Heparin inhibition of von Willebrand factor-dependent platelet function in vitro and in vivo

Platelets: A Potential Factor that Offers Strategies for Promoting Bone Regeneration

Bone defects represent a prevalent category of clinical injuries, causing significant pain and escalating health care burdens. Effectively addressing bone defects is thus of paramount importance. Platelets, formed from megakaryocyte lysis, have emerged as pivotal players in bone tissue repair, inflammatory responses, and angiogenesis. Their intracellular storage of various growth factors, cytokines, and membrane protein receptors contributes to these crucial functions. This article provides a comprehensive overview of platelets' roles in hematoma structure, inflammatory responses, and angiogenesis throughout the process of fracture healing. Beyond their application in conjunction with artificial bone substitute materials for treating bone defects, we propose the potential future use of anticoagulants such as heparin in combination with these materials to regulate platelet number and function, thereby promoting bone healing. Ultimately, we contemplate whether manipulating platelet function to modulate bone healing could offer innovative ideas and directions for the clinical treatment of bone defects.

Platelet volume indices and von Willebrand factor levels in blood exposed to polymer- or heparin-coated membrane oxygenators

INTRODUCTION

To understand the behavior of platelet volume indices and the von Willebrand factor (VWF), in vitro experiments using whole human blood were performed with extracorporeal circulation (ECC) circuits, including membrane oxygenators coated with acrylate copolymer (ACP) or immobilized heparin (IHP).

METHODS

Heparinized blood was circulated through two distinct experimental circuits: an ACP-coated reservoir and tubes, as well as membranes coated with either ACP or IHP (comprising five pieces of each type). The platelet distribution width, mean platelet volume (MPV), platelet large cell ratio (P-LCR), VWF quantity (VWFQ), and VWF activity (VWFA) were measured at 0, 8, 16, 24, and 32 h in each experiment. A two-way analysis of variance (ANOVA) was performed to determine whether the coating type or circulation duration affected the transition of each measurement.

RESULTS

Two-way ANOVA indicated that the transitions of MPV, P-LCR, and VWFA were significantly affected by the circulation duration (p = 0.030, 0.001, and <0.001, respectively) and that the transitions of VWFQ and VWFA were significantly affected by the coating type (p = 0.022 and 0.006, respectively). Factor interactions between the coating type and circulation duration were not observed for each transition (p > 0.05).

CONCLUSIONS

Our findings suggest that P-LCR is a good index for platelet activation in blood-circulating ECC and that VWFA and VWFQ are significantly attenuated in blood-circulating ECC with ACP-coated membranes, indicating the advantage of IHP coating regarding platelet activation.

Contributing factors to heparin resistance during cardiopulmonary bypass

Since the risk factors for heparin resistance (HR) before cardiopulmonary bypass (CPB) have not been fully clarified, this study investigated the contributing factors for HR after the initial unfractionated heparin (UFH) dose of 500 IU/kg. We retrospectively analyzed the data of 371 patients who underwent CPB surgery, with the initial UFH dose of 500 IU/kg, between May 2017 and December 2021. We defined HR as the failure to achieve activated clotting time (ACT) of > 480 s after the initial UFH dose of 500 IU/kg. HR was observed in 36 patients (9.7%) (HR group), while HR was not observed in 335 patients (control group). The HR group included significantly more patients with preoperative use of UFH, with significantly higher white blood cell counts, fibrinogen, fibrinogen degradation products, D-dimer, and C-reactive protein, and lower hemoglobin and albumin. The multivariable logistic regression analysis identified albumin (OR: 3.09, 95% CI 1.3504-7.0845, p = 0.0075) and fibrinogen (OR: 0.99, 95% CI 0.9869-0.9963, p = 0.0003) as independent predictors for HR. Using the Youden index, the cutoffs of albumin and fibrinogen were calculated as 3.8 g/dL and 303 mg/dL, respectively. The receiver operating characteristic curves showed the predictive performance of albumin (area under the curve (AUC): 0.78, sensitivity: 65%, specificity: 81%) and fibrinogen (AUC: 0.77, sensitivity: 56%, specificity: 88%). The incidence of HR after the initial UFH dose of 500 IU/kg was 9.7%. The preoperative albumin < 3.8 g/dL and fibrinogen > 303 mg/dL were independent predictors for HR.

Von Willebrand factor in diagnostics and treatment of cardiovascular disease: Recent advances and prospects

Von Willebrand factor (VWF) is a large multimeric glycoprotein involved in hemostasis. It is essential for platelet adhesion to the subendothelium of the damaged endothelial layer at high shear rates. Such shear rates occur in small-diameter arteries, especially at stenotic sites. Moreover, VWF carries coagulation factor VIII and protects it from proteolysis in the bloodstream. Deficiency or dysfunction of VWF predisposes to bleeding. In contrast, an increase in the concentration of high molecular weight multimers (HMWM) of VWF is closely associated with arterial thrombotic events. Severe aortic stenosis (AS) or hypertrophic obstructive cardiomyopathy (HOCM) can deplete HMWM of VWF and lead to cryptogenic, gastrointestinal, subcutaneous, and mucosal bleeding. Considering that VWF facilitates primary hemostasis and a local inflammatory response at high shear rates, its dysfunction may contribute to the development of coronary artery disease (CAD) and its complications. However, current diagnostic methods do not allow for an in-depth analysis of this contribution. The development of novel diagnostic techniques, primarily microfluidic, is underway. Such methods can provide physiologically relevant assessments of VWF function at high shear rates; however, they have not been introduced into clinical practice. The development and use of agents targeting VWF interaction with the vessel wall and/or platelets may be reasonable in prevention of CAD and its complications, given the prominent role of VWF in arterial thrombosis.

The Intriguing Connections between von Willebrand Factor, ADAMTS13 and Cancer

von Willebrand factor (VWF) is a complex and large protein that is cleaved by ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin type 1 motif, member 13), and together they serve important roles in normal hemostasis. Malignancy can result in both a deficiency or excess of VWF, leading to aberrant hemostasis with either increased bleeding or thrombotic complications, as respectively seen with acquired von Willebrand syndrome and cancer-associated venous thromboembolism. There is emerging evidence to suggest VWF also plays a role in inflammation, angiogenesis and tumor biology, and it is likely that VWF promotes tumor metastasis. High VWF levels have been documented in a number of malignancies and in some cases correlate with more advanced disease and poor prognosis. Tumor cells can induce endothelial cells to release VWF and certain tumor cells have the capacity for de novo expression of VWF, leading to a proinflammatory microenvironment that is likely conducive to tumor progression, metastasis and micro-thrombosis. VWF can facilitate tumor cell adhesion to endothelial cells and aids with the recruitment of platelets into the tumor microenvironment, where tumor/platelet aggregates are able to form and facilitate hematogenous spread of cancer. As ADAMTS13 moderates VWF level and activity, it too is potentially involved in the pathophysiology of these events. VWF and ADAMTS13 have been explored as tumor biomarkers for the detection and prognostication of certain malignancies; however, the results are underdeveloped and so currently not utilized for clinical use. Further studies addressing the basic science mechanisms and real word epidemiology are required to better appreciate the intriguing connections between VWF, ADAMTS13 and malignancy. A better understanding of the role VWF and ADAMTS13 play in the promotion and inhibition of cancer and its metastasis will help direct further translational studies to aid with the development of novel cancer prognostic tools and treatment modalities.

Heparin - Messias or Verschlimmbesserung?

A heightened risk of thrombosis noted early on with the severe acute respiratory syndrome coronavirus 2 infection led to the widespread use of heparin anticoagulation in the coronavirus disease 2019 (COVID-19) pandemic. However, reports soon started appearing in the literature where an apparent failure of heparin to prevent thrombotic events was observed in hospitalized patients with this viral infection. In this review, we explore the likely mechanisms for heparin failure with particular relevance to COVID-19. We also explore the role of anti-Xa assays and global hemostatic tests in this context. The current controversy of dosing heparin in this disease is detailed with some possible mechanistic reasons for anticoagulant failure. We hope that lessons learnt from the use of heparin in COVID-19 could assist us in the appropriate use of this anticoagulant in the future.

The heparin-von Willebrand factor interaction and conventional tests of haemostasis - the challenges in predicting bleeding in cardiopulmonary bypass

Bleeding is a significant complication of cardiopulmonary bypass (CPB), despite routine anticoagulation monitoring. This is likely to be multifactorial. In this prospective, single-centre cohort study of 30 patients undergoing CPB surgery, our aim was to characterise the changes in von Willebrand factor (VWF) function, platelet interaction and the global coagulation changes during and after CPB surgery and to determine whether bleeding can be predicted. Samples were taken at six time points before, during and after CPB surgery. We observed a significant rise in VWF antigen (VWF:Ag) throughout surgery, which continued postoperatively. The absolute VWF collagen-binding assays (VWF:CB) and VWF ristocetin cofactor (VWF:RCo) rose significantly but the VWF:CB/VWF:Ag and VWF:Ag/VWF:RCo fell significantly (P = 0·0015 and P = 0·0143), suggesting loss of large multimers. We detected a non-significant trend to loss of VWF:RCo after heparinisation and a significant recovery after protamine reversal which could reflect a direct heparin effect. There was a significant increase in the R and K times with a fall in alpha angle and maximum amplitude after heparin administration, using heparinase-thromboelastography (TEG). The parameters both significantly improved following protamine (P = 0·007 and P = 0·0054). The activated clotting time (ACT) and heparin anti-Xa level correlated poorly; neither predicted clinically significant bleeding. None of these parameters had a relationship with intraoperative blood loss or requirement for blood product replacement.

Systemic mastocytosis with subcutaneous hemorrhage and edema in a Greyhound dog: case report and review of diagnostic criteria

Systemic mastocytosis, characterized by infiltration of multiple organs by neoplastic mast cells, is a well-described entity in human medicine with specific criteria for diagnosis, but is ill defined in veterinary literature. Hemostatic disorders are reported in humans affected by systemic mastocytosis but have not been well described in veterinary literature. A 5-y-old, spayed female Greyhound dog had a 1-mo history of progressive ventral cutaneous edema, hemorrhage, and pain. Cytology of an antemortem aspirate from the subcutis of the ventral abdomen was suggestive of mast cell neoplasia, but no discrete mass was present. The dog was euthanized and submitted for autopsy; marked subcutaneous edema and hemorrhage were confirmed. The ventral abdominal panniculus and dermis superficial to the panniculus carnosus were infiltrated by a dense sheet of neoplastic mast cells. The neoplastic cells contained toluidine blue-positive granules and formed aggregates within the bone marrow and several visceral organs, including the liver, spleen, heart, and kidney. Diffuse edema and hemorrhage is an unusual presentation of mast cell tumors in dogs. Antemortem tests, including complete blood count, coagulation profile, and viscoelastic coagulation testing, were suggestive of a primary hemostatic defect. We discuss here the diagnostic criteria used in humans, how these can be applied to veterinary patients, and the limitations of the current diagnostic framework.

Shear Stress-Induced Activation of von Willebrand Factor and Cardiovascular Pathology

The von Willebrand factor (vWF) is a plasma protein that mediates platelet adhesion and leukocyte recruitment to vascular injury sites and carries coagulation factor VIII, a building block of the intrinsic pathway of coagulation. The presence of ultra-large multimers of vWF in the bloodstream is associated with spontaneous thrombosis, whereas its deficiency leads to bleeding. In cardiovascular pathology, the progression of the heart valve disease results in vWF deficiency and cryptogenic gastrointestinal bleeding. The association between higher plasma levels of vWF and thrombotic complications of coronary artery disease was described. Of note, it is not the plasma levels that are crucial for vWF hemostatic activity, but vWF activation, triggered by a rise in shear rates. vWF becomes highly reactive with platelets upon unfolding into a stretched conformation, at shear rates above the critical value (more than 5000 s⁻¹), which might occur at sites of arterial stenosis and injury. The activation of vWF and its counterbalance by ADAMTS-13, the vWF-cleaving protease, might contribute to complications of cardiovascular diseases. In this review, we discuss vWF involvement in complications of cardiovascular diseases and possible diagnostic and treatment approaches.

Von Willebrand factor and cancer; metastasis and coagulopathies

Von Willebrand factor (VWF) is a multimeric procoagulant plasma glycoprotein that mediates platelet adhesion along the endothelium. In addition to its role maintaining normal hemostasis, more recently novel biological functions for VWF have been described, including inflammation, angiogenesis, and metastasis. Significantly increased plasma VWF levels have been reported across a variety of cancer patient cohorts. Given that VWF is established as a risk factor for venous thrombosis, this is of direct clinical importance. Moreover, elevated VWF has also been observed localized within the tumor microenvironment, correlating with advanced disease stage and poorer clinical outcome. Critically, evidence suggests that elevated VWF levels in cancer patients may not only contribute to cancer associated coagulopathies but may also mediate cancer progression and metastasis. Studies have shown that VWF can promote pro-inflammatory signaling, regulate angiogenesis and vascular permeability, which may facilitate tumor cell growth and extravasation across the vessel wall. Endothelial secreted VWF multimers contribute to the adhesion and transendothelial migration of tumor cells key for tumor dissemination. In support of this, VWF inhibition attenuated metastasis in vivo. Perhaps most intriguingly, specific tumor cells have been reported to acquire de novo VWF expression which increases tumor-platelet heteroaggregates and confers enhanced metastatic activity. Current knowledge on the roles of VWF in cancer and in particular its contribution to metastasis and cancer associated coagulopathies is summarized in this review.

DNA binds to a specific site of the adhesive blood-protein von Willebrand factor guided by electrostatic interactions

Neutrophils release their intracellular content, DNA included, into the bloodstream to form neutrophil extracellular traps (NETs) that confine and kill circulating pathogens. The mechanosensitive adhesive blood protein, von Willebrand Factor (vWF), interacts with the extracellular DNA of NETs to potentially immobilize them during inflammatory and coagulatory conditions. Here, we elucidate the previously unknown molecular mechanism governing the DNA–vWF interaction by integrating atomistic, coarse-grained, and Brownian dynamics simulations, with thermophoresis, gel electrophoresis, fluorescence correlation spectroscopy (FCS), and microfluidic experiments. We demonstrate that, independently of its nucleotide sequence, double-stranded DNA binds to a specific helix of the vWF A1 domain, via three arginines. This interaction is attenuated by increasing the ionic strength. Our FCS and microfluidic measurements also highlight the key role shear-stress has in enabling this interaction. Our simulations attribute the previously-observed platelet-recruitment reduction and heparin-size modulation, upon establishment of DNA–vWF interactions, to indirect steric hindrance and partial overlap of the binding sites, respectively. Overall, we suggest electrostatics—guiding DNA to a specific protein binding site—as the main driving force defining DNA–vWF recognition. The molecular picture of a key shear-mediated DNA–protein interaction is provided here and it constitutes the basis for understanding NETs-mediated immune and hemostatic responses.

Management of a Pediatric Patient With a Left Ventricular Assist Device and Symptomatic Acquired von Willebrand Syndrome Presenting for Orthotopic Heart Transplant

We present the successful perioperative management of an 11-year-old patient presenting for heart transplant with a left ventricular assist device, symptomatic acquired von Willebrand syndrome, and recent preoperative intracranial hemorrhage. A brief review of the pathophysiology of acquired von Willebrand syndrome is included. As the number of pediatric patients supported with ventricular assist devices continues to increase, the management of symptomatic acquired von Willebrand syndrome during the perioperative period is an important consideration for anesthesiologists.

Miniaturized Test Loop for the Assessment of Blood Damage by Continuous-Flow Left-Ventricular Assist Devices

Although the hemocompatibility of left-ventricular assist devices (LVADs) has continuously improved, assessment of hemolysis remains mandatory in pre-clinical testing. The ASTM-F1841 has standardized this assessment since 1997. However, the recommended usage of fresh, non-pooled human blood is hardly feasible with the test loop volume specified therein, when testing the device under test versus a predicate device as required by the international standard 10993-4. In this study, we compared ASTM-conforming (ASTM) and downscaled (mini) test loops with a one-third priming volume for the assessment of blood damage at the ASTM operating point. Blood damage was assessed for HeartMate 3 and BPX-80 in 6 experiments with heparinized porcine slaughterhouse blood for 6 h. We analyzed plasma free hemoglobin (pfHb), von Willebrand factor (vWF) concentration and collagen-binding functionality and calculated indices of hemolysis and vWF-ratios. The mini test loops provided significantly higher pfHb increase and consistently stronger vWF-ratio decrease and yielded a significantly better differentiation of the pumps. Interestingly, indices of hemolysis were generally lower in the mini set-up, indicating less adverse effects by the mini loop itself. Thus, we propose our mini test loop as suitable tool for clinically relevant standardized assessment of blood damage by future LVADs with single-donation human blood.

The endothelial glycocalyx anchors von Willebrand factor fibers to the vascular endothelium

The dynamic change from a globular conformation to an elongated fiber determines the ability of von Willebrand factor (VWF) to trap platelets. Fiber formation is favored by the anchorage of VWF to the endothelial cell surface, and VWF-platelet aggregates on the endothelium contribute to inflammation, infection, and tumor progression. Although P-selectin and ανβ3-integrins may bind VWF, their precise role is unclear, and additional binding partners have been proposed. In the present study, we evaluated whether the endothelial glycocalyx anchors VWF fibers to the endothelium. Using microfluidic experiments, we showed that stabilization of the endothelial glycocalyx by chitosan oligosaccharides or overexpression of syndecan-1 (SDC-1) significantly supports the binding of VWF fibers to endothelial cells. Heparinase-mediated degradation or impaired synthesis of heparan sulfate (HS), a major component of the endothelial glycocalyx, reduces VWF fiber-dependent platelet recruitment. Molecular interaction studies using flow cytometry and live-cell fluorescence microscopy provided further evidence that VWF binds to HS linked to SDC-1. In a murine melanoma model, we found that protection of the endothelial glycocalyx through the silencing of heparanase increases the number of VWF fibers attached to the wall of tumor blood vessels. In conclusion, we identified HS chains as a relevant binding factor for VWF fibers at the endothelial cell surface in vitro and in vivo.

Primary Thromboprophylaxis with Heparins for Arteriovenous Fistula Failure in Pediatric Patients

Background

To reduce the incidence of early arteriovenous fistula failure (AVF) due to thrombosis in pediatric hemodialysis (HD) patients, a primary thromboprophylaxis (PTP) protocol was initiated at author's institution in June 2005. The goal of this study is to report author's experience with this protocol one year later.

Methods and Results

19 AVFs (14 patients, Historical group) and 8 AVFs (7 patients, PTP group) were created prior to and after initiation of PTP respectively. PTP consisted of unfractionated heparin (5–10 units/kg/hr) infusion postoperatively, followed by subcutaneous low molecular weigh heparin (LMWH) until AVF was matured. LMWH dosing was “Prophylactic” (0.5 mg/kg/d, anti-factor Xa levels: peak 0.25–0.5 and trough < 0.3 units/mL) and “Therapeutic” (1 mg/kg/d, anti-factor Xa level: peak 0.5-1 and trough < 0.5 units/mL) based on thrombosis predisposition. In Historical group, 12 AVFs did not receive thromboprophylaxis (No-treatment group), 5 received 81 mg aspirin/day (Aspirin group), and 2 received LMWH. In No-treatment group 10/12 AVFs failed: 9 thromboses and 1 stenosis. In Aspirin group 1/5 AVFs failed due to thrombosis. In PTP group 1/8 AVFs failed due to stenosis; the first 2 AVFs developed hematoma prompting a reduction in LMWH dose and monitoring trough anti-factor Xa levels, one AVF required thrombectomy after LMWH was transiently held. The incidence of thrombosis was less in PTP group (12.5%) when comparing to No-treatment group (83%) (p < 0.05).

Conclusion

PTP is a feasible option to prevent early thrombosis at AVF. Close clinical and laboratory monitoring including trough anti-factor Xa levels is required to adjust optimum anticoagulation. Larger studies are needed to clarify safety and efficacy of our PTP protocol.

Preparation and evaluation of a novel oral delivery system for low molecular weight heparin

OBJECTIVE

The objective of the present work was to prepare and evaluate a novel oral formulation for systemic delivery of low molecular weight heparin (LMWH). The formulation consisted of Eudragit S 100-coated positively charged liposomes encapsulating LMWH and a penetration enhancer.

MATERIALS AND METHODS

Positively charged liposomes were first prepared by the thin film hydration method using lipid (soy phosphotidylcholine and cholesterol) and stearyl amine (SA) in the optimum ratio of 16:1, along with cetylpyridinium chloride (CPC) as a penetration enhancer. Prepared liposomes were coated with negatively charged Eudragit S 100 (0.3% w/v). The formulations were studied for various in vitro and in vivo properties. Differential scanning calorimetry (DSC), x-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) studies, and in vitro drug release were used for in vitro characterization of the formulations. Ex vivo permeation studies were performed by using distal small intestine of rat. Oral absorption studies were conducted with the rat model.

RESULTS

Coating of the liposomes was confirmed by SEM and particle size determination studies. In vitro release studies of coated liposomes have demonstrated that the release of LMWH was in the following order: Stomach < small intestine < distal small intestine < colon. Ex vivo permeation studies have shown a fivefold increase in permeation of LMWH with Eudragit S 100-coated liposomes compared to uncoated, uncharged liposomes. Oral absorption studies have showed that with Eudragit-coated liposomes, the oral bioavailability of LMWH was improved, compared to plain LMWH solution. This is revealed by a threefold increase in the area under the curve (AUC) of the plasma concentration time curve.

CONCLUSION

A novel formulation for oral delivery of LMWH was thus successfully prepared and evaluated.

Heparin-Induced Thrombocytopenia

Heparin's anticoagulation effect makes it the principal component in the immediate treatment of all thrombotic diseases. After hemorrhage, the most important complication of heparin is thrombocytopenia, the decrease in the number of blood platelets. Ten cases of "heparin-induced thrombocytopenia" are described, and guidelines for diagnosing heparin-induced thrombocytopenia in patients who present for cardiac and vascular surgery are given. Methods of treating patients with heparin-induced thrombocytopenia, including use of arachidonic acid derivatives, antiplatelet agents, other anticoagulants, heparinoids, natural and synthetic thrombin inhibitors, hirudin, and defibrinogenation with ancrod, are discussed.

Comparison of Unfractionated Versus Low Molecular Weight Heparin for Deep Vein Thrombosis Prophylaxis During Breast and Pelvic Cancer Surgery: Efficacy, Safety, and Follow-up

In a prospective, double-blind randomized trial the efficacy and safety of low molecular weight heparin and un fractionated heparin were compared for the prevention of post operative deep vein thrombosis in patients undergoing major surgery for breast and pelvic cancer. Three hundred fifty-eight patients were randomly allocated to the two treatment groups. Thirty-four of these were excluded from the study. Of the re maining 324 patients, 164 received 5000 IU unfractionated heparin three times daily and 160 received 3000 anti-Xa units of low molecular weight heparin once daily. Treatment was started 2 to 5 hours preoperatively and continued for 7 days. The occurrence of deep vein thrombosis was determined by impedence plethysmography and/or venography. Deep vein thrombosis was diagnosed in 10 (6.3%) of 160 patients taking low molecular weight heparin and 10 (6.1 %) of 164 patients treated with unfractionated heparin (relative risk: 1.03; 95% confidence interval 0.44-2.39; p = 1.0). Major bleeding events occurred in 27 (16.8%) patients in the low molecular weight heparin group and 47 (28.7%) in the unfractionated heparin group (relative risk: 0.59; 95% confidence interval: 0.39-0.89; p = .01). Severe intraoperative bleeding was observed in three patients in the unfractionated heparin group and in none of the patients in the low molecular weight heparin group. Excessive intraoperative bleeding was less frequent in the low molecular weight heparin group (9.4% vs. 14%, p = .23) as was wound hematoma (11.3 vs. 17.7%, p = .12). Bleeding episodes with low molecular weight heparin were less likely to lead to further surgery for evacuation of hematoma (0.6% vs. 2.5%, p = .37). Perioperative death rate was 3.1 % in the low molecular weight heparin group and 1.8% in unfractionated heparin group (rela tive risk: 1.72, 95% confidence interval: 0.42-7.07; p = .49). Follow-up data were available for 316 patients for an average of 20 months after surgery. There were 35 further cancer deaths (11 low molecular weight heparin group, 24 unfractionated heparin group) and 23 patients with late onset thromboembolic complications (11 low molecular weight heparin group, 12 un fractionated heparin group). The two drugs were of similar efficacy but the frequency of major bleeding was 41.1% less in the low molecular weight heparin group.

Venovenous Extracorporeal Membrane Oxygenation in Intractable Pulmonary Insufficiency: Practical Issues and Future Directions

Venovenous extracorporeal membrane oxygenation (vv-ECMO) is a highly invasive method for organ support that is gaining in popularity due to recent technical advances and its successful application in the recent H1N1 epidemic. Although running a vv-ECMO program is potentially feasible for many hospitals, there are many theoretical concepts and practical issues that merit attention and require expertise. In this review, we focus on indications for vv-ECMO, components of the circuit, and management of patients on vv-ECMO. Concepts regarding oxygenation and decarboxylation and how they can be influenced are discussed. Day-to-day management, weaning, and most frequent complications are covered in light of the recent literature.

Biocompatibility of cardiopulmonary bypass circuit with new polymer Senko E-Ternal CoatingTM

Objective:

The aim of this study was to compare the biocompatibility of a new Senko E-Ternal coating (SEC) for cardiopulmonary bypass (CPB) circuits with the well-established poly-2-methoxyethyl acrylate (PMEA) coating.

Methods:

Forty patients undergoing aortic valve replacement were randomly assigned to either an SEC-coated group (n = 20) or a PMEA-coated group (n = 20). Clinical data and the following markers were analyzed: platelet count, platelet factor (PF) 4, fibrinogen, fibrinogen degradation products (FDPs), antithrombin III (AT III), thrombin-antithrombin complex (TAT), plasminogen, complement hemolytic activity (CH50), complement 3 (C3) and interleukin-6 (IL-6). Blood samples were obtained at five time points in both groups.

Results:

CPB time, aortic cross-clamp time and blood loss and transfusion were similar in both groups. There were no significant differences between the groups in terms of platelet count, PF4 and all coagulation and fibrinolytic parameters (FDP, AT III, TAT, and plasminogen) at any time points. Inflammatory markers (CH50, C3 and IL-6) were also similar in both groups at all time points.

Conclusions:

The SEC-coated circuit demonstrated equivalent biocompatibility to the PMEA-coated circuit. SEC-coated circuits are, therefore, favorably comparable to PMEA-coated circuits for clinical use in CPB.

Interaction of von Willebrand factor with platelets and the vessel wall

The initiation of thrombus formation at sites of vascular injury to secure haemostasis after tissue trauma requires the interaction of surface-exposed von Willebrand factor (VWF) with its primary platelet receptor, the glycoprotein (GP) Ib-IX-V complex. As an insoluble component of the extracellular matrix (ECM) of endothelial cells, VWF can directly initiate platelet adhesion. Circulating plasma VWF en-hances matrix VWF activity by binding to structures that become exposed to flowing blood, notably collagen type I and III in deeper layers of the vessel along with microfibrillar collagen type VI in the subendothelium. Moreover, plasma VWF is required to support platelet-to-platelet adhesion - i. e. aggregation - which promotes thrombus growth and consolidation. For these reasons, understanding how plasma VWF interaction with platelet receptors is regulated, particularly any distinctive features of GPIb binding to soluble as opposed to immobilized VWF, is of paramount importance in vascular biology. This brief review will highlight knowledge acquired and key problems that remain to be solved to elucidate fully the role of VWF in normal haemostasis and pathological thrombosis.

von Willebrand factor directly interacts with DNA from neutrophil extracellular traps

OBJECTIVE

Inflammatory conditions provoke essential processes in the human vascular system. It leads to the formation of ultralarge von Willebrand factor (VWF) fibers, which are immobilized on the endothelial cell surface and transform to highly adhesive strings under shear conditions. Furthermore, leukocytes release a meshwork of DNA (neutrophil extracellular traps) during the process of the recently discovered cell death program NETosis. In the present study, we characterized the interaction between VWF and DNA and possible binding sites to underline the role of VWF in thrombosis and inflammation besides its function in platelet adhesion.

APPROACH AND RESULTS

Both functionalized surfaces and intact cell layers of human umbilical vein endothelial cells were perfused with isolated, protein-free DNA or leukocytes from whole blood at distinct shear rates. DNA-VWF interaction was monitored using fluorescence microscopy, ELISA-based assays, molecular dynamics simulations, and electrostatic potential calculations. Isolated DNA, as well as DNA released by stimulated leukocytes, was able to bind to shear-activated, but not inactivated, VWF. However, DNA-VWF binding does not alter VWF degradation by a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13. Moreover, DNA-VWF interaction can be blocked using unfractionated and low-molecular-weight heparin, and DNA-VWF complexes attenuate platelet binding to VWF. These findings were supported using molecular dynamics simulations and electrostatic calculations of the A1- and A2-domains.

CONCLUSIONS

Our findings suggest that VWF directly binds and immobilizes extracellular DNA released from leukocytes. Therefore, we hypothesize that VWF might act as a linker for leukocyte adhesion to endothelial cells, supporting leukocyte extravasation and inflammation.

The role of platelets in coagulation dysfunction in xenotransplantation, and therapeutic options

Xenotransplantation could resolve the increasing discrepancy between the availability of deceased human donor organs and the demand for transplantation. Most advances in this field have resulted from the introduction of genetically engineered pigs, e.g., α1,3-galactosyltransferase gene-knockout (GTKO) pigs transgenic for one or more human complement-regulatory proteins (e.g., CD55, CD46, CD59). Failure of these grafts has not been associated with the classical features of acute humoral xenograft rejection, but with the development of thrombotic microangiopathy in the graft and/or consumptive coagulopathy in the recipient. Although the precise mechanisms of coagulation dysregulation remain unclear, molecular incompatibilities between primate coagulation factors and pig natural anticoagulants exacerbate the thrombotic state within the xenograft vasculature. Platelets play a crucial role in thrombosis and contribute to the coagulation disorder in xenotransplantation. They are therefore important targets if this barrier is to be overcome. Further genetic manipulation of the organ-source pigs, such as pigs that express one or more coagulation-regulatory genes (e.g., thrombomodulin, endothelial protein C receptor, tissue factor pathway inhibitor, CD39), is anticipated to inhibit platelet activation and the generation of thrombus. In addition, adjunctive pharmacologic anti-platelet therapy may be required. The genetic manipulations that are currently being tested are reviewed, as are the potential pharmacologic agents that may prove beneficial.

GpIbα-VWF blockade restores vessel patency by dissolving platelet aggregates formed under very high shear rate in mice

Interactions between platelet glycoprotein (Gp) IIb/IIIa and plasma proteins mediate platelet cross-linking in arterial thrombi. However, GpIIb/IIIa inhibitors fail to disperse platelet aggregates after myocardial infarction or ischemic stroke. These results suggest that stability of occlusive thrombi involves additional and as-yet-unidentified mechanisms. In the present study, we investigated the mechanisms driving platelet cross-linking during occlusive thrombus formation. Using computational fluid dynamic simulations and in vivo thrombosis models, we demonstrated that the inner structure of occlusive thrombi is heterogeneous and primarily determined by the rheological conditions that prevailed during thrombus growth. Unlike the first steps of thrombus formation, which are GpIIb/IIIa-dependent, our findings reveal that closure of the arterial lumen is mediated by GpIbα-von Willebrand Factor (VWF) interactions. Accordingly, disruption of platelet cross-linking using GpIbα-VWF inhibitors restored vessel patency and improved outcome in a mouse model of ischemic stroke, although the thrombi were resistant to fibrinolysis or traditional antithrombotic agents. Overall, our study demonstrates that disruption of GpIbα-VWF interactions restores vessel patency after occlusive thrombosis by specifically disaggregating the external layer of occlusive thrombi, which is constituted of platelet aggregates formed under very high shear rates.

Review article: heparin sensitivity and resistance: management during cardiopulmonary bypass

Heparin resistance during cardiac surgery is defined as the inability of an adequate heparin dose to increase the activated clotting time (ACT) to the desired level. Failure to attain the target ACT raises concerns that the patient is not fully anticoagulated and initiating cardiopulmonary bypass may result in excessive activation of the hemostatic system. Although antithrombin deficiency has generally been thought to be the primary mechanism of heparin resistance, the reasons for heparin resistance are both complex and multifactorial. Furthermore, the ACT is not specific to heparin's anticoagulant effect and is affected by multiple variables that are commonly present during cardiac surgery. Due to these many variables, it remains unclear whether decreased heparin responsiveness as measured by the ACT represents inadequate anticoagulation. Nevertheless, many clinicians choose a target ACT to assess anticoagulation, and interventions aimed at achieving the target ACT are routinely performed in the setting of heparin resistance. Treatments for heparin resistance/alterations in heparin responsiveness include additional heparin or antithrombin supplementation. In this review, we discuss the variability of heparin potency, heparin responsiveness as measured by the ACT, and the current management of heparin resistance.

Polyphosphate binds to human von Willebrand factor in vivo and modulates its interaction with glycoprotein Ib

BACKGROUND

Polyphosphate, a phosphate polymer released by activated platelets, has recently been described as a potent modulator of blood coagulation and fibrinolysis. In blood plasma, polyphosphate binds to and alters the biological functions of factor XII, fibrin(ogen), thrombin and factor VII activating protease.

OBJECTIVES

The aim of the present study is to investigate whether polyphosphate also binds to von Willebrand factor (VWF) and alters some of its activities.

METHODS/RESULTS

When studying patients with type 1 von Willebrand disease (VWD) and their healthy relatives, we discovered a significant correlation between von Willebrand factor (VWF) and platelet polyphosphate levels. We have also found polyphosphate in preparations of VWF isolated from normal platelets and plasma. Surface plasmon resonance and electrophoretic mobility assays indicated that polyphosphate interacts with VWF in a dose- and time-dependent manner. Treatment of normal plasma with active exopolyphosphatase decreased the VWF ristocetin cofactor (VWF:RCo) activity, a functional measure of VWF binding to platelet glycoprotein receptor Ib. VWF collagen binding and multimerization were unaltered after polyphosphate depletion. Moreover, addition of polyphosphate increased the deficient VWF:RCo activity presented by plasma from patients with type 1 VWD.

CONCLUSIONS

Our results reveal that a new role is played by polyphosphate in hemostasis by its interaction with VWF, and suggest that this polymer may be effective in the treatment of some types of VWD.

Change in heparin potency and effects on the activated clotting time in children undergoing cardiopulmonary bypass

BACKGROUND

Heparin is the anticoagulant most commonly used for cardiopulmonary bypass (CPB), and the activated clotting time (ACT) is its primary monitor. In October 2009, the Food and Drug Administration changed the United States Pharmacopeia (USP) monograph for unfractionated heparin to incorporate new quality tests and a new potency assay and reference standard. This latter change was anticipated by in vitro tests to reduce heparin potency by 10% in each USP unit dose. After integration of the "new" heparin into our practice, we subjectively noticed less prolongation of the ACT with our routine heparin bolus before the initiation of CPB. We performed this investigation to provide objective evidence of a reduction in the level of anticoagulation achieved with use of the new heparin as assessed by ACT values and to document the occurrence of having an ACT below our institutional threshold before the initiation of CPB.

METHODS

A retrospective chart review was performed on all children who underwent CPB at Children's Healthcare of Atlanta between July 1, 2008, and June 30, 2009, before the release of the new heparin ("old heparin" [OH] group) and between June 1, 2010, and May 31, 2011, after complete integration of the new heparin ("new heparin" [NH] group). Baseline ACTs and ACTs after the administration of 400 U/kg of heparin were recorded for both the OH and NH groups. We determined the number of patients in each group having an ACT <480 seconds after the initial heparin bolus but before the initiation of CPB. Additionally, patients were divided into 3 age groups (<1 month, 1 to 12 months, and >1 year) to analyze similar ACT changes.

RESULTS

Postheparin ACTs were significantly lower in the NH group than in the OH group. There were significantly more patients having an ACT <480 seconds after the initial heparin bolus in the NH group (OH: 68 of 557 [12.2%] versus NH: 140 of 491 patients [28.5%]; P < 0.0001). The change remained significant when assessed across the age groups.

CONCLUSIONS

In this investigation we provide objective evidence that the level of anticoagulation after the initial pre-CPB heparin bolus as assessed by the ACT is significantly less with use of the new heparin. This reduction remained consistent across 3 age groups and was associated with a more frequent occurrence of ACTs below our institutional threshold for the initiation of CPB. Consideration should be given to increasing the initial weight-based heparin dose administered before CPB.

Platelet aggregation in humans and nonhuman primates: relevance to xenotransplantation

INTRODUCTION

Platelet activation/aggregation plays a key role in the dysregulation of coagulation and the development of thrombotic microangiopathy in nonhuman primate recipients of pig xenografts. As a preliminary to the study of anti-platelet therapy in vitro and in vivo, the present study aimed to compare platelet aggregation in whole blood from humans, baboons, and cynomolgus monkeys.

METHODS

Using "Chrono-log" technology (two-sample four-channel Chrono-log Whole Blood Aggregometer), we studied aggregation of platelets in healthy humans (n = 8), baboons (n = 5), and monkeys (n = 8). Whole blood (WB) samples were collected, and platelet aggregation was assessed using three different volumes of blood (1, 0.5, and 0.25 ml). Platelet activation was induced using collagen (at 3 and 5 μg/ml), ristocetin (at 0.5 and 1.0 mg/ml), adenosine diphosphate (ADP; at 10, 20, and 40 μm), or thrombin (at 1 and 5 IU/ml). Inhibition of agonist-induced platelet aggregation by heparin and low molecular weight heparin (LMWH) (at 1, 10, and 100 IU/ml) was evaluated.

RESULTS

Mean platelet counts were 222.1, 263.2, and 276.1 (×10(3) /μl) in humans, baboons, and monkeys, respectively. In all three species, platelet aggregation was induced by collagen, ristocetin, ADP, or thrombin in a dose-dependent manner. A blood volume of 0.5 ml provided the most consistent results with all agonists in all three species. Dilution studies indicated that there was a significant positive correlation between platelet count and percent aggregation of platelets (P < 0.05). Collagen (3 and 5 μg/ml), ADP (10, 20, and 40 μm), and thrombin (1 and 5 IU/ml) induced significantly greater platelet aggregation in humans than in baboons. ADP (20 and 40 μm) and thrombin (1 and 5 IU/ml) induced significantly greater platelet aggregation in monkeys than in baboons. There was no species difference with ristocetin (0.5 or 1.0 mg/ml). In all species, thrombin (1 or 5 IU) induced greater platelet aggregation than any of the other reagents. Heparin at 1 IU/ml and LMWH at 10 IU/ml in all species almost completely abrogated thrombin-induced platelet aggregation. Heparin at 100 IU/ml effectively inhibited platelet aggregation induced by collagen, but only partially inhibited aggregation induced by ADP or ristocetin. LMWH only partially inhibited aggregation induced by collagen, ristocetin, and ADP.

CONCLUSIONS

The "Chrono-log" technology proved to be a reliable method of evaluating platelet activation and aggregation in vitro in primates. Species differences may play a role in platelet aggregation, with the monkey being more comparable to the human than the baboon, although overall trends were similar. In all species, thrombin induced greater platelet aggregation than other agonists. Even a concentration of heparin of 1 IU/ml, which is probably the maximal concentration that is clinically-applicable, prevented platelet aggregation induced by thrombin, but was less effective in preventing aggregation induced by collagen, ADP, or, particularly, ristocetin.

Parenteral anticoagulants: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines

This article describes the pharmacology of approved parenteral anticoagulants. These include the indirect anticoagulants, unfractionated heparin (UFH), low-molecular-weight heparins (LMWHs), fondaparinux, and danaparoid, as well as the direct thrombin inhibitors hirudin, bivalirudin, and argatroban. UFH is a heterogeneous mixture of glycosaminoglycans that bind to antithrombin via a unique pentasaccharide sequence and catalyze the inactivation of thrombin, factor Xa, and other clotting enzymes. Heparin also binds to cells and plasma proteins other than antithrombin causing unpredictable pharmacokinetic and pharmacodynamic properties and triggering nonhemorrhagic side effects, such as heparin-induced thrombocytopenia (HIT) and osteoporosis. LMWHs have greater inhibitory activity against factor Xa than thrombin and exhibit less binding to cells and plasma proteins than heparin. Consequently, LMWH preparations have more predictable pharmacokinetic and pharmacodynamic properties, have a longer half-life than heparin, and are associated with a lower risk of nonhemorrhagic side effects. LMWHs can be administered once daily or bid by subcutaneous injection, without coagulation monitoring. Based on their greater convenience, LMWHs have replaced UFH for many clinical indications. Fondaparinux, a synthetic pentasaccharide, catalyzes the inhibition of factor Xa, but not thrombin, in an antithrombin-dependent fashion. Fondaparinux binds only to antithrombin. Therefore, fondaparinux-associated HIT or osteoporosis is unlikely to occur. Fondaparinux exhibits complete bioavailability when administered subcutaneously, has a longer half-life than LMWHs, and is given once daily by subcutaneous injection in fixed doses, without coagulation monitoring. Three additional parenteral direct thrombin inhibitors and danaparoid are approved as alternatives to heparin in patients with HIT.

Bivalirudin is inferior to heparin in preservation of intraoperative autologous blood

INTRODUCTION

Bivalirudin is used as an alternative to heparin in cardiac surgery, and may be superior to heparin with regard to platelet function. Bivalirudin however, is prone to cleavage by thrombin resulting in coagulation in areas of stasis.

MATERIAL AND METHODS

We compared the preservation of platelet function and the quality of anticoagulation in autologous blood of 26 cardiac surgical patients collected intraoperatively and anticoagulated ex vivo with either bivalirudin or heparin, with supplementation of bivalirudin over time and prevention of stasis.

RESULTS

We found in both preservatives a reduction in ADP-induced platelet aggregation response over a period of 105 minutes (median, IQR: 73-141) as measured by Multiplate®. Supplementation of additional bivalirudin (23 ± 1.1 μg/ml/hr) and prevention of stasis was not able to prevent thrombin generation. We found a 5-fold increase in levels of prothrombin fragment 1+2 in bivalirudin preserved autologous blood as compared to heparin preserved blood (F(1+2) levels median 8.9 nM [quartile percentiles 4.2-12.4] vs 1.3 nM [0.6-2.1], P=0.001 Mann-Whitney, n=10).

CONCLUSIONS

Our study suggests that preservation of platelet function in autologous blood anticoagulated with bivalirudin is not a suitable alternative to heparin.

Crystal structures of aprotinin and its complex with sucrose octasulfate reveal multiple modes of interactions with implications for heparin binding

The crystal structures of aprotinin and its complex with sucrose octasulfate (SOS), a polysulfated heparin analog, were determined at 1.7-2.6A resolutions. Aprotinin is monomeric in solution, which associates into a decamer at high salt concentrations. Sulfate ions serve to neutralize the basic amino acid residues of aprotinin to stabilize the decameric aprotinin. Whereas SOS interacts with heparin binding proteins at 1:1 molar ratio, SOS was surprisingly found to induce strong agglutination of aprotinins. Five molecules of aprotinin interact with one molecule of the sulfated sugar, which is stabilized by electrostatic interactions between the positively charged residues of aprotinin and sulfate groups of SOS. The multiple binding modes of SOS with five individual aprotinin molecules may represent the diverse patterns of potential heparin binding to aprotinin, reflecting the interactions of densely packed protein molecules along the heparin polymer.

Platelet adhesion under flow

Platelet-adhesive mechanisms play a well-defined role in hemostasis and thrombosis, but evidence continues to emerge for a relevant contribution to other pathophysiological processes, including inflammation, immune-mediated responses to microbial and viral pathogens, and cancer metastasis. Hemostasis and thrombosis are related aspects of the response to vascular injury, but the former protects from bleeding after trauma, while the latter is a disease mechanism. In either situation, adhesive interactions mediated by specific membrane receptors support the initial attachment of single platelets to cellular and extracellular matrix constituents of the vessel wall and tissues. In the subsequent steps of thrombus growth and stabilization, adhesive interactions mediate platelet-to-platelet cohesion (i.e., aggregation) and anchoring to the fibrin clot. A key functional aspect of platelets is their ability to circulate in a quiescent state surveying the integrity of the inner vascular surface, coupled to a prompt reaction wherever alterations are detected. In many respects, therefore, platelet adhesion to vascular wall structures, to one another, or to other blood cells are facets of the same fundamental biological process. The adaptation of platelet-adhesive functions to the effects of blood flow is the main focus of this review.

Platelet-dense granule deficiency causes postoperative hemorrhage in patients receiving enoxaparin: a novel observation with dramatic clinical implications

BACKGROUND

Inherent hemorrhage risk has impeded the universal adoption of low-molecular-weight heparin (LMWH) for venous thromboembolic prophylaxis in surgical patients. Coagulation pathway parameters and platelet numbers routinely are evaluated preoperatively; scant attention has been directed toward evaluation of platelet function. We hypothesized that administration of LMWH may unmask latent platelet dysfunction and result in postoperative hemorrhage.

METHODS

Postoperative hemorrhage occurred in 15 (3.5%) of 423 consecutive patients undergoing laparoscopic gastric bypass. All patients received LMWH (enoxaparin, 40 mg) preoperatively. Hematologic evaluation included measurement of von Willebrand's factor level and activity, factor VIII level, and electron microscopic enumeration of platelet-dense granules.

RESULTS

All patients had normal preoperative platelet counts and coagulation profiles. Ten patients underwent hematologic evaluation: coagulation pathway parameters were normal in all; however, all patients had a markedly decreased number of platelet-dense granules.

CONCLUSIONS

Platelet-dense granule deficiency may cause postoperative hemorrhage in patients receiving LMWH.

Identification of surface epitopes of human coagulation factor Va that are important for interaction with activated protein C and heparin

Inactivation of factor Va (FVa) by activated protein C (APC) is a key reaction in the down-regulation of thrombin formation. FVa inactivation by APC is correlated with a loss of FXa cofactor activity as a result of three proteolytic cleavages in the FVa heavy chain at Arg306, Arg506, and Arg679. Recently, we have shown that heparin specifically inhibits the APC-mediated cleavage at Arg506 and stimulates cleavage at Arg306. Three-dimensional molecular models of APC docked at the Arg306 and Arg506 cleavage sites in FVa have identified several FVa amino acids that may be important for FVa inactivation by APC in the absence and presence of heparin. Mutagenesis of Lys320, Arg321, and Arg400 to Ala resulted in an increased inactivation rate by APC at Arg306, which indicates the importance of these residues in the FVa-APC interaction. No heparin-mediated stimulation of Arg306 cleavage was observed for these mutants, and stimulation by protein S was similar to that of wild type FVa. With this, we have now demonstrated that a cluster of basic residues in FVa comprising Lys320, Arg321, and Arg400 is required for the heparin-mediated stimulation of cleavage at Arg306 by APC. Furthermore, mutations that were introduced near the Arg506 cleavage site had a significant but modest effect on the rate of APC-catalyzed FVa inactivation, suggesting an extended interaction surface between the FVa Arg506 site and APC.