Heparin-induced thrombocytopenia: a ten-year retrospective

Outcomes of heparin-induced thrombocytopenia type II in aneurysmal subarachnoid hemorrhage patients: A US nationwide analysis

BACKGROUND

Despite the widespread use of heparin during and following endovascular procedures in the management of aneurysmal subarachnoid hemorrhage (SAH) patients, limited research has explored the incidence and impact of heparin-induced thrombocytopenia (HIT) on SAH.

METHODS

Descriptive statistics, multivariate regressions, and propensity score-matching were employed to compare clinical characteristics, comorbidities, interventions, complications, and outcomes of HIT in SAH patients identified within the US National Inpatient Sample database from 2010 to 2019.

RESULTS

Among 76 387 SAH patients from 2010 to 2019, 166 (0.22%) developed HIT. HIT was identified as a significant predictor of prolonged length of stay (OR 6.799, 95% CI 3.985 to 11.6, P<0.01) and poor functional outcomes (OR 2.541, 95% CI 1.628 to 3.966, P<0.01) after adjusting for relevant factors. HIT incidence was higher in patients with elevated SAH severity scores (1.42 vs 1.06, P<0.01), younger patients (58.04 vs 61.39 years, P=0.01), overweight individuals (0.4% vs 0.2%, P<0.01), those on long-term anticoagulants (10.84% vs 5.72%, P<0.01), or with a cerebrospinal fluid drainage device (external ventricular drain, ventriculoperitoneal shunt; P<0.01). HIT patients showed increased rates of endovascular coiling, ventricular drain placement, shunt placement, deep vein thrombosis, urinary tract infection, acute kidney injury, pulmonary embolism, venous sinus thrombosis, pneumonia, and cerebral vasospasm (all P<0.01).

CONCLUSION

SAH patients with HIT exhibited various comorbidities and increased rates of complications, which may contribute to extended hospital stays. This nationwide study aids clinical suspicion and highlights HIT's impact on SAH patients.

Identification of disulfide bond-linking sites in biosynthesized platelet factor 4 by establishing a partial reduction method without alkylation

Platelet factor 4 (PF4), a specific protein primarily found in megakaryocytes and platelet α-granules, plays an essential role in the coagulation process. It carries a high positive charge and thus has a unique ability to readily form complexes with negatively charged heparin. This interaction between PF4 and heparin plays a crucial role in platelet aggregation and thrombosis, resulting in heparin-induced thrombocytopenia (HIT). HIT is often diagnosed through various diagnostic tests that utilize exogenous PF4 detecting antibodies against PF4/heparin complexes. Besides, PF4 was recently found to have the potential to restore cognitive function. Therefore, a comprehensive characterization of biosynthetic PF4 standards is crucial for the diagnosis and management of HIT. In this study, a bacterial expression system was established to efficiently produce recombinant human PF4 (rhPF4). This PF4 was characterized using liquid chromatography-high resolution mass spectrometry, confirming it is completely identical to native PF4 in terms of disulfide connectivity and sequence. The in vitro chemotaxis assay indicates that it possesses biological activity. Furthermore, PF4 contains two disulfide bonds, which are crucial for its structural integrity and function. A partial reduction method was successfully developed to assign the disulfide bond connectivity as Cys10-Cys36 and Cys12-Cys52.

Characterization of complexes of PF4 and heparins by size-exclusion chromatography coupled with multi-angle light scattering detector

Heparin-induced thrombocytopenia (HIT) is an immune complication of heparin therapy. Antibodies binding to complexes of platelet factor 4 (PF4) and heparin is the trigger of HIT. A method using size exclusion chromatography with multi-angle laser light scattering detector (SEC-MALS) was developed in this work. The soluble ultra-large complex (ULC) was separated from the small complex (SC) and their molecular weights (MWs) were firstly measured. The complexes of PF4 and three heparins with different MW, including unfractionated heparin (UFH), dalteparin (Daltep) and enoxaparin (Eno) were characterized using this method. The contents and the sizes of ULC increased gradually when heparins were added to PF4 to certain amounts. While, they reduced after more heparins were added. It is the first time to measure the MWs of the biggest ULC of PF4-heparins as millions of Dalton. at the proper ratios of PF4 to heparin (PHR). Meanwhile, those mixtures at those certain PHRs induced the higher expression of CD83 and CD14 markers on dendritic cells (DCs) suggesting that they had stronger immunogenicity and is critical for HIT.

Regulatory T Cells Control PF4/Heparin Antibody Production in Mice

Heparin-induced thrombocytopenia is a relatively common drug-induced immune disorder that can have life-threatening consequences for affected patients. Immune complexes consisting of heparin, platelet factor 4 (PF4), and PF4/heparin-reactive Abs are central to the pathogenesis of heparin-induced thrombocytopenia. Regulatory T (Treg) cells are a subpopulation of CD4 T cells that play a key role in regulating immune responses, but their role in controlling PF4/heparin-specific Ab production is unknown. In the studies described in this article, we found that Foxp3-deficient mice lacking functional Treg cells spontaneously produced PF4/heparin-specific Abs. Following transplantation with bone marrow cells from Foxp3-deficient but not wild-type mice, Rag1-deficient recipients also produced PF4/heparin-specific Abs spontaneously. Adoptively transferred Treg cells prevented spontaneous production of PF4/heparin-specific Abs in Foxp3-deficient mice and inhibited PF4/heparin complex-induced production of PF4/heparin-specific IgGs in wild-type mice. Treg cells suppress immune responses mainly through releasing anti-inflammatory cytokines, such as IL-10. IL-10-deficient mice spontaneously produced PF4/heparin-specific Abs. Moreover, bone marrow chimeric mice with CD4 T cell-specific deletion of IL-10 increased PF4/heparin-specific IgG production upon PF4/heparin complex challenge. Short-term IL-10 administration suppresses PF4/heparin-specific IgG production in wild-type mice. Taken together, these findings demonstrate that Treg cells play an important role in suppressing PF4/heparin-specific Ab production.

Comparative Analysis of the Suspected Heparin-Induced Thrombocytopenia Level in Korea

The primary objective of our study was to evaluate the frequency of suspected heparin-induced thrombocytopenia (HIT) among patients treated with different formulations of heparin and investigate the factors that affect the incidence of HIT. This study is an electronic medical record (EMR)-based large-scale retrospective cohort study conducted from 2009 to 2014 in Korea. After hospitalization, patient platelet count was determined before heparin was prescribed, and all platelet count values obtained during hospitalization were extracted. Suspected HIT was estimated by three 4Ts scores (acute thrombocytopenia, timing onset and other possible causes), which when combined yielded a high probability of HIT. Among 6046 patients enrolled in this study, HIT was suspected in 641 cases (10.6%) and a statistically significant increase in HIT incidence rate was observed for three heparins used (p < 0.001). Dalteparin (HR = 0.55, p = 0.036) and enoxaparin (HR = 0.40, p < 0.001) showed a relatively low HIT incidence rate, compared to unfractionated heparin. Majority of suspected HIT cases (76.9 and 66.7%) occurred in days 8-10 and 5-7 of dalteparin and enoxaparin treatments, respectively. Most of the patients medicated with dalteparin were cancer patients; however, no statistically significant relationship was observed between HIT occurrence and cancer. HIT can cause serious complications, making early diagnosis crucial. Clinical practitioners first prescribing heparin should focus on preventing and detecting complications early by conducting frequent, regular platelet counts before and after heparin administration.

Hirudin therapy during thrombolysis for venous thrombosis in heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is the most common drug-related thrombocytopenia. Thromboembolic complications occur in approximately 50% of patients with HIT and result in limb amputation and death in up to 20% and 30% respectively. Because patients with a history of HIT may require future intravenous anticoagulation but have a high-risk of thromboembolism if re-challenged with heparin, alternative therapies are necessary when further anticoagulation is indicated. The use of direct thrombin inhibitors in HIT patients who also require thrombolytic therapy offers unique challenges to anticoagulant monitoring and safety. We present a case of progressive ileofemoral deep venous thrombosis in a patient with a history of HIT in order to review the combined use of hirudin and thrombolysis in this setting.

Predictive Genomics of Adverse Events After Cardiac Surgery

Significant, yet highly individual, thrombotic and inflammatory responses to surgery provide an excellent opportunity for insight into the genomic impact upon a patient's postoperative course. Cardiac surgery elicits the most profound perioperative disturbance and is associated with the highest incidence of adverse outcomes of any elective surgical procedure. Thus, cardiac surgical patients are an ideal population in which to evaluate the influence of complex traits on perioperative morbidity and mortality. This review describes the application of fundamental genetics upon the occurrence of adverse outcomes after cardiac surgery and cardiac transplantation. Specific emphases include a brief primer of the principles of genetics concentrating on the effects of variation within the human genome upon clinical outcomes and the differences between so-called Mendelian traits and complex traits. Four important clinical diseases dealt with in this review as examples of the impact of genetic factors on clinical outcomes are the genetics of heparin-induced thrombocytopenia, heart transplantation rejection and vasculopathy, atrial fibrillation, and infection.

Laboratory Diagnosis of Heparin-Induced Thrombocytopenia in Asian Indians as Investigated With Functional and Immunologic Methods

Heparin-induced thrombocytopenia (HIT) was looked for in 33 Indian patients undergoing cardiovascular surgery who received unfractionated heparin (UFH). Platelet counts were performed before the initiation of UFH therapy and 5 to 16 days after administration of the therapy. A decrease in patients' platelet count >35% of the baseline value or <100,000/μL was considered to be suggestive of HIT syndrome. Heparin-induced platelet aggregation (HIPA) and enzyme-linked immunosorbent assay (ELISA) tests were performed in all the patients to detect antibodies against heparin/PF4 complex. Thrombocytopenia was found to be present in 10 (30%) patients. Of these, two patients were found to be positive as detected by HIPA and ELISA tests. These two patients were considered to have classic HIT syndrome. One of these had bleeding while the other was asymptomatic. Among the 23 non-thrombocytopenic patients and controls, HIT antibodies were not detectable by HIPA test in all but absent by ELISA in only 21 of them. Two of 23 non-thrombocytopenic patients had positive results on the ELISA test. Because the latter had negative test results on HIPA, they were considered to be false-positive results. Thus, HIT syndrome develops in approximately 6% of Indian patients treated with UFH.

Thrombocytopenia in Critical Care Patients

Thrombocytopenia is a common laboratory finding in the intensive care unit (ICU) patient. Because the causes can range from laboratory artifact to life-threatening processes such as thrombotic thrombocytopenic purpura (TTP), identifying the cause of thrombocytopenia is important. In the evaluation of the thrombocytopenia patient, one should incorporate all clinical clues such as why the patient is in the hospital, medications the patient is on, and other abnormal laboratory findings. One should ensure that the patient does not suffer from heparin-induced thrombocytopenia (HIT) or one of the thrombotic microangiopathies (TMs). HIT can present in any patient on heparin and requires specific testing and antithrombotic therapy. TMs cover a spectrum of disease ranging from TTP to pregnancy complications and can have a variety of presentations. Management of disseminated intravascular coagulation depends on the patient’s condition and complication. Other causes of ICU thrombocytopenia include sepsis, medication side effects, post-transfusion purpura, catastrophic anti phospholipid antibody disease, and immune thrombocytopenia.

Critical role for mouse marginal zone B cells in PF4/heparin antibody production

Heparin-induced thrombocytopenia (HIT) is an immune-mediated disorder that can cause fatal arterial or venous thrombosis/thromboembolism. Immune complexes consisting of platelet factor 4 (PF4), heparin, and PF4/heparin-reactive antibodies are central to the pathogenesis of HIT. However, the B-cell origin of HIT antibody production is not known. Here, we show that anti-PF4/heparin antibodies are readily generated in wild-type mice on challenge with PF4/heparin complexes, and that antibody production is severely impaired in B-cell-specific Notch2-deficient mice that lack marginal zone (MZ) B cells. As expected, Notch2-deficient mice responded normally to challenge with T-cell-dependent antigen nitrophenyl-chicken γ globulin but not to the T-cell-independent antigen trinitrophenyl-Ficoll. In addition, wild-type, but not Notch2-deficient, B cells plus B-cell-depleted wild-type splenocytes adoptively transferred into B-cell-deficient μMT mice responded to PF4/heparin complex challenge. PF4/heparin-specific antibodies produced by wild-type mice were IgG2b and IgG3 isotypes. An in vitro class-switching assay showed that MZ B cells were capable of producing antibodies of IgG2b and IgG3 isotypes. Lastly, MZ, but not follicular, B cells adoptively transferred into B-cell-deficient μMT mice responded to PF4/heparin complex challenge by producing PF4/heparin-specific antibodies of IgG2b and IgG3 isotypes. Taken together, these data demonstrate that MZ B cells are critical for PF4/heparin-specific antibody production.

Rational design and characterization of platelet factor 4 antagonists for the study of heparin-induced thrombocytopenia

Patients with heparin-induced thrombocytopenia (HIT) remain at risk for recurrent thromboembolic complications despite improvements in management. HIT is caused by antibodies that preferentially recognize ultralarge complexes (ULCs) of heparin and platelet factor 4 (PF4) tetramers. We demonstrated previously that a variant PF4(K50E) forms dimers but does not tetramerize or form ULCs. Here, we identified small molecules predicted to bind PF4 near the dimer-dimer interface and that interfere with PF4 tetramerization. Screening a library of small molecules in silico for binding at this site, we identified 4 compounds that inhibited tetramerization at micromolar concentrations, designated PF4 antagonists (PF4As). PF4As also inhibited formation of pathogenic ULCs, and 3 of these PF4As promoted the breakdown of preformed ULCs. To characterize the ability of PF4As to inhibit cellular activation, we developed a robust and reproducible assay that measures cellular activation by HIT antibodies via FcγRIIA using DT40 cells. PF4As inhibit FcγRIIA-dependent activation of DT40 cells by HIT antibodies as well as platelet activation, as measured by serotonin release. PF4As provide new tools to probe the pathophysiology of HIT. They also may provide insight into the development of novel, disease-specific therapeutics for the treatment of thromboembolic complications in HIT.

Management of acquired bleeding problems in cancer patients

Cancer patients can have acquired bleeding problems for many reasons. In this review, an approach to the evaluation and management of the bleeding patient is discussed. Specific issues including coagulation defects, thrombocytopenia, platelet dysfunction, and bleeding complications of specific hematological malignancies due to anticoagulation, are discussed.

Thrombocytopenia and other hot topics

A frequent issue in the management of cancer patients with thrombosis is thrombocytopenia, whether related to chemotherapy, heparin, or other drugs. The oncologist will have multiple new anticoagulants to choose from in the future, but for now, the effect of these experimental agents on cancer thrombosis is unknown. Despite the effectiveness of anticoagulation in cancer patients, survival benefit remains controversial. Given the ongoing clinical questions and new drugs, anticoagulant therapy in patients with cancer promises to have an exciting future.

Management of acquired bleeding problems in cancer patients

Cancer patients can have acquired bleeding problems for many reasons. In this review, an approach to the bleeding patient in the Emergency Department is discussed. Specific issue including coagulation defects, thrombocytopenia, platelet dysfunction, bleeding complications of specific hematological malignancies and due to anticoagulation, are discussed.

The prevention and treatment of venous thromboembolism with LMWHs and new anticoagulants

As the risk factors for thrombosis are becoming better understood, so is the need for anticoagulation. The inherent difficulties with warfarin are such that a low-molecular-weight heparin (LMWH) is often the key therapeutic. However, there are several different species of LMWH available to the practitioner, which leads to the need for an objective guide. New agents are coming onto the marketplace, and these may supersede both warfarin and the heparins. The current report will review the biochemistry and pharmacology of different LWMHs and identify which are more suitable for the different presentations of venous thromboembolism. It will conclude with a brief synopsis of new agents which may supersede warfarin and heparin.

Heparin-induced thrombocytopenia. A contemporary clinical approach to diagnosis and management

Thrombocytopenia following heparin administration can be associated with an immune reaction, now referred to as heparin-induced thrombocytopenia (HIT). HIT is essentially a prothrombotic disorder mediated by an IgG antiplatelet factor 4/heparin antibody, which induces platelet, endothelial cell, monocyte, and other cellular activation, leading to thrombin generation and thrombotic complications. Indeed, HIT can also be regarded as a serious adverse drug effect. Importantly, HIT can be a life-threatening and limb-threatening condition frequently associated with characteristically severe and extensive thromboembolism (both venous and arterial) rather than with bleeding. This article provides an overview of HIT, with an emphasis on the clinical diagnosis and management.

Review: Recent Advances in Argatroban-Warfarin Transition in Patients With Heparin-induced Thrombocytopenia

Heparin-induced thrombocytopenia is a devastating, life-threatening, immune-mediated complication of therapy with unfractionated heparin, and less frequently, with low molecular weight heparin. Direct thrombin inhibitors are now standard therapy for the prevention of thrombosis in heparin-induced thrombocytopenia. Argatroban, a small synthetic molecule that inhibits thrombin at its active site, is increasingly used as the direct thrombin inhibitors of choice. Transition to longer term oral anticoagulation needs to be instituted after the platelet count has risen, because of the persistent risk of thrombosis. Although guidelines available in the literature outline the management of heparin-induced thrombocytopenia, they are not presented in a concise and comprehensive manner easily followed by physicians. This article reviews current recommendations, relevant studies, and clinical management trials carried out on patients with heparin-induced thrombocytopenia and provides updated, detailed guidelines for treatment of heparin-induced thrombocytopenia with emphasis on a key part of the management, the argatroban—warfarin transition.

Platelet transfusions in heparin-induced thrombocytopenia: a report of four cases and review of the literature

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is a complication of heparin therapy associated with thrombocytopenia and thrombosis. The diagnosis of HIT is based on clinical criteria and laboratory tests, including the serotonin release assay (SRA). Because HIT patients are thrombocytopenic, platelet (PLT) transfusions may be contemplated; however, many published reviews have concluded that PLT transfusions are contraindicated in HIT because they may precipitate thrombotic events. This study reports four patients with clinically suspected HIT who received PLT transfusions without complications, and the literature regarding this subject has been reviewed.

STUDY DESIGN AND METHODS

Patients with a SRA ordered for suspected HIT were retrospectively identified. Charts of patients with positive SRAs who received a PLT transfusion when HIT was clinically suspected were reviewed for evidence of PLT transfusion safety and efficacy. A comprehensive search of the published literature regarding PLT transfusions in patients with HIT was conducted.

RESULTS

A SRA was performed on 189 patients with suspected HIT. Thirteen patients tested positive and 4 of these received a PLT transfusion. No patient developed a thrombotic complication. All 4 patients had adequate posttransfusion PLT increments. Two of the 3 patients with active bleeding had cessation of bleeding after transfusion. Review of the literature revealed no case of a complication clearly attributable to PLT transfusion.

CONCLUSION

Four patients with clinically suspected HIT and a positive SRA were transfused PLTs both efficaciously and safely. These outcomes, combined with the results of the literature review, suggest that PLT transfusions should not be withheld when clinically indicated in patients with HIT.

Thrombocytopenia associated with antithrombotic therapy in patients with cardiovascular diseases: diagnosis and treatment

Agents with antiplatelet and anticoagulant activity have been proved to be effective in reducing the incidence of complications following acute coronary syndrome, percutaneous coronary intervention, and cardiopulmonary bypass. However, these agents, including heparin, glycoprotein IIb/IIIa receptor inhibitors, and thienopyridines, are associated with increased risk of bleeding and thrombocytopenia and have been administered together with increasing frequency in a variety of cardiovascular settings. Therefore, clinicians must be familiar with the safety and rational use of these potent antithrombotic agents. Clinical features of thrombocytopenia range from bleeding to thrombosis, even death, and therapy is very different depending on the underlying cause. Additionally, patients may sometimes need urgent intervention or surgery. Thus, it is essential to quickly discriminate the etiology and start appropriate therapy. This review highlights the pathogenesis, clinical and laboratory manifestation, differential diagnosis, and treatment of antithrombotic drug-induced thrombocytopenia in cardiovascular diseases.

Serologic Results in >1000 Patients With Suspected Heparin-Induced Thrombocytopenia

Heparin-induced thrombocytopenia (HIT) can lead to life-threatening and limb-threatening thrombosis. HIT is thought to be initiated by the interaction of pathogenic antibodies toward a complex platelet factor 4 (PF4) and heparin (PF4:H), which can activate platelets and predispose to thrombosis. As such, the laboratory diagnosis of HIT includes antigenic and functional assays to detect antibodies directed at PF4:H complexes. We performed a retrospective analysis of 1017 consecutive samples tested by serotonin-release assay and by enzyme-linked immunosorbent assay (ELISA). Most samples showed no serologic evidence of HIT, whereas 4% to 5% of samples demonstrated both antigenic and functional serological evidence for HIT. Approximately 12% to 18% of samples showed immunologic evidence of anti-PF4:H antibodies but without functional evidence of serotonin release in vitro. Interestingly, a small minority of samples (0.7%) caused serotonin release but were negative in the ELISA. The results are presented using cutoff values established at our hospital and for the ELISA manufacturer. This study provides a pretest probability of the serologic results from an antigenic assay (ELISA) and a functional assay (serotonin-release assay) in patients clinically suspected of having HIT.

[Heparin-induced thrombocytopenia]

Hemorrhage is the most common and best-recognized complication of heparin treatment. However, a potentially more dangerous complication is the development of heparin-induced thrombocytopenia (HIT). All patients exposed to heparin, irrespective of the dose and route of administration, are at risk of developing HIT. It is due to the formation of antibodies against the heparin-platelet factor 4 complex, which cause secondary activation of platelets, coagulation and, finally, increased thrombin production. The main symptom is the sudden onset of thrombocytopenia involving a drop in the platelet count to less than 50% of the basal level, with or without the appearance of thrombotic complications some 5 to 14 days after the start of heparin therapy. Heparin-induced thrombocytopenia can be detected early in patients receiving heparin by monitoring the platelet count. Demonstration of heparin-dependent platelet activation using an antigen or functional assay confirms the clinical diagnosis. Once the diagnosis of HIT has been confirmed serologically or there is a high level of suspicion of HIT, heparin must be suspended and treatment with an alternative anticoagulant should be considered. This review contains a discussion of the diagnosis and treatment of this syndrome.

No Difference in Risk for Thrombocytopenia During Treatment of Pulmonary Embolism and Deep Venous Thrombosis With Either Low-Molecular-Weight Heparin or Unfractionated Heparin

BACKGROUND

Low-molecular-weight heparin (LMWH) is a popular alternative to unfractionated heparin (UH) for the treatment of pulmonary embolism (PE) and deep vein thrombosis (DVT), in part based on the perception of a lower risk for heparin-induced thrombocytopenia (HIT). To investigate the evidence supporting this perception, we performed a metaanalysis to compare the incidence of thrombocytopenia between LMWH and UH during PE and/or DVT treatment.

METHODS

Randomized trials comparing LMWH with UH for PE and/or DVT treatment were searched for in the MEDLINE database, bibliographies, and by correspondence with published investigators. Two reviewers independently selected high-quality studies and extracted data regarding heparin-associated thrombocytopenia (HAT), HIT confirmed by laboratory testing, and heparin-induced thrombocytopenia with thrombosis (HITT). Outcome rates between LMWH and UH were compared using a binomial, generalized linear mixed model with a logit link and Gaussian random effects for study.

RESULTS

Thirteen studies involving 5,275 patients met inclusion criteria. There were no statistically significant differences in HAT rates between the two treatments (LMWH, 1.2%; UH, 1.5%; p = 0.246). The incidence of documented HIT and HITT was too low to make an adequate comparison between groups.

CONCLUSIONS

Our review disclosed no statistically significant difference in HAT between LMWH and UH and insufficient evidence to conclude that HIT and HITT rates were different between them. There was no evidence from randomized comparative trials to support the contention that patients receiving treatment for PE or DVT with UH are more prone to these complications than those receiving LMWH.

Procoagulant Microparticles

Apoptosis and vascular cell activation are main contributors to the release of procoagulant microparticles (MPs), deleterious partners in atherothrombosis. Elevated levels of circulating platelet, monocyte, or endothelial-derived MPs are associated with most of the cardiovascular risk factors and appear indicative of poor clinical outcome. In addition to being a valuable hallmark of vascular cell damage, MPs are at the crossroad of atherothrombosis processes by exerting direct effects on vascular or blood cells. Under pathological circumstances, circulating MPs would support cellular cross-talk leading to vascular inflammation and tissue remodeling, endothelial dysfunction, leukocyte adhesion, and stimulation. Exposed membrane phosphatidylserine and functional tissue factor (TF) are 2 procoagulant entities conveyed by circulating MPs. At sites of vascular injury, P-selectin exposure by activated endothelial cells or platelets leads to the rapid recruitment of MPs bearing the P-selectin glycoprotein ligand-1 and blood-borne TF, thereby triggering coagulation. Within the atherosclerotic plaque, sequestered MPs constitute the main reservoir of TF activity, promoting coagulation after plaque erosion or rupture. Lesion-bound MPs, eventually harboring proteolytic and angiogenic effectors are additional actors in plaque vulnerability. Pharmacological strategies aimed at modulating the release of procoagulant MPs appear a promising therapeutic approach of both thrombotic processes and bleeding disorders.

Prothrombotic factors enhance heparin-induced thrombocytopenia and thrombosis in vivo in a mouse model

BACKGROUND

Heparin-induced thrombocytopenia/thrombosis (HIT/T) is a common cause of life- and limb-threatening thrombosis. The development of antibodies that react with complexes of heparin and platelet factor 4 (PF4) is fundamental to the development of the disease. However, anti-PF4/heparin antibodies are far more common than is HIT/T and there is less understanding of the factors that contribute to thrombosis in only a subset of patients.

OBJECTIVES

Both qualitative and quantitative differences in multiple factors (e.g. antibodies, heparin and platelets) may influence the clinical course of patients who develop anti-PF4/heparin antibodies. We examined the hypothesis that host-specific factors, such as comorbid prothrombotic conditions, would exacerbate the pathologic effects of anti-PF4/heparin antibodies.

METHODS AND RESULTS

A mouse model transgenic for human Fcgamma RIIa and PF4 and null for mouse PF4 was used to study the influence of prothrombotic conditions on the effects of anti-PF4/heparin antibodies in vivo. To simulate a prothrombotic milieu, mice were fed a hypercholesterolemic diet (HD). HD-fed mice had elevated plasma cholesterol, increased platelet reactivity and increased endothelial activation relative to mice fed a standard diet (SD). Age- and sex-matched mice from each diet group were treated with an anti-PF4/heparin antibody and heparin. HD-fed mice developed more severe thrombocytopenia than similarly treated SD-fed mice. Mice with moderate to severe thrombocytopenia had elevated plasma levels of thrombin-antithrombin complexes, indicative of increased thrombin generation in vivo. Platelet-fibrin thrombi were observed in multiple organs of HD-fed mice that developed severe thrombocytopenia.

CONCLUSIONS

Host-specific factors, such as prothrombotic changes in platelet reactivity and/or endothelial activation, may influence the development of thrombosis in a subset of patients who develop anti-PF4/heparin antibodies.

[Thrombocytopathy and blood complications in uremia]

Bleeding diathesis and thrombotic tendencies are characteristic findings in patients with end-stage renal disease. The pathogenesis of uremic bleeding tendency is related to multiple dysfunctions of the platelets. The platelet numbers may be reduced slightly, while platelet turnover is increased. The reduced adhesion of platelets to the vascular subendothelial wall is due to reduction of GPIb and altered conformational changes of GPIIb/IIIa receptors. Alterations of platelet adhesion and aggregation are caused by uremic toxins, increased platelet production of NO, PGI(2), calcium and cAMP as well as renal anemia. Correction of uremic bleeding is caused by treatment of renal anemia with recombinant human erythropoietin or darbepoetin alpha, adequate dialysis, desmopressin, cryoprecipitate, tranexamic acid, or conjugated estrogens. Thrombotic complications in uremia are caused by increased platelet aggregation and hypercoagulability. Erythrocyte-platelet-aggregates, leukocyte-platelet-aggregates and platelet microparticles are found in higher percentage in uremic patients as compared to healthy individuals. The increased expression of platelet phosphatidylserine initiates phagocytosis and coagulation. Therapy with antiplatelet drugs does not reduce vascular access thrombosis but increases bleeding complications in endstage renal disease patients. Heparin-induced thrombocytopenia (HIT type II) may develop in 0-12 % of hemodialysis patients. HIT antibody positive uremic patients mostly develop only mild thrombocytopenia and only very few thrombotic complications. Substitution of heparin by hirudin, danaparoid or regional citrate anticoagulation should be decided based on each single case.

Argatroban: update

Unfractionated heparin has historically been used as the anticoagulant of choice in the management of a number of thrombotic diseases. Recognition of the limitations of heparin has led to the development of a newer class of anticoagulants, the direct thrombin inhibitors. Argatroban is a synthetic small molecule that selectively inhibits thrombin at its active site. In preclinical studies, argatroban has been shown to be more effective than heparin in preventing arterial thrombosis and in promoting vessel patency in conjunction with thrombolysis in a number of animal models. In clinical trials, argatroban has been shown to be as effective as heparin in the management of ST-segment elevation myocardial infarction in conjunction with thrombolysis. It has been shown to be an effective anticoagulant in patients undergoing percutaneous coronary interventions. In patients with heparin-induced thrombocytopenia and heparin-induced thrombocytopenia complicated by thrombosis, argatroban significantly decreases the risk of thrombotic events. Small studies have demonstrated a potential role for its use in ischemic stroke and hemodialysis. Additional studies are warranted to confirm argatroban's efficacy in a wide variety of clinical settings.

Prevalence of heparin-induced antibodies in patients with chronic renal failure undergoing hemodialysis

Heparin-induced thrombocytopenia (HIT) type II is a serious complication of heparin therapy. It presents initially as thrombocytopenia, and is associated with thrombosis in 20-50% of the cases. HIT is related to the presence of heparin-induced antibodies (HIA), which show specificity for the PF4-heparin (PF4-H) complex. The FcgammaRIIa receptor has been suggested to participate in the pathogenic mechanism of HIA. Since patients undergoing chronic hemodialysis (HD) are exposed repeatedly to heparin, we studied the prevalence of HIA and their eventual relationship with thrombocytopenia and/or thrombosis, and the possible participation of the FcgammaRIIa polymorphism. We studied 207 patients with chronic renal failure (CRF) undergoing HD. As a control we included 130 blood donors and 28 patients with CRF without HD. The HIA patients were studied with the use of a PF4-H ELISA. Additionally, in some positive cases for the previous test, a 14C- serotonin release assay (14C-SRA) was performed. The polymorphism FcgammaRIIa H/R131 was studied by polymerase chain reaction (PCR) with allele-specific primers. Thirty-seven patients (17.9%) undergoing HD presented with HIA. The majority of these antibodies were IgG, IgM, and IgA. The HIA investigated presented specificity against the PF4-H complex, but not against PF4 alone (P<0.001). Twelve out of 22 (54.5%) PF4-H antibodies were positive when tested with the 14C-SRA. The distribution of the FcgammaRIIa polymorphism in patients and healthy controls was 42.6% and 41.6% for H/H131, 41% and 48.9% for the H/R131 isoform, and 16.4% and 9.5% for the R/R131 isoform, respectively. No statistically significant difference in the FcgammaRIIa isoform distribution was found. Twenty-nine out of 156 patients (18.5%) presented thrombocytopenia, and 21/207 (12.4%) had thrombosis of the native vein arterio-venous fistula (AVF). We did not find any statistically significant between HIA and thrombocytopenia or thrombosis. An important proportion of patients with CRF undergoing HD developed HIA, but these cases were not associated with thrombocytopenia or thrombosis of AVF. The frequency of the FcgammaRIIa polymorphism did not statistically differ between HIT type II and normal controls.

Critical care clotting catastrophies

Coagulation problems are very common in intensive care patients. It is important to recognize potential problems, perform a rapid assessment, and start therapy. The author reviews general clinical and laboratory approaches to diagnosis and treatment of the bleeding patient and to correction of coagulopathies. This review outlines a set of often catastrophic coagulation problems, which may present both thrombotic and bleeding challenges. These include heparin induced thrombocytopenia, thrombotic thrombocytopenic purpura, and disseminated intravascular coagulation.

Heparin-induced cardiac tamponade and life-threatening hyperkalema in a patient with chronic hemodialysis

Heparin, a commonly used anticoagulant agent, is frequently used in patients undergoing hemodialysis. As with most medications, heparin has a significant side effect profile. Two of its most important side effects, major bleeding and hyperkalemia, may be devastating without immediate diagnosis and treatment. Major bleeding such as gastrointestinal, genitourinary or intracranial bleeding is occasionally encountered and rarely neglected. However, heparin-induced cardiac tamponade is rarely encountered and may be easily overlooked. Another side effect, heparin-induced hyperkalemia, an unusual but well-described side effect, is frequently forgotten until life-threatening arrhythmia has occurred. We report a case involving a 40-year-old male patient with uremia, who had received heparin for 10 days for deep vein thrombosis in the left lower extremity. Hemopericardium with cardiac tamponade and life-threatening hyperkalemia were both noted in this patient.

Alternative parenteral anticoagulation with argatroban, a direct thrombin inhibitor

Argatroban, a direct thrombin inhibitor, effectively inhibits free and clot-bound thrombin without the need of a cofactor and exerts dose-dependent anticoagulant effects that are rapidly active and rapidly reversible (elimination half-life: 39-51 min). Argatroban provides predictable parenteral anticoagulation and is well tolerated with an acceptably low bleeding risk in a variety of clinical settings, including heparin-induced thrombocytopenia, acute ischemic stroke, percutaneous coronary intervention and hemodialysis. This review will discuss the clinical pharmacology and utility of argatroban; in particular, clinical trial experiences will be discussed in patients with, or at risk of, heparin-induced thrombocytopenia (where heparins must be avoided) including those requiring hemodialysis or percutaneous coronary intervention, and in patients with acute ischemic stroke (where heparins are not generally recommended).

Ultralarge complexes of PF4 and heparin are central to the pathogenesis of heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia and thrombosis (HITT) is a severe complication of heparin therapy caused by antibodies to complexes between unfractionated heparin (UFH) and platelet factor 4 (PF4) that form over a narrow molar range of reactants and initiate antibody-induced platelet activation. We observed that UFH and tetrameric PF4 formed ultralarge (> 670 kDa) complexes (ULCs) only over a narrow molar range with an optimal ratio of PF4 to heparin of approximately 1:1. These ULCs were stable and visible by electron microscopy, but they could be dissociated into smaller complexes upon addition of heparin. ULCs formed inefficiently when PF4 was incubated with low-molecular-weight heparin, and none formed with the pentasaccharide fondaparinux sodium. In addition, mutation studies showed that formation of ULCs depended on the presence of PF4 tetramers. The ULCs were more reactive as determined by their capacity to bind to a HITT-like monoclonal antibody and showed greater capacity to promote platelet activation in an antibody- and FcgammaRIIA-dependent manner than were the smaller complexes. The capacity of PF4 to form ULCs composed of multiple PF4 tetramers arrayed in a lattice with several molecules of UFH may play a fundamental role in autoantibody formation, antibody-dependent platelet activation, and the propensity for thrombosis in patients with HITT.

Heparin-induced thrombocytopenia presenting with thrombosis of multiple saphenous vein grafts and myocardial infarction

We report herein a patient with coronary artery disease that developed heparin-induced thrombocytopenia after coronary artery bypass graft with resulting thrombosis of multiple saphenous vein grafts and myocardial infarction after heparin exposure. The patient required lepirudin and a cardiac catheterization with placement of stents.