Heparin

Structural characterization and functional effects of a circulating heparan sulfate in a patient with hepatocellular carcinoma

A circulating anticoagulant was isolated from the plasma of a 42-year-old man with cirrhosis and hepatocellular carcinoma who had an unusual coagulation test profile. The patient developed a fatal coagulopathy, unresponsive to protamine therapy or plasma exchange following liver biopsy. However, at presentation, routine hemostasis assays were normal. The patient had mucocutaneous bleeding but the sole laboratory abnormality was a prolonged thrombin time (TT = 99 s, normal 25-35 s). Protamine titration indicated activity equivalent to a heparin concentration of 6-7 U/ml. Antithrombin III (AT III) antigen and activity were markedly elevated. The anticoagulant activity, purified from plasma by DEAE chromatography, was identified as a glycosaminoglycan (GAG). GAG anti-thrombin activity was completely abolished by heparin lyase III. Based on the degree of sulfation and HPLC pattern, the GAG was classified as heparan sulfate. Low levels (4 microM) of purified GAG markedly prolonged the TT (>120 s) but not the activated partial thromboplastin time (PTT) (31.4 s). In a Factor Xa assay, the GAG exhibited a potency equivalent to 0.06 U of low molecular weight heparin per nmol of uronic acid. Patients with endogenous circulating glycosaminoglycans can present with unusual laboratory coagulation test profiles. These reflect complex dysfunction of hemostasis, leading to difficulty in providing diagnosis and effective care.

Drug-Induced Thrombocytopenia: Is it a Serious Concern for Glycoprotein IIb/IIIa Receptor Inhibitors?

Over the past decade, several glycoprotein IIb/IIIa receptor antagonists have been developed and tested clinically as adjuncts to coronary intervention and/or treatment of acute coronary syndromes. Thrombocytopenia associated with this class of compounds has been described in most large studies to date and when it occurs in combination with bleeding represents a major safety concern. Cases of thrombocytopenia caused by GP IIb/IIIa antagonists vary in their clinical presentation according to time of onset (following the first dose or delayed), severity (profound, i.e., <20,000 cells/mm(3), or mild), and may or may not be associated with clinically important bleeding. More than one etiology appears responsible for thrombocytopenia associated with GP IIb/IIIa antagonists, including acute, idiosyncratic, as well as delayed immune-mediated mechanisms. Comparison of the incidence of thrombocytopenia across the different agents currently being studied and the one agent commercially available is complicated by varying definitions of thrombocytopenia used to date; different clinical settings in which GP IIb/IIIa antagonists have been studied; use of concomitant medications such as heparin, which itself may cause thrombocytopenia; relatively infrequent occurrence of thrombocytopenia; and the limited number of patients exposed to these agents. Review of the large studies presented and published to date suggests that thrombocytopenia due specifically to GP IIb/IIIa receptor antagonists occurs in less than 5% of treated patients and may vary depending on the type of agent, concomitant therapy, and clinical scenario. Current standard management includes immediate cessation of the GP IIb/IIIa antagonist and, in severe cases, platelet transfusions. In cases with associated hemorrhage, other anticoagulants and antiplatelet agents should be discontinued and possibly reversed. There may be a role for IV IgG and steroids, especially for cases of thrombocytopenia that are immune-mediated; however, further investigations are necessary.

aPTT in Heparinized Patients: Influence of the Interval Between Sampling and Testing

Activated partial thromboplastin times (aPTTs) from 29 whole blood samples were drawn from patients receiv ing unfractionated heparin through a constant intravenous drip. Three aPTTs were determined for each whole blood sample. The first aPTT was performed on a separated portion of the original sample at approximately 30 min from the time of sam pling. A second was performed on the same separated sample at approximately 90 min from the time of sampling, and the third was performed on the original sample at 90 min. The mean aPTT of the set of 30-minute separated samples was 79.7 seconds. The mean aPTT of the 90-minute separated samples was 72.8 seconds. The mean aPTT of the 90-minute original sample was 60.8 seconds. These results demonstrate that an aPTT performed 30 min after sampling yields a value signifi cantly greater than an aPTT performed at 90 min. The lower aPTT values may lead to an unnecessary increase in heparin dosage. This situation underscores the need to further investi gate the aPTT as a foundation for the standardization of the protocol for the use of heparin.

Low-molecular-weight heparin for the treatment of venous thromboembolism

Unfractionated heparin (UFH) binds to several plasma, platelet, and endothelial proteins, producing a highly variable anticoagulant response. For this reason, frequent anticoagulation monitoring and dosage adjustment are necessary during UFH administration. In contrast, low-molecular-weight heparins (LMWHs) exhibit less protein binding and provide more predictable anticoagulation with reduced need for patient monitoring and dosage adjustment. Therefore, LMWHs are potentially useful for anticoagulation therapy on an outpatient basis. Several recent clinical trials have compared LMWHs (administered primarily on an outpatient basis) and UFH for the treatment of venous thromboembolism. These trials have indicated that LMWHs possess efficacy and safety characteristics similar to intravenous heparin but are easier to administer. LMWH preparations vary considerably in their methods of preparation and pharmacological properties, and the relative efficacies of these various LMWHs remain to be determined by direct comparisons in randomized clinical trials.

New therapies for unstable angina and non-Q-wave myocardial infarction: recent clinical trials

Thrombosis after the rupture of an atherosclerotic plaque often precipitates the acute coronary syndromes of unstable angina and myocardial infarction. The combination of aspirin and heparin has been shown to reduce the occurrence of both symptomatic and asymptomatic ("silent") ischemia, myocardial infarction, and death in patients with these syndromes. However, heparin and aspirin each have significant limitations as antithrombotic drugs. Additionally, coagulation abnormalities may persist for several months after an acute ischemic event, and long-term anticoagulation may be beneficial. Because of the need for frequent anticoagulation monitoring and dosage adjustment, the use of heparin is limited to short-term treatment during the acute in-hospital phase. Recently several novel antithrombotic treatments have been developed. The benefits of direct thrombin inhibitors, platelet fibrinogen receptor antagonists, and low-molecular-weight heparins in the treatment of acute coronary syndromes have been demonstrated in randomized clinical trials.

Pharmacology of the low-molecular-weight heparins

Unfractionated heparin (UFH) is widely used to both treat and prevent venous thromboembolism. More recently, UFH has been used to prevent death and myocardial infarction in patients with unstable angina or acute myocardial infarction and acute occlusion in those undergoing percutaneous coronary revascularization. However, its poor bioavailability (when administered in low doses subcutaneously), its mechanism of clearance, and its short half-life make its anticoagulant activity difficult to predict and maintain. To overcome these limitations, low-molecular-weight heparins (LMWHs) have been developed that have greater bioavailability and a longer half-life in plasma. Because LMWHs provide more predictable anticoagulant activity compared with subcutaneous UFH, it is not necessary to monitor the activated partial thromboplastin time during treatment. These newer agents are as effective as UFH in the prophylaxis and treatment of thromboembolic and cardiac disorders and, by allowing shorter hospital stays, are more cost effective. Thus LMWHs offer clear pharmacokinetic advantages over UFH. More studies are needed to determine the extent to which clinically available LMWHs can be used in place of UFH.

The importance of sampling site in the measurement of whole-blood platelet flow cytometry

PURPOSE

Flow cytometry is an emerging technology that may be of use in clarifying the defects of platelet function after cardiopulmonary bypass. However, the technique used for platelet sampling may affect results. The objective of this study was to evaluate the influence of the sampling site on the degree of expression of a variety of platelet-associated proteins.

METHODS

Whole-blood flow cytometric assays for the detection of platelet glycoprotein (GP) Ib, guanosine monophosphate (GMP)-140, thrombospondin, activated GPIIb/IIIa, and platelet-associated factor (FXIIIa) were developed. These markers were then measured in samples taken simultaneously from a peripheral vein, radial artery, and the side port of the central venous catheter, in eight patients about to undergo surgery.

RESULTS

When multiple samples from individual patients were assessed, the degree of activation with all of the activation assays (GMP-140, thrombospondin, activated GPIIb/IIIa, FXIIIa) was significantly greater in samples taken from the arterial catheter (p < 0.05) compared with the central venous catheter or the peripheral vein. The mean difference between sample sites was calculated in the study patients. Percent activation of FXIIIa from arterial blood was significantly greater than from the central vein and the peripheral vein (arterial-peripheral venous, 18.7 +/- 8.6; central venous-peripheral venous, 3.7 +/- 3.6; p = 0.005). There was no site-related difference in detected expression of platelet GPIb.

CONCLUSION

The site of platelet sampling significantly affects the degree of activation detected by flow cytometry. To approximate results that would be obtained from peripheral blood, samples should be taken from the side port of the central venous catheter and not from the arterial catheter in patients studied during surgery.

Thrombosis and cardiovascular disease

The early stages of venous thrombosis originate at selective sites of reduced blood flow in the apices of venous valves. Stagnation of venous flow results in the formation of a platelet-fibrin thrombus, which serves as the nidus for thrombus propagation. Stasis alone, however, does not result in thrombosis necessarily. The presence of activated coagulation factors is essential. The major predisposing factors to venous thrombus are activation of blood coagulation and venous stasis. In contrast to venous thrombus formation, vascular abnormalities are the most important causative factor in arterial thrombosis.

A peptide sequence of heparin/heparan sulfate (HP/HS)-interacting protein supports selective, high affinity binding of HP/HS and cell attachment

We previously have identified a novel cell surface heparan sulfate/heparin (HS/HP)-interacting protein (HIP) found in human uterine epithelia and a variety of other human epithelial and endothelial cells and cell lines (Liu, S., Smith, S. E., Julian, J., Rohde, L. H., Karin, N. J., and Carson, D. D. (1996) J. Biol. Chem. 271, 11817-11823; Rohde, L. H., Julian, J., Babaknia, A., and Carson, D. D. (1996) J. Biol. Chem. 271, 11824-11830). The amino acid sequence predicted for HIP revealed a potential HS/HP-binding motif. In the present studies, a synthetic peptide corresponding to this putative HS/HP-binding motif, HIP peptide, was synthesized and examined with regard to its HS/HP binding and cell attachment promoting activity. Results using solid phase binding assays demonstrate that HIP peptide binds HS/HP with high selectivity and has high affinity for bulk HP (50% saturation congruent with 300 nM) and even higher affinity for a subset of polysaccharides found in commercial [3H]HP (half-saturation congruent with 10 nM). Moreover, HIP peptide binds subsets of cell and extracellular matrix-associated HS and dermatan sulfate expressed by RL95 cells, a human uterine adenocarcinoma cell line. HIP peptide also binds a similar fraction of HS as well as dermatan sulfate expressed by JAR cells, a human choriocarcinoma cell line. In contrast to binding of cell- or extracellular matrix-associated HS, HIP peptide does not bind secreted or released forms of HS or DS from either RL95 or JAR cells to a significant extent. HS species that bind to HIP peptide are generally larger, have a higher negative charge density, and have a larger proportion of di- and trisulfated disaccharide units than HS species that do not bind to HIP peptide, demonstrating structural differences among these polysaccharides. This same peptide supports HS-dependent JAR cell attachment. Collectively, these data demonstrate that a linear peptide sequence found within HIP can account, at least in part, for the HS/HP binding and cell adhesion promoting activities of this protein.

Hirunorms, novel hirudin-like direct thrombin inhibitors

1. Hirunorms are new synthetic peptides designed to interact with thrombin in a similar way to the natural inhibitor hirudin. 2. Hirunorms are specific and efficient in vitro inhibitors of thrombin activity. 3. Hirunorms are potent anticoagulant and antithrombotic agents in in vivo experimental models devoid of hemorrhagic effects at doses that are active in preventing thrombosis.

Guidelines for antithrombotic therapy in pediatric patients

Because of the relatively low incidence of TEs in children, the diagnostic and therapeutic approaches used are largely extrapolated from guidelines for adults. Features that differ in children compared with adults include underlying disorders, high incidence of CVL-related DVT in the upper venous system, and response to SH, warfarin, and thrombolytic agents. There is a paucity of information on the risk/benefit ratio of the therapeutic interventions and long-term outcome. Clinical trials are urgently needed to clarify optimal management for pediatric patients with TEs.

Dalteparin for deep venous thrombosis: a hospital-in-the-home program

OBJECTIVE

To assess the efficacy, safety and cost savings of home treatment of lower-limb deep venous thrombosis (DVT).

SETTING

A hospital-in-the-home treatment program.

PATIENTS

One hundred patients with acute lower limb DVT (53 proximal, 47 distal), and no contraindication to home treatment, were entered into the program from March 1995 to February 1997.

INTERVENTION

All patients received dalteparin, 200 units/kg subcutaneously, once daily for a minimum of five days, with commencement of oral anticoagulation (warfarin) on Day 2. Patients with proximal DVT had lung ventilation-perfusion scans performed and were admitted to hospital for at least 24 hours. Patients with distal DVT were discharged directly to home treatment.

MAIN OUTCOME MEASURES

Clinical responses and the results of sequential duplex ultrasonography at one week, one month, three months and six months.

RESULTS

There were no major, but six minor, bleeding complications, two of which led to dalteparin being withdrawn. Sixteen patients had lung ventilation-perfusion scans showing a high probability of pulmonary embolism. All were asymptomatic, and follow-up for at least three months showed no symptomatic thromboembolic events. Duplex ultrasonography showed progression of thrombosis in the first week of therapy in 13.2% of distal and 2.7% of proximal thromboses. Thereafter, distal DVT improved at a much greater rate than proximal DVT; after six months complete resolution was seen in 60.7% of distal and 18.5% of proximal thromboses, respectively. Cost saving was $197 per bed-day equivalent compared with inpatient care. At 15 months' follow-up, swelling and/or pain was reported by 49% of patients with distal DVT and 66% of those with proximal DVT.

CONCLUSIONS

Once-daily dalteparin therapy for DVT in a hospital-in-the-home setting was safe, efficacious and cost effective. However, DVT resolution is a slow process, with significant long term morbidity.

Thrombin generation, inhibition and clinical outcomes in patients with acute myocardial infarction treated with thrombolytic therapy and heparin: results from the GUSTO-I Trial. GUSTO-I Hemostasis Substudy Group. Global Utilization of Streptokinase and TPA for Occluded Coronary Arteries

OBJECTIVES

We sought to assess the effects of antithrombotic therapy after thrombolysis for acute myocardial infarction on markers of thrombin generation and activity and to determine the relation of these markers with clinical outcomes.

BACKGROUND

Thrombin activation and generation often occur with thrombolysis for acute myocardial infarction. Antithrombotic regimens have been developed to reduce the resulting thrombotic complications.

METHODS

We sampled plasma markers of thrombin generation and activity after thrombolysis in 292 patients. We assessed the relations of these markers with clinical outcomes at 30 days.

RESULTS

Fibrinopeptide A (FPA), a marker of thrombin activity toward fibrinogen, was elevated at baseline (12.3 ng/ml) and increased to 18.4 ng/ml by 90 min after streptokinase and subcutaneous heparin treatment. With intravenous heparin, this increase was attenuated, but intravenous heparin did not prevent thrombin generation, as measured by prothrombin fragment 1.2 (F1.2). Heparin level, measured by anti-Xa activity, correlated with activated partial thromboplastin time (aPTT, r = 0.62 to 0.67). Thrombin activity, measured by FPA, was as closely related to aPTT as to the heparin level. Baseline levels of F1.2 were significantly related to the risk of death or reinfarction at 30 days (p = 0.008); values 12 h after enrollment also were related to 30-day mortality (p = 0.05).

CONCLUSIONS

Although intravenous heparin partly suppresses the increased thrombin activity associated with thrombolysis, it does not inhibit thrombin generation. The aPTT was as good a measure of suppression of thrombin activity as the heparin level itself. Hematologic markers of thrombin generation were found to be related to the subsequent risk of thrombotic events.

Mycophenolate mofetil reduces production of interferon-dependent major histocompatibility complex induction during allograft rejection, probably by limiting clonal expansion

The immunosuppressive drug mycophenolate mofetil (MMF) acts by releasing mycophenolic acid (MPA), which inhibits the enzyme inosine monophosphate dehydrogenase (IMPDH) and thus inhibits de novo purine synthesis. Unlike cyclosporine (CsA), MMF has no direct effect on cytokine gene expression in vitro. We examined the effect of MMF, in comparison to CsA, on in vivo production of interferon-gamma (IFN-gamma) in mice. Two stimuli for IFN-gamma induction were used: (1) allogeneic P815 mastocytoma ascites tumour cells and (2) bacterial lipopolysaccharide (LPS). The allogeneic response is dependent on clonal expansion of T cells, while the LPS response is polyclonal and T cell independent. Since major histocompatibility complex (MHC) induction in mouse kidney is IFN-gamma dependent, we assessed the in vivo induction of IFN-gamma indirectly by measuring MHC induction in mouse kidneys in three systems: radiolabelled antibody binding assay, immunoperoxidase staining in tissue sections, and Northern blotting for steady-state MHC mRNA levels. IFN-gamma steady-state mRNA levels were assessed by reverse transcriptase polymerase chain reaction (RT-PCR). In the allogeneic response, MMF (40-160 mg/kg/day) reduced the production of IFN-gamma in a dose-dependent fashion. MHC class I and II induction was reduced by 35% to 74% and 30% to 74%, respectively. However, MMF had less effect on the induction of MHC by a nonimmune stimulus, bacterial LPS, whereas CsA reduced the induction of IFN-gamma in both responses. We conclude that MMF reduces the IFN-dependent induction of MHC in vivo during specific immune responses, probably by limiting clonal expansion, while preserving nonspecific cytokine production in response to LPS.

Intraoperative monitoring of danaparoid sodium anticoagulation during cardiovascular operations

PURPOSE

Patients with cardiovascular disorders frequently need anticoagulation for diagnostic studies, surgical procedures, and therapy. Heparin-induced thrombocytopenia is a relatively common complication of heparin therapy that can result in thrombosis and subsequent limb loss or death, necessitating use of alternative anticoagulants.

METHODS

Two patients who needed cardiac surgery had thrombocytopenia induced by exposure to heparin and heparin-coated tubing. Several assays were examined for their ability to monitor intraoperative anticoagulation of a factor Xa inhibitor, danaparoid sodium.

RESULTS

In vitro, celite and kaolin activated dotting times and activated partial thromboplastin time were prolonged linearly in the presence of increasing concentrations of danaparoid sodium. Aprotinin did not alter the linearity of the response but did alter its slope. In vivo, activated clotting times and activated partial thromboplastin time were insensitive to clinically significant changes in danaparoid sodium plasma levels during cardiopulmonary bypass. Correction in activated partial thromboplastin time lagged 2 hours behind clinically important changes in anti-factor Xa levels. Only anti-factor Xa levels were adequate to monitor intraoperative danaparoid sodium levels.

CONCLUSION

Anticoagulation for cardiopulmonary bypass can be successfully performed with danaparoid sodium and intraoperative anti-factor Xa monitoring.

Thrombin binds to soluble fibrin degradation products where it is protected from inhibition by heparin-antithrombin but susceptible to inactivation by antithrombin-independent inhibitors

BACKGROUND

Thrombolytic therapy induces a procoagulant state characterized by elevated plasma levels of fibrinopeptide A (FPA), but the responsible mechanism is uncertain.

METHODS AND RESULTS

Washed plasma clots were incubated in citrated plasma in the presence or absence of tissue plasminogen activator (t-PA), and FPA generation was monitored as an index of unopposed thrombin activity. FPA levels are almost twofold higher in the presence of t-PA than in its absence. This primarily reflects the action of thrombin bound to soluble fibrin degradation products because (a) there is progressive FPA generation even after clots are removed from t-PA-containing plasma, and (b) clot lysates produce concentration-dependent FPA generation when incubated in citrated plasma. Using thrombin-agarose affinity chromatography, (DD)E and fragment E but not D-dimer were identified as the thrombin-binding fibrin fragments, indicating that the thrombin-binding site is located within the E domain. Heparin inhibits thrombin bound to fibrin degradation products less effectively than free thrombin. In contrast, D-Phe-Pro-ArgCH2Cl, hirudin and hirugen inhibit free thrombin and thrombin bound to fibrin degradation products equally well.

CONCLUSIONS

Thrombin bound to soluble fibrin degradation products is primarily responsible for the increase in FPA levels that occurs when a clot undergoes t-PA-induced lysis. Like clot-bound thrombin, thrombin bound to fibrin derivatives is protected from inhibition by heparin but susceptible to inactivation by direct thrombin inhibitors. These findings help to explain the superiority of direct thrombin inhibitors over heparin as adjuncts to thrombolytic therapy.

Indium-111 DTPA-heparin: radiolabeling, pharmacokinetics, and biodistribution following intravenous administration in rat and rabbit

Heparin was coupled to DTPA using the bicyclic anhydride and labeled with Indium-111. This resulted in a radiochemically pure preparation (greater than 95% activity in one peak) as determined by high pressure liquid radiochromatography and did not affect the anticoagulant properties of heparin. Biodistribution in the rat at 1, 20, and 60 minutes after intravenous injection showed rapid blood clearance with uptake in the liver followed by bone and kidney when expressed as percent injected total dose per organ and liver followed by kidney and spleen when expressed as percent injected total dose per gram. Blood elimination in the rabbit was 18.5 minutes which decreased to 7.5 minutes when followed by the injection of protamine. Radioactivity cleared from the liver and lungs as a single exponential with a half-time of 30 minutes, but there was very rapid increase of radioactivity in the lungs, peaking at 1-2 minutes, following the injection of protamine. Indium-111 DTPA-heparin may be used to study in vivo pharmacokinetics and biodistribution of heparin.

The variable anticoagulant response to unfractionated heparin in vivo reflects binding to plasma proteins rather than clearance

The anticoagulant response to fixed doses of unfractionated heparin is variable in patients with acute illness, and some patients with venous thromboembolism require high doses of heparin to achieve a therapeutic anticoagulant response. To investigate the mechanism responsible for the variable anticoagulant response to heparin in acute illness, heparin clearance and nonspecific protein binding were compared in control and endotoxin-treated rabbits. The plasma half-life (t 1/2) of radiolabeled heparin increased in a dose-dependent fashion. At all doses of heparin studied, the t 1/2 of radiolabeled heparin was unaffected by experimental endotoxemia when compared with control animals. In contrast, the amount of heparin recovered was lower in the plasma of endotoxemic animals because of increased binding to plasma proteins. A chemically modified heparin with low affinity for antithrombin III was added ex vivo or in vivo to displace heparin bound nonspecifically to plasma proteins. The proportion of heparin bound to plasma proteins was significantly greater in the plasma of endotoxemic animals than in controls. These findings indicate that acute inflammation alters heparin recovery but does not affect heparin clearance. The variability of the anticoagulant response to heparin seen in patients with thromboembolism may, in part, be due to this effect of the underlying disease process.

A novel protamine variant reversal of heparin anticoagulation in human blood in vitro

PURPOSE

Protamine reversal of heparin anticoagulation during cardiovascular surgery may cause severe hypotension and pulmonary hypertension. A novel protamine variant, [+18RGD], has been developed that effectively reverses heparin anticoagulation without toxicity in canine experiments. Heretofore, human studies have not been undertaken. This investigation hypothesized that [+18RGD] would effectively reverse heparin anticoagulation of human blood in vitro.

METHODS

Fifty patients who underwent anticoagulation therapy during vascular surgery had blood sampled at baseline and 30 minutes after receiving heparin (150 IU/kg). Activated clotting times were used to define specific quantities of [+18RGD] or protamine necessary to completely reverse heparin anticoagulation in the blood sample of each patient. These defined amounts of [+18RGD] or protamine were then administered to the heparinized blood samples, and percent reversals of activated partial thromboplastin time, thrombin clotting time, and antifactor Xa/IIa levels were determined. In addition, platelet aggregation assays, as well as platelet and white blood cell counts were performed.

RESULTS

[+18RGD] and protamine were equivalent in reversing heparin as assessed by thrombin clotting time, antifactor Xa, antifactor IIa levels, and white blood cell changes. [+18RGD], when compared with protamine, was superior in this regard, as assessed by activated partial thromboplastin time (94.5 +/- 1.0 vs 86.5 +/- 1.3% delta, respectively; p < 0.001) and platelet declines (-3.9 +/- 2.9 vs -12.8 +/- 3.4 per mm3, respectively; p = 0.048). Platelet aggregation was also decreased for [+18RGD] compared with protamine (23.6 +/- 1.5 vs 28.5 +/- 1.9%, respectively; p = 0.048).

CONCLUSIONS

[+18RGD] was as effective as protamine for in vitro reversal of heparin anticoagulation by most coagulation assays, was statistically more effective at reversal than protamine by aPTT assay, and was associated with lesser platelet reductions than protamine. [+18RGD], if less toxic than protamine in human beings, would allow for effective clinical reversal of heparin anticoagulation.

Guidelines for management of left-sided prosthetic valve thrombosis: a role for thrombolytic therapy. Consensus Conference on Prosthetic Valve Thrombosis

OBJECTIVES

We sought to form a consensus recommendation for management of prosthetic valve thrombosis (PVT) from previous case and uncontrolled reports from a consensus of international specialists.

BACKGROUND

PVT and thromboembolism relate to inadequate anticoagulation and valve type and location. PVT is suspected by history (dyspnea) and auscultation (muffled valve sounds or new murmurs) and confirmed by Doppler echocardiography showing a marked valve gradient.

METHODS

A consensus conference was held to recommend management of left-sided PVT.

RESULTS

Transesophageal Doppler echocardiography is used to visualize abnormal leaflet motion and the size, location and mobility of thrombus. Thrombolysis is used for high risk surgical candidates with left-sided PVT (New York Heart Association functional class III or IV) because cerebral thromboembolism may occur in 12% of patients. Duration of thrombolysis depends on resolution of pressure gradients and valve areas to near normal by Doppler echocardiography performed every few hours. Lysis is stopped after 72 or 24 h if there is no hemodynamic improvement (operation indicated). Heparin infusion with frequent measurement of activated partial thromboplastin time (aPTT) begins when aPTT is more than twice control levels and can be converted to warfarin (international normalized ratio [INR] 2.5 to 3.5) plus aspirin (81 to 100 mg/day). Patients in functional class I or II have lower surgical mortality, and those with large immobile thrombi on the prosthetic valve or left atrium have responded to endogenous lysis with combined subcutaneous heparin every 12 h (aPTT 55 to 80 s) plus warfarin (INR 2.5 to 3.5) for 1 to 6 months. Operation is advised for nonresponders or patients with mobile thrombi.

CONCLUSIONS

Thrombolysis, followed by heparin, warfarin and aspirin, is advised for high risk surgical candidates with left-sided PVT.

Cytokeratin 18 is expressed on the hepatocyte plasma membrane surface and interacts with thrombin-antithrombin complexes

During experiments to identify putative hepatic receptors for thrombin-antithrombin (TAT) complexes, a 45-kDa protein was identified by ligand blotting. Following gel purification, amino acid sequencing revealed the 45-kDa TAT-binding polypeptide to be cytokeratin 18 (CK18). The presence of CK18 on the surface of intact rat hepatoma cells was demonstrated by binding of 125I-anti-CK18 antibodies. Anti-CK18 antibodies reduced the binding and internalization of 125I-TAT by rat hepatoma cells. Immunocytochemical analysis, to determine the location of CK18 in vivo, revealed a periportal gradient of CK18 staining; with hepatocytes around the portal triads demonstrating striking pericellular staining. In addition, anti-CK18 IgG associated with perfused livers to a significantly greater extent than preimmune IgG. Taken together, these data provide evidence that CK18 is found on the extracellular surface of hepatocytes and could play a role in TAT removal. Finally, these data, in conjunction with recent reports of CK8 (Hembrough, T. A., Li, L., and Gonias, S. L. (1996) J. Biol. Chem. 271, 25684-25691) and CK1 cell membrane surface expression (Schmaier, A. H. (1997) Thromb. Hemostasis 78, 101-107), indicate a novel role for these proteins as putative cellular receptors or cofactors to cellular receptors.

Preoperative assessment of the elderly patient

Preoperative assessment of the elderly patient for surgery is vital to the success of the surgical procedure. A thorough evaluation must first begin with an understanding of the physiologic and pathophysiologic changes unique to the elderly patient and the aging skin. A complete preoperative assessment entails assessing the patient and dermatologic problem, preparing the patient and caregivers for the surgery and its expected outcomes, and highlighting issues and problems that need to be managed prior to the procedure. With the continued growth of the geriatric population, all dermatologic surgeons should be aware of the special issues related to their geriatric patients. With heightened awareness of and screening for potential pitfalls in the elderly surgical patient, adverse outcomes can be avoided.

The continuing debate: conservative or interventional therapy for unstable coronary artery disease

A "look-and-act" approach, using a strategy of early intervention, and a "watchful waiting" approach, using medical therapy, are compared for the management of patients with unstable coronary artery disease (UCAD). Results from clinical trials are used to illustrate the advantages of antiplatelet and antithrombotic therapy and the problems associated with early intervention. Finally, a clinical management strategy for patients with UCAD is outlined. It is concluded that watchful waiting appears to be the most effective way of managing both patients and resources.

Low-molecular-weight heparin in the treatment of patients with venous thromboembolism

BACKGROUND

Low-molecular-weight heparin is known to be safe and effective for the initial treatment of patients with proximal deep-vein thrombosis. However, its application to pulmonary embolism or previous episodes of thromboembolism has not been studied.

METHODS

We randomly assigned 1021 patients with symptomatic venous thromboembolism to fixed-dose, subcutaneous low-molecular-weight heparin (reviparin sodium) or adjusted-dose, intravenous unfractionated heparin. Oral anticoagulant therapy with a coumarin derivative was started concomitantly and continued for 12 weeks. Approximately one third of the patients had associated pulmonary embolism. The outcome events studied over the 12 weeks were symptomatic recurrent venous thromboembolism, major bleeding, and death. We sought to determine whether low-molecular-weight heparin is at least equivalent to unfractionated heparin in patients with venous thromboembolism.

RESULTS

Twenty-seven of the 510 patients assigned to low-molecular-weight heparin (5.3 percent) had recurrent thromboembolic events, as compared with 25 of the 511 patients assigned to unfractionated heparin (4.9 percent). The difference of 0.4 percentage point indicates that the two therapies have equivalent value according to our predetermined definition of equivalence. Sixteen patients assigned to low-molecular-weight heparin (3.1 percent) and 12 patients assigned to unfractionated heparin (2.3 percent) had episodes of major bleeding (P= 0.63), and the mortality rates in the two groups were 7.1 percent and 7.6 percent, respectively (P=0.89).

CONCLUSIONS

Fixed-dose, subcutaneous low-molecular-weight heparin is as effective and safe as adjusted-dose, intravenous unfractionated heparin for the initial management of venous thromboembolism, regardless of whether the patient has pulmonary embolism or a history of venous thromboembolism.

New frontiers in the management of unstable coronary artery disease

Thrombotic occlusion is responsible for most acute manifestations of coronary artery disease, including unstable angina and non-Q-wave myocardial infarction. Antiplatelet therapy plays a major role in reducing the risk of ischemic events in such patients. Since thrombin generation is vital to the pathogenesis of thrombosis, recent studies have focused on thrombin inhibition in the management of acute ischemia. Heparin is the most widely used anticoagulant for acute management of thrombosis and is the treatment of choice in preventing and treating venous thromboembolism. Given in therapeutic doses intravenously, it is more effective than aspirin in reducing the risk of death or myocardial infarction in patients with unstable angina. Low-molecular-weight (LMW) heparins have improved pharmacologic and pharmacokinetic properties over standard heparin that may result in greater efficacy and safety. LMW heparins may be given in a fixed dose subcutaneously without monitoring, resulting in greater clinical utility and cost-effectiveness compared with standard heparin. Given subcutaneously in fixed, weight-adjusted doses they are more effective and safer than intravenous heparin in treating deep-vein thrombosis. Several studies have evaluated LMW heparins in unstable angina. In a small open trial, LMW heparin (nadroparin) reduced the risk of acute myocardial infarction compared with aspirin alone or a combination of aspirin and standard heparin. In 2 large clinical trials, LMW heparin (dalteparin) has been shown to be effective in the management of unstable angina with a 63% reduction in risk of death or acute myocardial infarction over patients treated with aspirin alone (Fragmin during Instability in Coronary Artery Disease; FRISC) and to be as effective as intravenous heparin (Fragmin in Unstable Coronary Artery Disease; FRIC).

Therapeutic options for severe pulmonary hypertension

Pulmonary hypertension occurs as a consequence of numerous and varied conditions, all of which result in an elevation of pulmonary vascular resistance. Over the past decade, significant progress has been made in understanding the factors which contribute to the progressive nature of pulmonary vascular disease, and in identifying new treatments for pulmonary hypertension. The majority of these therapeutic options are pharmacologic, but for specific circumstances, surgical therapy may be a consideration. This article discusses nonspecific therapies for all patients with pulmonary hypertension, vasodilator therapy (including screening for vasodilator responsiveness, standard oral agents, and newer intravenous or inhalational therapies) and surgical options applicable to specific situations.

An evaluation of empiric vs. nomogram-based dosing of heparin in an intensive care unit

OBJECTIVE

To compare the time to achieve therapeutic anticoagulation with heparin using two dosing methods.

DESIGN

A retrospective before and after time series.

SETTING

An 11-bed medical-surgical intensive care unit of a tertiary, teaching hospital.

PATIENTS

Critically ill patients who required full dose anticoagulation as part of treatment of hemodynamic or respiratory failure.

INTERVENTION

The use of a weight-based dosing nomogram with independent adjustment of infusion rates by nursing staff utilizing the nomogram dosing directions. This nomogram was compared with prior empiric heparin dosing by physicians.

MEASUREMENTS AND MAIN RESULTS

The time to achieve a therapeutic activated partial thromboplastin time (PTT) (> 60 secs) was significantly less during use of the nomogram than during use of the empiric dosing method. The number of adjustments required to achieve the therapeutic PTT was less during use of the nomogram. Heparin dosing using the nomogram resulted in larger initial heparin infusion rates (unit/kg) and significantly higher initial PTT results. There was no difference in the initial bolus dose, time to first measured PTT, number of PTT measurements outside the therapeutic range, or signs of toxicity from subtherapeutic or supratherapeutic anticoagulation.

CONCLUSIONS

Use of a weight-based heparin dosing nomogram by intensive care unit nursing staff can shorten the time to achieve therapeutic anticoagulation compared with empiric dosing by physicians.

Monitoring of hemostasis in cardiac surgical patients: impact of point-of-care testing on blood loss and transfusion outcomes

Patients undergoing cardiac surgery with cardiopulmonary bypass (CPB) are at increased risk for excessive perioperative blood loss requiring transfusion of blood products. Strategies to optimize administration of heparin and protamine and the assessment of their effects on coagulation are evolving in cardiac surgical patients. Two recent evaluations have focused on the use of multiple point-of-care (POC) coagulation assays for patient-specific adjustment of heparin and protamine dosage. These studies indicate that blood loss and transfusion requirements in cardiac surgical patients may be reduced with more accurate control of heparin anticoagulation and its reversal. Blood component administration in patients with excessive post-CPB bleeding is generally empiric in part, related to turnaround times of laboratory-based tests. Methods are now available for rapid, POC assessment of coagulation to allow appropriate, targeted therapy for acquired hemostatic abnormalities. Recent studies indicate that a rapid evaluation of thrombocytopenia and coagulation factor deficiencies with POC tests can facilitate the optimal administration of pharmacologic and transfusion-based therapy in patients who exhibit excessive bleeding after CPB. POC tests that assess platelet function have been developed, and their use may facilitate identification of which patients at risk for excessive blood loss may respond to pharmacologic interventions such as desmopressin acetate or antifibrinolytic agents.

Antithrombin III during cardiac surgery: effect on response of activated clotting time to heparin and relationship to markers of hemostatic activation

This study was designed to determine if, and to what extent, antithrombin III (AT) levels affect the response of the activated clotting time (ACT) to heparin in concentrations used during cardiac surgery, and to characterize the relationship between AT levels and markers of activation of coagulation during cardiopulmonary bypass (CPB). After informed consent, blood specimens obtained from eight normal volunteers (Phase I) were used to measure the response of the kaolin and celite ACT to heparin after in vitro addition of AT (200 U/dL) and after dilution with AT-deficient plasma to yield AT concentrations of 20, 40, 60, 80, and 100 U/dL. In Phase II, blood specimens collected before the administration of heparin and prior to discontinuation of CPB, were used to measure the response of the kaolin ACT to heparin (preheparin only), AT concentration, and a battery of coagulation assays in 31 patients undergoing repeat or combined cardiac surgical procedures. In Phase I, strong linear relationships were observed between kaolin (slope = 1.04 AT - 2, r2 = 0.78) and celite (slope = 1.36 AT + 6, r2 = 0.77) ACT slopes and AT concentrations below 100 U/dL. In the pre-CPB period of Phase II, only factors V (partial r = -0.49) and VIII (partial r = -0.63) were independently associated with heparin-derived slope using multivariate analysis; an inverse relationship was observed between AT and fibrinopeptide A levels (r = -0.41) at the end of CPB. Our findings indicate that the responsiveness of whole blood (ACT) to heparin at the high concentrations used with CPB is progressively reduced when the AT concentration decreases below 80 U/dL. Because AT is variably, and sometimes extensively, reduced in many patients before and during CPB, AT supplementation in these patients might be useful in reducing excessive thrombin-mediated consumption of labile hemostatic blood components, excessive microvascular bleeding, and transfusion of blood products.

IMPLICATIONS

Heparin, a drug with anticoagulant properties, is routinely given to patients undergoing cardiac surgery to prevent clot formation within the cardiopulmonary bypass circuit. However, when levels are reduced, heparin is not as effective. Findings within this study indicate that administration of antithrombin III may help to preserve the hemostatic system during cardiopulmonary bypass.

Treatment of venous thrombo-embolism

Clinically suspected deep vein thrombosis (DVT) or pulmonary thromboembolism (PE) should be initially treated with heparin, and an objective diagnosis obtained. In pregnancy, heparin is usually continued until delivery, following which warfarin is substituted. In the absence of pregnancy, warfarin is substituted and usually continued for 3 months after a first thrombo-embolic event. Low molecular weight heparins are increasingly preferred to unfractionated heparin in non-pregnant patients with acute DVT, because of efficacy when given by daily subcutaneous injection without routine monitoring of coagulation assays, greater efficacy, and lower risks of major bleeding and of mortality. Unfractionated heparin requires monitoring by the APTT (target ratio 1.5-2.5), and warfarin requires monitoring by the International Normalized Ratio (INR) of the prothrombin time (target ratio 2.0-3.0). Graduated elastic compression stockings reduce post-thrombotic leg symptoms after DVT. Secondary prevention is important in future high risk situations.

Sulodexide versus calcium heparin in the medium-term treatment of deep vein thrombosis of the lower limbs

Thirty adult patients with distal, monolateral deep vein thrombosis of the lower limbs were randomly treated for sixty days either with subcutaneous Ca-Heparin or with Sulodexide, administered IM for ten days and orally for fifty days. The thrombus accretion above the knee, the venous pressures of the affected leg, the clinical symptomatology, and some laboratory coagulative tests were monitored throughout the administration period. Local tolerability of the two treatments was also evaluated. The two applied treatments evidenced a net antithrombotic activity, preventing thrombus accretion above the knee, improving with the same efficacy the venous pressures in the affected legs, and similarly reducing clinical symptoms, with a quick and statistically significant trend toward normalization. Blood fibrinogen was significantly lowered by both drugs, while only Ca-heparin yielded a prolongation of activated partial thromboplastin time. Local tolerability of treatments was better for the mainly oral Sulodexide administrations, while subcutaneous Ca-heparin often induced small, though transient, hematomas.

Induction of the acute-phase reaction increases heparin-binding proteins in plasma

We have previously demonstrated that the nonspecific binding of unfractionated heparin (UFH) to plasma proteins has a marked modulating effect on its anticoagulant activity. Since some heparin-binding proteins are also acute-phase-reactant proteins, we explored the possibility that the induction of the acute-phase response can increase the plasma concentrations of heparin-binding proteins. The recovery of a fixed amount of UFH or low-molecular-weight heparin (LMWH) added in vitro to rat plasma samples obtained at various time intervals after the administration of intravenous endotoxin or subcutaneous turpentine was compared with that of saline-treated control animals. The anti-factor Xa activity was measured in the plasma samples before and after the addition of a chemically modified low-affinity heparin (LAH) to displace the proportion of the added heparin that is reversibly bound to plasma proteins. Our results show that at 6 hours post-endotoxin and at 24 hours post-turpentine treatment, virtually no anti-factor Xa activity could be measured in the plasma samples, while the expected levels were obtained for control plasma. After the addition of LAH to displace protein-bound UFH, essentially the same anti-factor Xa levels were measured in the plasma from all three treatment groups. These results indicate that induction of the acute-phase reaction can dramatically increase the levels of heparin-binding proteins in rat plasma. In addition, we compared the anti-factor Xa recovery of UFH with that of an LMWH from the plasma of endotoxin- and saline-treated rats and demonstrated that LMWH binds less to plasma proteins than UFH, even in plasma in which the levels of heparin-binding proteins are markedly elevated. The recovery of a fixed amount of UFH added in vitro to human plasma from septic patients was also reduced, but not to the same extent as seen in rat plasma. Removal of candidate heparin-binding and acute-phase proteins by immunodepletion indicated that vitronectin plays an important role in the nonspecific binding of UFH in patient plasma.

Danaparoid in the prevention of thromboembolic complications

OBJECTIVE

To review the therapies used to prevent postoperative thromboembolic complications with a focus on the role of danaparoid, a new low-molecular-weight glycosaminoglycan.

DATA SOURCES

A MEDLINE search was performed to identify pertinent English-language literature including studies, abstracts, and review articles. Key search terms included danaparoid, heparinoid, lomoparin, heparin, prophylaxis, thrombosis, embolism, thromboembolism, and thromboembolic and postoperative complications. The manufacturer of danaparoid was contracted for additional information related to this compound.

STUDY SELECTION AND DATA EXTRACTION

All identified articles were reviewed for possible inclusion in this review. Comparisons primarily focused on data obtained from prospective, randomized, controlled, blind clinical trials. Another important consideration was the use of venography to determine the presence of deep venous thrombosis.

DATA SYNTHESIS

Various therapies are available for the prevention of postoperative thromboembolic complications. Effective pharmacologic treatments currently available include adjusted-dose heparin, warfarin, aspirin, dextran, and low-molecular-weight heparins (LMWHs). Until recently, warfarin was considered the drug of choice for thromboprophylaxis in high-risk patients, including patients undergoing orthopedic surgical procedures. Because of their comparable efficacy and greater ease of use, LMWHs are gaining favor over warfarin in this patient population. In well-designed clinical trials involving patients undergoing elective total hip replacement or fractured hip surgery, danaparoid has demonstrated greater efficacy than other active treatments, including warfarin, dextran, aspirin, and heparin plus dihydroergotamine. While studies comparing danaparoid with LMWHs have not yet been published, danaparoid may be more useful in patients with heparin-associated thrombocytopenia.

CONCLUSIONS

Danaparoid is an antithrombotic agent with characteristics that distinguish it from heparin and LMWHs. Based on the efficacy and safety data reviewed, danaparoid should be considered one of the drugs of choice for the prevention of thromboembolic complications in patients undergoing orthopedic hip procedures and the drug of choice for the management of any patient with heparin-induced thrombocytopenia who requires anticoagulant therapy.

Enoxaparin, a low-molecular-weight heparin suppresses prothrombin activation more effectively than unfractionated heparin in patients treated for venous thromboembolism

Although unfractionated heparin (UFH) is standard therapy for venous thromboembolism, there is a clinical failure rate of up to 10%. Newer treatment strategies include low-molecular-weight heparins (LMWH). As part of an international study comparing the efficacy and safety of enoxaparin, a LMWH versus UFH in patients with venous thromboembolism, we studied the effects of enoxaparin and UFH on the plasma concentrations of two activation peptides, fragment 1 + 2 (F1 + 2), and thrombin-antithrombin III (TAT) complexes. We hypothesized that enoxaparin would be more effective in suppressing activation of coagulation. Intravenous heparin was given by bolus injection followed by infusion. There were 2 enoxaparin treatment groups (1 mg/kg s.c., bid and 1.5 mg/kg s.c. daily). Plasma samples were obtained from 11 patients in the enoxaparin group and 6 patients in the heparin group prior to and at 6 hour intervals after initiating therapy. Clinical characteristics of the enoxaparin and UFH patient groups were similar. TAT concentrations were not statistically different between groups at any treatment interval. However, plasma F1 + 2 concentrations differed significantly (p < 0.05); concentrations in the enoxaparin group were consistently lower over time than in the UFH group. These results suggest that this LMWH is more effective in suppressing ongoing thrombosis in vivo than UFH in patients with venous thrombosis.

A Histomorphometric Comparison of the Effects of Heparin and Low-Molecular-Weight Heparin on Cancellous Bone in Rats

Long-term heparin treatment causes osteoporosis through, an as yet, undefined mechanism. To investigate this phenomenon and to determine the relative benefits of low-molecular-weight heparin (LMWH) use, we treated rats with once daily subcutaneous injections of either unfractionated heparin (1.0 U/g or 0.5 U/g), the LMWH, Tinzaparin (1.0 U/g or 0.5 U/g), or placebo (saline) for a period of 32 days. The effects on bone were then compared both histomorphometrically and biochemically by measuring urinary type I collagen cross-linked pyridinoline (PYD) and serum alkaline phosphatase, markers of bone resorption and formation, respectively. Histomorphometric analysis of the distal third of the right femur, in the region proximal to the epiphyseal growth plate, demonstrated that both heparin and LMWH decrease cancellous bone volume in a dose-dependent fashion, but that heparin causes significantly more cancellous bone loss than does LMWH. Although both heparin and LMWH decrease osteoblast and osteoid surface to a similar extent, only heparin increases osteoclast surface. In support of these histomorphometric findings, biochemical markers of bone turnover demonstrated that both heparin and LMWH treatment produce a dose-dependent decrease in serum alkaline phosphatase, consistent with reduced bone formation, whereas only heparin causes a transient increase in urinary PYD, consistent with an increase in bone resorption. Based on these observations, we conclude that heparin decreases cancellous bone volume both by decreasing the rate of bone formation and increasing the rate of bone resorption. In contrast, LMWH, causes less osteopenia than heparin because it only decreases the rate of bone formation.

A heparin-binding synthetic peptide of heparin/heparan sulfate-interacting protein modulates blood coagulation activities

We have previously identified and characterized a heparin-binding cell surface protein (heparin/heparan sulfate-interacting protein, or HIP) present on epithelial and endothelial cells. A synthetic peptide mimicking a heparin-binding domain of HIP is now shown to bind a small subset of heparin molecules with high affinity and, therefore, presumably recognizes a specific structural motif in the heparin molecule. Further analyses revealed that the heparin molecules exhibiting a high affinity for the HIP peptide also show an extremely high affinity for antithrombin III (AT-III), a cofactor required for heparin's anticoagulant activity. The HIP peptide was shown to compete with AT-III for binding to heparin and to neutralize the anticoagulant activity of heparin in blood plasma assays. Furthermore, the heparin subfraction that binds to the HIP peptide with high affinity exhibits an extremely high anticoagulant activity. We conclude that although the HIP peptide shows no sequence similarity with AT-III, the two proteins recognize the same or similar structural motifs in heparin.

Hemofiltration during cardiopulmonary bypass: the effect on anti-Xa and anti-IIa heparin activity

Previous studies have demonstrated that heparin concentrations during cardiopulmonary bypass (CPB) may vary considerably, which may be related to variability in redistribution, cellular and plasma protein binding, and clearance of heparin. The purpose of this study was to determine whether hemofiltration removes lower molecular weight fractions of heparin from plasma and thus contributes to variability of blood levels of heparin. Twenty patients undergoing cardiac surgery with CPB were enrolled in this study after informed consent was obtained. The study was subdivided into two phases. The first 10 patients were enrolled in Phase I, which was designed to determine whether hemofiltration removes lower molecular weight fractions of heparin from blood. Blood specimens obtained from the inflow line and outflow lines of the hemofiltration unit were used to measure complete blood counts (CBC) and plasma heparin activity by anti-Xa and anti-IIa assays. Phase II was designed to evaluate the effect of hemofiltration on circulating plasma heparin activity. In Phase II, blood specimens were obtained from 10 patients via the arterial cannula of the extracorporeal circuit prior to and after hemofiltration for measurements of CBCs, anti-Xa plasma heparin, as well as whole blood heparin concentration using an automated protamine titration assay (Hepcon instrument, Medtronic Inc., Parker, CO). Ultrafiltrate and reservoir volumes were measured in both phases of the study. Hemofiltration did not remove lower (anti-Xa measurable) molecular weight heparin, but it resulted in a considerable increase in heparin activity in the outflow line, as measured by both anti-Xa and anti-IIa assays. The plasma anti-Xa heparin activity obtained after hemofiltration (5 +/- 1.8 U/mL) was substantially (P = 0.003) greater than heparin activity obtained prior to hemofiltration (3.9 +/- 1.7 U/mL). The increase in heparin activity with hemofiltration was directly related to ultrafiltrate volume (r = 0.63, P < 0.0001) and hematocrit (r = 0.73, P < 0.0001). Hemofiltration increases heparin concentration and may contribute to variability in heparin activity during CPB. Point-of-care heparin concentration methods would allow identification of the anticipated rise in heparin concentration, with the apparent clinical implication of a reduced need for supplemental heparin to maintain a target whole blood heparin concentration.

Low-molecular-weight heparin: from the bench to the orthopedic patient

ABSTRACTStandard heparin is widely used for the prevention and treatment of venous thromboembolism; however, it has several limitations including variable dose response, dose-dependent clearance and inhibition of platelet function. To overcome these disadvantages, standard heparin, which is composed of glycosaminoglycans of various molecular weights, has been fractionated into its low-molecular-weight component. Low-molecularweight heparin (LMWH) exhibits less binding to plasma proteins and endothelial cells than standard heparin resulting in a more predictable dose response profile, a dose-independent mechanism of clearance and a longer plasma half-life. LMWH also has a lower binding affinity for platelets and produces less microvascular bleeding. Evidence from randomized clinical trials demonstrates that LMWH is effective in the prevention of deep vein thrombosis (DVT) in high-risk orthopedic patients. There is also considerable evidence of its efficacy and safety in the initial treatment of proximal DVT. Recent studies have demonstrated the feasibility of home treatment with LMWH, which offers the advantage of greater clinical utility compared with current antithrombotic regimens and hence the possibility of cost savings.

Effect of nonspecific binding to plasma proteins on the antithrombin activities of unfractionated heparin, low-molecular-weight heparin, and dermatan sulfate

BACKGROUND

Nonspecific binding to plasma proteins decreases the anti-factor Xa (anti-Xa) activity of unfractionated heparin (UFH) but not that of low-molecular-weight heparin (LMWH). However, plasma proteins could influence the anti-thrombin (anti-IIa) activity of LMWH. To explore this possibility, we compared the effects of plasma proteins on the anti-IIa activities of UFH and LMWH. We also examined their effects on the anti-IIa activity of dermatan sulfate (DS) because, like UFH, DS binds to plasma proteins.

METHODS AND RESULTS

There was almost complete recovery of anti-IIa activity when UFH, LMWH, or DS was added to plasma from each of 20 healthy volunteers. The addition of a chemically modified heparin with low affinity for antithrombin III to plasma containing UFH increased the anti-IIa activity in a concentration-dependent fashion by displacing UFH from plasma proteins. In contrast, addition of low-affinity heparin had no effect on the anti-IIa activity of LMWH. LMWH does not bind to plasma proteins because the bulk of the LMWH chains are < 6000 D, and only heparin fractions > 6000 D bind nonspecifically to plasma proteins. As further evidence that plasma proteins do not influence the anti-IIa activity of LMWH, the rate of thrombin inhibition in plasma in the presence of LMWH is virtually identical to that in buffer containing physiological amounts of the major antithrombins. In contrast, with UFH or DS, the rate of thrombin inhibition is twofold slower in plasma than in buffer.

CONCLUSIONS

Nonspecific binding of UFH to plasma proteins most likely contributes to the variable anti-IIa response to UFH in patients with thromboembolic disease. Although DS also binds to plasma proteins, the clinical significance of this finding is unclear. In contrast, because LMWH does not bind to plasma proteins, the anti-IIa activity of LMWH should be just as predictable as its anti-Xa activity.