haemostasis and thrombosis

In contrast to an obsolete notion that erythrocytes, or red blood cells (RBCs), play a passive and minor role in hemostasis and thrombosis, over the past decades there has been increasing evidence that RBCs have biologically and clinically important functions in blood clotting and its disorders. This review summarizes the main mechanisms that underlie the involvement of RBCs in hemostasis and thrombosis in vivo, such as rheological effects on blood viscosity and platelet margination, aggregation and deformability of RBCs; direct adhesion and indirect biochemical interactions with endothelial cells and platelets, etc. The ability of stored and pathologically altered RBCs to generate thrombin through exposure of phosphatidylserine has been emphasized. The procoagulant and prothrombotic potential of RBC-derived microparticles transfused with stored RBCs or formed in various pathological conditions associated with hemolysis has been described along with prothrombotic effects of free hemoglobin and heme. Binding of fibrinogen or fibrin to RBCs may influence their effects on fibrin network structure, clot mechanical properties, and fibrinolytic resistance. Recent data on platelet-driven clot contraction show that RBCs compressed by platelets pulling on fibrin form a tightly packed array of polyhedral erythrocytes, or polyhedrocytes, which comprises a nearly impermeable barrier important for hemostasis and wound healing. RBCs may perform dual roles, both helping to stem bleeding but at the same time contributing to thrombosis in a variety of ways.