Heparin

Coagulation and anticoagulation in extracorporeal membrane oxygenation

The hemostatic system poses a major problem in extracorporeal membrane oxygenation (ECMO). The foreign surface in the extracorporeal circuit activates platelets and the clotting system. To avoid loss of platelets and activation of the clotting system, anticoagulation is necessary. In addition, in many patients on ECMO, preexisting clotting disorders are present. Therefore, bleeding and/or thrombosis are frequent complications in ECMO patients that require specific treatment and may even necessitate termination of ECMO. Most ECMO centers use heparin for anticoagulation and the activated clotting time (ACT) for monitoring. Reduction of problems with hemostasis may be obtained with less thrombogenic surfaces, new anticoagulants with a short half-life, platelet inhibitors, protease inhibitors, or selective anticoagulation in the extracorporeal circuit. While there will probably never be a complete nonthrombogenic surface available and all anticoagulants will have some risk of bleeding, improvement can be obtained by a combination of measures including better surfaces, more sophisticated anticoagulation regimens, and close laboratory monitoring.

Heparin inhibits ligand binding to the leukocyte integrin Mac-1 (CD11b/CD18)

BACKGROUND

The clinical benefits of heparin reach beyond its anticoagulative properties. Recently, it has been described that leukocytes adhere on immobilized heparin mediated by the integrin Mac-1 (CD11b/CD18, alphaMbeta2, or CR3). Because inhibition of this versatile adhesion molecule could explain various aspects of the beneficial clinical effects of heparin, we evaluated whether soluble heparin modulates Mac-1 function in vitro and in vivo.

METHODS AND RESULTS

Binding of unfractionated heparin to Mac-1 on PMA-stimulated monocytes and granulocytes was directly demonstrated in flow cytometry, whereas no binding of heparin was detected on unstimulated leukocytes. Unfractionated heparin inhibited binding of the soluble ligands fibrinogen, factor X, and iC3b to Mac-1. Adhesion of the monocytic cell line THP-1 and of peripheral monocytes and granulocytes to immobilized ICAM-1 was impaired by unfractionated heparin, to the same extent as with inhibition of Mac-1 by monoclonal antibodies such as c7E3. Low-molecular-weight heparin also inhibits binding of fibrinogen to Mac-1. Additionally, flow cytometry of whole blood preparations of patients treated with unfractionated heparin revealed an inhibitory effect of heparin on the binding of fibrinogen to Mac-1 that correlates (n= 48, r=0.63, P<0.001) to the extent of prolongation of the activated partial thromboplastin time.

CONCLUSIONS

We describe a pharmacologically relevant property of heparin that may contribute to its benefits in clinical use. The binding of heparin to Mac-1 and the resulting inhibition in binding of Mac-1 ligands may directly modulate coagulation, inflammation, and cell proliferation.

Thrombin generation after the abrupt cessation of intravenous unfractionated heparin among patients with acute coronary syndromes: potential mechanisms for heightened prothrombotic potential

OBJECTIVES

The purpose of this study was to determine the mechanistic basis for thrombin generation and increased prothrombotic potential after the abrupt cessation of intravenous (i.v.) unfractionated heparin among patients with acute coronary syndromes.

BACKGROUND

A "rebound" increase in prothrombotic potential has been observed biochemically and clinically after the abrupt cessation of unfractionated heparin (UFH) among patients with acute coronary syndromes. Although the mechanism is unknown, tissue factor and the extrinsic coagulation cascade, both operative in atherosclerotic vascular disease and arterial thrombosis, are thought to be centrally involved.

METHODS

In a single-center, pilot study, 30 patients with either unstable angina or non-ST segment elevation myocardial infarction who had received a continuous i.v. infusion of UFH for 48 h were randomly assigned to: 1) abrupt cessation, 2) i.v. weaning over 12 h or 3) subcutaneous weaning over 12 h.

RESULTS

Thrombin generation (prothrombin fragment 1.2) was evident within 1 h of UFH cessation, increased progressively (by nearly two-fold) at 24 h (p = 0.002) and correlated inversely with tissue factor pathway inhibitor concentration (r = -0.61). Thrombin generation was greatest among patients randomized to abrupt cessation (1.6-fold increase at 24 h) and least in those with i.v. weaning.

CONCLUSIONS

Thrombin generation after the abrupt cessation of UFH may represent a drug-induced impairment of physiologic vascular thromboresistance in response to locally generated tissue factor. A dosing strategy of abbreviated i.v. weaning attenuates but does not prevent heparin rebound among patients with acute coronary syndromes.

Direct measurement of unfractionated heparin using a biochemical assay

A number of investigations have noted that functional biological assays for heparin are not always reliable and may not reflect the actual biochemical level of heparin in patients receiving anticoagulant therapy. This creates the possibility that patients receiving anticoagulant treatment may have an excess or deficiency of circulating levels of heparin. To address this problem, we have developed a direct biochemical measurement of heparin. The heparin assay uses fluorophore-assisted carbohydrate electrophoresis (FACE) to directly measure the predominate disaccharide of unfractionated heparin. In this study, unfractionated heparin was measured in vitro throughout a wide range of heparin concentrations in plasma. Seven in vivo pharmacokinetic studies in five normal subjects given 3,000 USP units of unfractionated heparin intravenously showed a three-phase elimination process with higher peak plasma levels and shorter elimination times than predicted from previous studies. At these doses, heparin is largely eliminated intact through urinary excretion. Body weight has a significant effect on heparin kinetics. When we compared the direct biochemical assay with two biological clotting assays, we found the latter can overestimate biochemical heparin concentrations. The FACE assay, due to its sensitivity, is also able to measure circulating levels of endogenous heparin in plasma and urine. Direct heparin measurement using the FACE technique is practical and useful for studies of the correlation of biochemical and biological activities.

Low molecular weight heparin in acute coronary syndromes

Traditionally, unfractionated heparin has played an important role in the treatment of acute coronary syndromes. Low molecular weight heparin (LMWH), is a promising new type of heparin, which is fractionated to include only heparin molecules of lower molecular weight. LMWHs are administered subcutaneously and do not require monitoring of the activated partial thromboplastin time, making them much easier to use. LMWHs are combined inhibitors of both thrombin and Factor Xa inhibitors. Several recent large trials in unstable angina and non-Q wave myocardial infarction have shown that LMWH is effective, and one agent has been shown to be superior to unfractionated heparin in reducing death, myocardial infarction, or recurrent angina. They also are very low cost (approximately $50 per day) and appear to be very cost effective in the treatment of unstable angina. Thus, LMWHs appear to be the new anticoagulant agent in acute coronary syndromes.

Increased use of prophylactic vena cava filters in trauma patients failed to decrease overall incidence of pulmonary embolism

BACKGROUND

Recent studies have reported that placement of vena cava filters (VCFs) early after injury may decrease the incidence of pulmonary embolism (PE) in high-risk trauma patients.

STUDY DESIGN

This was a retrospective review of all trauma patients with placement of VCFs admitted to a single level-1 trauma center between 1989 and 1997. Two cohorts corresponding to years of high or low prophylactic VCF use (PVCF) were compared.

RESULTS

Records were reviewed for 299 trauma patients identified as having had placement of a VCE Two hundred forty-eight filters were placed before the diagnosis of PE. During years of low PVCF use, the overall PE incidence was 0.31%; during years of high PVCF use, the incidence of PE was higher at 0.48% (p = 0.045, chi-square).

CONCLUSIONS

Increased use of PVCFs failed to decrease the overall rate of PE in our trauma patient population.

Deep-vein thrombosis of the lower limbs: diagnosis and management

Acute deep venous thrombosis (DVT) of the lower extremities is a serious and potentially fatal disorder which often complicates the course of severely ill, hospitalized patients but may also affect ambulatory and otherwise healthy people. It is uncommon in young individuals and becomes more frequent with advancing age. The clinically important problems associated with DVT are death from pulmonary embolism (PE), morbidity resulting from the acute event, the post-thrombotic syndrome, and the inconvenience and side-effects of investigations and treatment. Furthermore, an often underemphasized problem is the anxiety that may occur in those patients who have suffered a thrombotic episode.

HEPTEST: a suitable method for monitoring heparin during pregnancy

Methods of monitoring heparin in pregnancy are problematic. The aim of this study was to assess the plasma HEPTEST as a rapid and reliable test for heparin monitoring in pregnancy. HEPTEST, activated partial thromboplastin time (APTT) and chromogenic anti-Xa assays were performed on individual heparin-spiked plasma samples from two groups: normal non-pregnant women (n = 6) and normal pregnant women during the third trimester (n = 6). Heparin activity curves were established in plasma from both groups for low (< 0.3 IU/ml), intermediate (0.3-0.7 IU/ml) and high (> 0.7 IU/ml) heparin concentrations and validated by comparison with the anti-Xa chromogenic assay. Both the APTT and HEPTEST demonstrated good correlation with anti-Xa levels across all heparin concentrations in both plasma groups (r range = 0.879-0.945). In comparison with the APTT, the HEPTEST showed better correlation with anti-Xa levels at low concentrations of heparin (r values 0.933 vs. 0.772, respectively). For both the APTT and HEPTEST there were significant differences between the clotting times in pregnant and non-pregnant plasma at a number of heparin concentrations. This data supports the plasma HEPTEST as an acceptable alternative to the chromogenic anti-Xa assay for monitoring heparin thromboprophylaxis in pregnancy.

Antithrombins and the importance of good control

The generation of thrombin and the formation of platelet rich intra-coronary thrombus in response to atherosclerotic plaque rupture is pathognomonic of acute coronary syndromes. An understanding of the process of thrombin generation and the unique relationship between the structure and function of thrombin is essential to developing more effective anti-thrombotic strategies than the use of standard unfractionated heparin in the acute coronary syndromes. The mechanisms of action of heparin, low molecular weight heparins (LMWHs) and the newer direct anti-thrombins, recombinant hirudin and Hirulog, are reviewed. Evidence from the currently available phase 2 and 3 trials of these drugs regarding their efficacy in the acute coronary syndromes is also reviewed.

Physicians' and Nurses' Evaluation of a Pharmacist-Managed Heparin Protocol Service

A total of 60 physicians and 90 nurses in a 300-bed, non-teaching, community-based hospital were surveyed to determine their attitudes and impressions of pharmacists' management of an inpatient, intravenous, unfractionated heparin dosing and monitoring service. Written questionnaires were used to gather their opinions of: (1) the value of having pharmacists provide the heparin protocol service, (2) the effectiveness of having pharmacists provide therapeutic management of thromboembolic disorders, and (3) the effectiveness of communications with pharmacists. Survey response rates were 82% and 53% for nurses and physicians, respectively. For all three service aspects evaluated, responses from nurses were significantly higher (p < 0.05) than physicians. Data from this study strongly support pharmacist-provided therapeutic management of thromboembolic disorders and show that nurses and physicians place a high value on pharmaceutical care services.

Point-of-care and standard laboratory coagulation testing during cardiovascular surgery: balancing reliability and timeliness

OBJECTIVE

The use of point-of-care technology has increased faster than efforts to validate its effectiveness compared to standard laboratory testing modalities. To address this issue with a current point-of-care coagulation system (HEMOCHRON Jr, International Technidyne Corporation (ITC), Edison, NJ), we designed a study to test the hypothesis that data obtained from point-of-care coagulation equipment correlates with data obtained from standard laboratory coagulation equipment. One of the potential advantages gained using point-of-care testing is the ability to obtain more rapid results. To address this issue, turnaround time, defined as the elapsed time (in minutes) from when the sample was acquired from the patient until the investigators knew the results, was also determined.

METHODS

Following Human Investigation Committee approval and informed consent, a prospective study was conducted to compare results obtained from point-of-care coagulation equipment with those results obtained from standard laboratory coagulation equipment. The study was performed in three groups of patients undergoing cardiovascular surgery, each requiring different levels of anticoagulation.

RESULTS

Of the 83 patients who met the inclusion criteria, the correlation (combining data from groups 1-3) between results obtained from point-of-care and standard laboratory prothrombin time was r = 0.867, p < 0.001. The correlation (group 3) between point-of-care and standard laboratory international normalized ratio was r = 0.943, p < 0.001. The correlation (combining data from groups 1 & 2) between point-of-care and standard laboratory activated partial thromboplastin time was r = 0.825, p < 0.001. Median turnaround time for the standard laboratory was 90 minutes, with a mean turnaround time of 74 to 78 minutes, depending upon the group. In contrast, the median turnaround time for point-of-care testing was two minutes and 14 seconds.

CONCLUSIONS

The results from this study population reveal that data obtained from point-of-care prothrombin time, international normalized ratio and activated partial thromboplastin time results correlate with results obtained from standard laboratory coagulation testing. The value of obtaining reliable results in a timely fashion offers a potential advantage for point-of-care testing in dinical situations, such as in the operating room, where saving time may translate into financial savings.

Deep venous thrombosis and pulmonary embolism in bilateral lower-extremity amputee patients

OBJECTIVE

To determine the incidence of deep venous thrombosis and pulmonary emboli and the value of an inferior vena cava filter in patients with bilateral lower-extremity amputations, and to determine the incidence of pulmonary emboli after filter placement.

DESIGN

Retrospective study with a follow-up of 3 to 64 months.

SETTING

Inner-city university hospital.

PATIENTS

Twenty-seven consecutive patients with bilateral lower-limb amputation.

RESULTS

Age, sex, and race were assessed, and had no impact on the incidence of pulmonary embolus in these patients with lower-extremity amputation.

CONCLUSION

No clinical objective evidence of pulmonary emboli occurred after placement of an inferior vena caval filter.

Anticoagulant therapy in unstable angina

The goal of anticoagulant therapy in unstable angina is to prevent progression of a subocclusive coronary thrombus to complete occlusion of the coronary artery, thereby preventing myocardial infarction and death. Although these have been many advances in therapy with anticoagulants, considerable morbidity and mortality remains. Also, although combination therapy with potent novel anticoagulants and antiplatelet agents may be an alternative strategy, this needs to be balanced against the risks of hemorrhagic complications. More precise and biologically relevant methods of monitoring anticoagulant effect, along with appropriately modified doses given in combination offers promise.

New antithrombotic agents

The development of new antithrombotic agents has been stimulated by clinical needs and by advances in biotechnology that have made it possible to produce drugs that target specific steps in thrombogenesis. Heparin has pharmacokinetic and biophysical limitations that are overcome by new anticoagulants. Of these, low-molecular-weight heparin and direct inhibitors of thrombin have been evaluated clinically. Coumarins require careful laboratory monitoring because of concerns about safety. Orally active direct inhibitors of thrombin and factor Xa may replace coumarins. Aspirin is of limited efficacy because it inhibits only one pathway of platelet activation. Inhibitors of adenosine diphosphate receptor and glycoprotein IIb/IIIa antagonists are more effective than aspirin and are used in the clinic.

Influence of stent length and heparin coating on platelet activation: a flow cytometric analysis in a pulsed floating model

Platelets are involved in acute and subacute thrombotic occlusions of coronary stents and also may play a role in the pathophysiology of in-stent restenosis. This study sought to investigate the expression of activation dependent glycoproteins on platelets by flow cytometry and time until stent thrombosis in an in vitro model of stent thrombosis. Coronary stents were placed in parallel silicon tubings with circulating citrated platelet rich plasma to measure 1) influence of stent length on platelet antigens; 2) influence of heparin coating on platelet antigens; and 3) time until stent thrombosis. After recalcification aliquots of platelet-rich plasma were taken over 10 minutes in 2-minute intervals and immediately fixed and stabilized. For flow cytometric analysis monoclonal antibodies to CD41a (glycoprotein IIb/ IIIa), CD42b (glycoprotein Ib-V-IX), CD62p (P-selectin), and CD63 (glycoprotein 53) were used. Within 2 minutes after start of circulation, the expression of CD62p and CD63 increased. Longer stents resulted in more platelet activation than shorter stents (25 mm vs. 15 mm; p<0.001. Time until stent thrombosis was reduced (25 mm vs. 15 mm; p<0.05). Heparin coating did not significantly influence flow cytometry detectable platelet activation but prolonged time until stent thrombosis (coated vs. uncoated; p<0.005). In control tubing systems without stents platelet activation was less pronounced (p<0.0001). Antibodies to CD41a and CD42b did not show significant changes. In this model platelet activation detected by flow cytometry and time until stent thrombosis were dependent on stent length and coating. In vitro testing could be useful to optimize stent design and material.

Optimal management of bleeding and transfusion in patients undergoing cardiac surgery

Patients undergoing cardiac surgery with cardiopulmonary bypass (CPB) are at increased risk for excessive perioperative blood loss requiring transfusion of blood products. Point-of-care evaluation of platelets, coagulation factors, and fibrinogen can enable physicians to rapidly assess bleeding abnormalities, facilitate the optimal administration of pharmacological and transfusion-based therapy, and also identify patients with surgical bleeding. The ability to reduce the unnecessary use of blood products in this setting has important implications for emerging issues in blood inventory and blood costs. The ability to decrease surgical time, along with exploration rates, has important consequences for health care costs in an increasingly managed health care environment.

Localization and characterization of antithrombin in human kidneys

Antithrombin is a serine protease inhibitor that is critical in maintaining a thromboresistant vasculature. The association between low serum antithrombin concentration and renal disease suggests that the kidney plays a role in the conservation of plasma antithrombin. We used immunohistochemical techniques to determine the spatial distribution, heparin binding characteristics, and intracellular and intercellular localization of antithrombin in biopsy specimens (n = 53) of human donor kidneys obtained at the time of transplantation. In the renal cortex, double antibody techniques demonstrated the presence of intracellular antithrombin in proximal tubule epithelial cells. The reactivity was granular and was co-localized with vesicle-like structures. Distal and collecting tubules did not demonstrate intraepithelial antithrombin reactivity. No tubule structures in the medullary region demonstrated intracellular antithrombin, but all these structures showed intense basement membrane antithrombin reactivity. Double antibody techniques also demonstrated that the heparin binding domain of intraepithelial antithrombin was occupied. Semiquantitative scores for intraepithelial antithrombin were significantly decreased in renal biopsy specimens obtained 30 min after anastomosis compared with biopsies from the same organ obtained before anastomosis. These findings suggest that antithrombin, probably in association with heparin or heparan sulfate, is internalized by renal proximal epithelial cells. Although the ultimate fate of intraepithelial antithrombin is not known, this may represent a mechanism by which the kidney helps to maintain plasma antithrombin concentrations.

Efficacy and tolerability of a very low molecular weight heparin compared with standard heparin in patients with unstable angina: a pilot study

BACKGROUND

Unstable angina is an active thrombotic process that involves thrombus formation and platelets. It requires a rapid and intensive treatment with anticoagulants and antiplatelets.

HYPOTHESIS

The aim of the present study was to compare the efficacy of a very low molecular weight heparin, OP 2000, with standard heparin in the treatment of unstable angina. Tolerance and safety were also assessed.

METHODS

The study population included 120 consecutive hospitalized patients with unstable angina randomized for treatment with very low molecular weight heparin or with standard heparin. The dosage of the study drug was 200 mg intramuscular (i.m.) the first day followed by 150 mg IM/day. The control drug was standard heparin starting at a dosage of 5,000 UI/ml intravenously (i.v.) and followed by continuous infusion at an activated partial thromboplastin time-adjusted dosage. The primary end points were death, acute myocardial infarction, urgent revascularization, and recurrence of angina. Tolerability was assessed using bleeding parameters, thrombocytopenia, and allergic reactions.

RESULTS

Fourteen clinical events were reported in the study group compared with 25 events in the control group (p < 0.05). No adverse events were reported in either group.

CONCLUSION

During the acute phase of unstable angina, treatment with a very low molecular weight heparin plus aspirin was more effective than treatment with standard heparin plus aspirin.

Vasoflux, a new anticoagulant with a novel mechanism of action

BACKGROUND

Heparin and direct thrombin inhibitors, such as hirudin, have limitations in the treatment of acute coronary syndromes. Heparin does not inactivate fibrin-bound thrombin, whereas hirudin fails to block thrombin generation. In contrast, Vasoflux is a novel anticoagulant that inactivates fibrin-bound thrombin and attenuates factor Xa generation.

METHODS AND RESULTS

Vasoflux is prepared by depolymerization of heparin, restricting molecular size to between 3000 and 8000 Da, and reducing antithrombin affinity by periodate oxidation. Vasoflux catalyzes fibrin-bound thrombin inactivation by heparin cofactor II (HCII) and inhibits factor IXa activation of factor X independently of antithrombin and HCII. Compared with other anticoagulants in a thrombogenic extracorporeal circuit, Vasoflux maintains filter patency at concentrations that produce an activated clotting time (ACT) of 220 seconds. In contrast, to maintain filter patency, heparin, low-molecular-weight heparin (LMWH), and hirudin require concentrations that produced an ACT of 720, 415, and >1500 seconds, respectively, whereas dermatan sulfate was ineffective at concentrations that produced an ACT of 360 seconds.

CONCLUSIONS

Vasoflux is more effective than heparin and LMWH because it inactivates fibrin-bound thrombin and is superior to hirudin and dermatan sulfate because it also blocks factor Xa generation.

Deep-vein thrombosis

Deep-vein thrombosis is an important complication of several inherited and acquired disorders, but may also occur spontaneously. Prevention of recurrent venous thrombosis and pulmonary embolism is the main reason for accurate diagnosis and adequate treatment. This seminar discusses only symptomatic deep-vein thrombosis. The diagnosis can be confirmed by objective tests in only about 30% of patients with symptoms. Venous thromboembolic complications happen in less than 1% of untreated patients in whom the presence of venous thrombosis is rejected on the basis of serial ultrasonography or ultrasonography plus either D-dimer or clinical score. Initial anticoagulant treatment (intravenous or subcutaneous heparin) should continue until oral anticoagulant treatment, started concurrently, increases the international normalised ratio above 2.0 for more than 24 h. The optimum duration of oral anticoagulant treatment is unresolved, but may be guided by the presence of temporary or persistent risk factors or presentation with recurrent venous thromboembolism.

A randomized, multicenter trial of weight-adjusted intravenous heparin dose titration and point-of-care coagulation monitoring in hospitalized patients with active thromboembolic disease. Antithrombotic Therapy Consortium Investigators

BACKGROUND

Therapy with intravenous unfractionated heparin improves clinical outcome in patients with active thromboembolic disease, but achieving and maintaining a therapeutic level of anticoagulation remains a major challenge for clinicians.

METHODS

A total of 113 patients requiring heparin for at least 48 hours were randomly assigned at 7 medical centers to either weight-adjusted or non-weight-adjusted dose titration. They were separately assigned to either laboratory-based or point-of-care (bedside) coagulation monitoring.

RESULTS

Weight-adjusted heparin dosing yielded a higher mean activated partial thromboplastin time (aPTT) value 6 hours after treatment initiation than non-weight-adjusted dosing (99.9 vs 78.8 seconds; P =.002) and reduced the time required to exceed a minimum threshold (aPTT >45 seconds) of anticoagulation (10.5 vs 8.6 hours; P =.002). Point-of-care coagulation monitoring significantly reduced the time from blood sample acquisition to a heparin infusion adjustment (0.4 vs 1.6 hours; P <.0001) and to reach the therapeutic aPTT range (51 to 80 seconds) (16.1 vs 19.4 hours; P =.24) compared with laboratory monitoring. Although a majority of patients participating in the study surpassed the minimum threshold of anticoagulation within the first 12 hours and reached the target aPTT within 24 hours, maintaining the aPTT within the therapeutic range was relatively uncommon (on average 30% of the overall study period) and did not differ between treatment or monitoring strategies.

CONCLUSIONS

Weight-adjusted heparin dosing according to a standardized titration nomogram combined with point-of-care coagulation monitoring using the BMC Coaguchek Plus System represents an effective and widely generalizable strategy for managing patients with thromboembolic disease that fosters the rapid achievement of a desired range of anticoagulation. Additional work is needed, however, to improve on existing patient-specific strategies that can more effectively sustain a therapeutic state of anticoagulation.

Catheter-directed thrombolysis for thromboembolic disease during pregnancy: a viable option

Anticoagulation with intravenous heparin has been the standard treatment for the management of gestational thromboembolic complications. Catheter-directed thrombolysis is an encouraging approach for the treatment of thromboembolic disease and has not been previously reported during pregnancy. One gravid woman with pulmonary embolism, critically ill, and hemodynamically compromised, and two gravid women with iliofemoral venous thrombosis, who failed to respond to standard treatment with intravenous heparin, were treated with catheter-directed urokinase. All three patients experienced rapid resolution of symptoms and successful pregnancy outcomes. In our three patients, catheter-directed thrombolysis for thromboembolic disease during pregnancy allowed rapid resolution of hemodynamic abnormalities and/or resolution of thrombus. Catheter-directed thrombolysis offered a reasonably safe alternative to prolonged medical management in these young, otherwise healthy, patients. Long-term, it may prevent the postphlebitic syndrome.

Activated partial thromboplastin time (aPTT) monitoring to achieve therapeutic anticoagulation before and after introducing a nomogram for adjunctive heparin treatment with thrombolytic therapy for acute myocardial infarction

In patients with acute myocardial infarction (AMI) receiving thrombolytic therapy and i.v. unfractionated heparin, anticoagulant levels are frequently outside the target range. We evaluated the effects on anticoagulant levels before (group A) and after (group B) the introduction of a heparin nomogram in consecutive AMI-patients, receiving thrombolytic therapy and adjunctive heparin treatment. The target activated partial thromboplastin time (aPTT) was defined as 60-90 s. During the first 72 h after admission, the total number of aPTTs within the target range and the time taken to achieve the range were compared. The incidence of bleeding complications was assessed. Group A consisted of 56 and group B of 55 patients. The number of patients within the target range at 72 h (44 versus 51; chi2=4.51; P=0.034) was significantly higher in group B. No difference was found between total aPTTs within the target range (26% in group A versus 30% in group B; P=ns). Bleeding complications were slightly less in group B (7 versus in group A versus 2 patients in group B; P=ns). We concluded that the introduction of a nomogram resulted in significantly more patients with aPTTs within the target range. However, a substantial number of aPTTs before and after introduction of the nomogram were outside the target range. Moreover, this retrospective study shows that previously acquired prospective data (which showed a marked improvement of anticoagulation using a heparin nomogram) are not necessarily reproduced in the real life clinical setting.

Investigation of the anticoagulant mechanisms of a covalent antithrombin-heparin complex

Recently, we developed a covalent antithrombin-heparin complex (ATH) as a possible treatment for respiratory distress syndrome. ATH reacted rapidly with thrombin and efficiently catalyzed the inhibition of either thrombin or factor Xa by exogenous antithrombin. In order to investigate mechanisms for the conjugate's unusual anticoagulant properties, changes in fluorescence due to covalent linkage or addition of exogenous antithrombin were studied in relation to reaction with thrombin derivatives or factor Xa. The emission spectrum of ATH was similar to that of antithrombin plus heparin mixtures. ATH quickly inhibited thrombin or factor Xa activities, as measured by a fluorogenic substrate. Fluorescein-labeled heparin was displaced from either thrombin or active site blocked thrombin by ATH, indicating that thrombin must bind to the conjugate's heparin moiety. Interaction of thrombin with ATH's heparin component was confirmed by a slow reaction rate of conjugate with a thrombin mutant that has weak heparin binding. Total intrinsic fluorescence increased when exogenous antithrombin was added to ATH, indicating that the catalytic mechanism may occur through a second inhibitor binding site. Thus, ATH reacts directly with thrombin through a bridge mechanism and probably catalyzes the reaction of thrombin with antithrombin by a second binding sequence on its heparin chain.

Low molecular weight heparins: current use and indications

Thromboembolic complications are associated with significant morbidity and mortality in postoperative patients. For many years, unfractionated heparin has been used successfully in primary and secondary prophylaxis of these complications. In recent times, however, the usefulness of LMWHs has caught the attention of clinicians because of improved bioavailability, predictable anticoagulation, ease of administration, and the lack of need for monitoring anticoagulation. In clinical situations, LMWHs have been tested and proved to be safe and equipotent or supe rior when compared with unfractionated heparin or warfarin (Table 5). It is clear from clinical trials that LMWHs are superior in primary prophylaxis of DVT in orthopaedic surgical procedures, treatment of unstable angina, and in patients with multiple traumas. LMWHs were also tested and found to be an acceptable alternative to unfractionated heparin in both the primary prophylaxis of DVT in high risk general surgical procedures and in the treatment of patients with DVT and pulmonary embolism. However, the role of LMWHs in ischemic heart diseases, valvular heart diseases, postcoronary angioplasty, and vascular surgery remains to be proved. The major impact of LMWHs would be in allowing clinicians to treat PE and DVT in an outpatient setting, which would directly impact medical economics. LMWHs are associated with similar complications as unfractionated heparin is, but the complications occur less frequently. Currently, the main limitation in using LMWHs in place of unfractionated heparin or warfarin is its cost. However, taking into account the cost incurred by hospitalization and longterm monitoring of anticoagulation in patients treated with unfractionated heparin, certain trials have proved the cost of LMWHs to be the same or less than the cost of unfractionated heparin overall. We envision that LMWHs will be widely used in the future and will bring welcomed change in the treatment of thromboembolic diseases.

Venous thromboembolic events in pediatric patients. Diagnosis and management

Venous thromboembolism is a rapidly increasing secondary complication in children being treated for serious, life-threatening, primary diseases. Most current management guidelines and recommendations for imaging techniques have been extrapolated from the results of trials in adults. This may be less than optimal for children as there are important differences. The purpose of this article is to summarize the information on venous thromboembolism in children, and offer some guidelines for diagnosis, prophylaxis, and therapeutic intervention based on the best available evidence.

Evaluation of the activated partial thromboplastin time (APTT) sensitivity to heparin using five commercial reagents: implications for therapeutic monitoring

Heparin is an effective drug for prevention and treatment of thromboembolic conditions. Although several biological assays have been proposed for monitoring unfractionated heparin therapy, the measurement of the activated partial thromboplastin time (APTT) is the most widely employed test, and the overall risk of thromboembolic episodes was markedly reduced by maintaining APTT ratios above 1.5. However, the adjustment of the heparin therapy on the basis of APTT presents several questions which are still unresolved. Major discrepancies were found in APTTs performed using different reagents in both ex vivo and in vitro heparinized samples and occasionally with different lots of the same reagents; poor correlation was observed between APTT values and plasma heparin concentrations. In order to gain further insights into this phenomenon, we analysed the sensitivity to heparin of five commercial reagents for APTT measurement in 19 ex vivo heparinized samples. Differences were observed; correlation coefficients ranged from 0.820 to 0.985 and slopes of linear regressions from 0.26 to 1.14. Moreover, unsatisfactory correlations were obtained when APTT ratios were compared with heparin plasma concentrations in the same patients' samples. In the heparin therapeutic range of 0.35 - 0.70 U/ml, reagent-dependent differences were observed in the corresponding APTT values. These results point out a critical role of the assay methodology in monitoring heparin therapy by APTT. We suggest that reference materials and methods should be urgently identified, a universally agreed scale for reporting results should be established and reference ranges for the unfractionated heparin therapy should be reconsidered taking on account the reagent employed.

Heparin in acute ischaemic stroke. The T wave is negative and it's time to stop

The use of heparin for acute ischaemic stroke has long been controversial. Although it may have some theoretical benefits in preventing fibrin formation and thrombus propagation, data from 16 randomised controlled trials involving more than 22,000 patients show that heparin produces no significant net benefit in eventual outcome.

Use of antithrombotic agents during pregnancy

Anticoagulant therapy is indicated during pregnancy for the prevention and treatment of VTE, for the prevention and treatment of systemic embolism in patients with mechanical heart valves, and, in combination with aspirin, for the prevention of pregnancy loss in women with APLA and previous pregnancy losses. Several questions concerning anticoagulant therapy remain unanswered. Oral anticoagulants are fetopathic, but the true risks of the warfarin embryopathy and CNS abnormalities are unknown. There is some evidence that warfarin embryopathy occurs only when oral anticoagulants are administered between the 6th and the 12th weeks of gestation and that oral anticoagulants may not be fetopathic when administered in the first 6 weeks of gestation. Oral anticoagulant therapy should be avoided in the weeks before delivery because of the risk of serious perinatal bleeding caused by the trauma of delivery to the anticoagulated fetus. The safety of aspirin during the first trimester of pregnancy is still a subject of debate. There is a concern about the efficacy of unfractionated heparin in the prevention of arterial embolism in pregnant women with mechanical heart valves. Finally, the role of LMWH and heparinoids and appropriate dosing have still to be determined. Because it is safe for the fetus, heparin is the anticoagulant of choice during pregnancy for situations in which its efficacy is established. The evidence for the efficacy of heparin for the prevention and treatment of VTE disorders during pregnancy is based on level IV studies. There is some doubt that heparin is effective for the prevention of systemic embolism in patients with mechanical heart valves. Low doses of heparin or poorly controlled heparin therapy are not effective in preventing systemic embolism in patients with mechanical heart valves.

Heparin dosing in patients undergoing coronary intervention

Unfractionated heparin remains an essential component of the antithrombotic regimen in patients undergoing coronary intervention, although the timing, dosing, and duration of heparin therapy have evolved over the past several years. Complications associated with heparin use include bleeding events, which occur in 3.9-16.4% of patients receiving conventional heparin. Less commonly, clinically significant thrombocytopenia develops, related to the duration of heparin administration. In patients undergoing coronary intervention who do not receive platelet glycoprotein (GP) IIb/IIIa inhibitors, sufficient heparin should be given to achieve an activated clotting time (ACT) of 250-300 seconds with the HemoTec device and 300-350 seconds with the Hemochron device. There is a general trend to use lower, weight-adjusted heparin dosing (70-100 units/kg) to avoid excessive levels of anticoagulation, with additional heparin boluses to achieve a therapeutic ACT level. When GP IIb/IIIa inhibitors are used, weight-adjusted heparin dosing can be decreased to 70 units/kg to achieve a target ACT of 200 seconds with either the HemoTec or Hemochron device. After uncomplicated coronary intervention, there appears to be little value associated with continued heparin therapy, and the risk of bleeding complications clearly increases with longer durations and higher levels of anticoagulation after coronary intervention.

A randomized comparison of bivalirudin and heparin in patients undergoing coronary angioplasty for postinfarction angina. Hirulog Angioplasty Study Investigators

The outcome of coronary angioplasty performed for unstable angina is determined, in part, by the acuteness and severity of the clinical presentation. The risk of abrupt vessel closure is increased in patients with postinfarction angina. The Hirulog Angioplasty Study compared the efficacy and safety of bivalirudin with weight-adjusted heparin in patients undergoing percutaneous transluminal coronary angioplasty (PTCA) for unstable or postinfarction angina. We report the results of the intent-to-treat analysis using adjudicated data for the prespecified group of 741 patients who underwent angioplasty within 2 weeks of documented myocardial infarction. Patients received either bivalirudin or heparin immediately before angioplasty. The primary efficacy endpoint was procedural failure defined as abrupt vessel closure, death, myocardial infarction, or revascularization during hospitalization. Bivalirudin significantly (p = 0.004) decreased the incidence of procedural failure compared with heparin (5.1% vs 10.8%, odds ratio 0.45; 95% CI 0.25-0.79). The improved efficacy of bivalirudin was replicated for each individual clinical endpoint. The incidence of major bleeding was significantly (p = 0.001) lower in bivalirudin-treated patients compared with heparin-treated patients (2.4% vs 11.8%, respectively). The benefits observed with bivalirudin are of similar magnitude as those reported for platelet glycoprotein (GP) IIb/IIIa inhibitors, such as abciximab. Bivalirudin may be a more effective foundation anticoagulant than heparin in patients undergoing coronary angioplasty for postinfarction angina.

Direct thrombin inhibitors for treatment of arterial thrombosis: potential differences between bivalirudin and hirudin

Given the central role of thrombin in arterial thrombogenesis, most treatment strategies for acute coronary syndromes are aimed at inhibiting its generation or blocking its activity. Although heparin has been widely used, it has limitations in the setting of arterial thrombosis. These limitations reflect the inability of heparin to inactivate thrombin bound to fibrin, a major stimulus for thrombus growth. In addition, the anticoagulant response to heparin varies from patient to patient, and heparin is neutralized by platelet Factor IV, large quantities of which are released from platelets activated at sites of plaque rupture. Consequently, heparin requires careful laboratory monitoring to ensure an adequate anticoagulant effect. Direct thrombin inhibitors, such as hirudin and bivalirudin, overcome the limitations of heparin. These agents inhibit fibrin-bound thrombin, as well as fluid-phase thrombin, and produce a predictable anticoagulant response. Bivalirudin has both safety and potential efficacy advantages over hirudin. Bivalirudin appears to have a wider therapeutic window than hirudin, possibly because bivalirudin only transiently inhibits the active site of thrombin. The better safety profile of bivalirudin permits administration of higher doses, which may give it an efficacy advantage. Hirudin prevents thrombin from activating protein C, thereby suppressing this natural anticoagulant pathway. In contrast, bivalirudin may promote protein C activation by transiently inhibiting thrombin until it can be bound by thrombomodulin. Differences between bivalirudin and hirudin, as well as other direct thrombin inhibitors, highlight the pitfalls of considering all direct thrombin inhibitors to have equivalent risk-benefit profiles.

Oral delivery of anticoagulant doses of heparin. A randomized, double-blind, controlled study in humans

BACKGROUND

Parenteral heparin is the anticoagulant of choice in hospitalized patients. Continued anticoagulation is achieved by subcutaneous administration of low-molecular-weight heparin or with an orally active anticoagulant such as warfarin. An oral heparin formulation would avoid the inconvenience of subcutaneous injection and the unfavorable drug interactions and adverse events associated with warfarin. A candidate delivery agent, sodium N-[8(-2-hydroxybenzoyl)amino]caprylate (SNAC), was evaluated with escalating oral heparin doses in a randomized, double-blind, controlled clinical study for safety, tolerability, and effects on indexes of anticoagulation.

METHODS AND RESULTS

Increases in activated partial thromboplastin time (aPTT), anti-factors IIa and Xa, and tissue factor pathway inhibitor (TFPI) concentrations were detected when normal volunteers were dosed with 10.5 g SNAC/20000 IU heparin by gavage in some subjects. For the entire group, 30000 IU SNAC and heparin elevated TFPI from 74.9+/-7.6 to 254.2+/-12.3 mg/mL (P<0.001) 1 hour after dosing (P<0.001). Similar changes occurred in anti-factor IIa and anti-factor Xa. aPTT rose from 28+/-0.5 to 42.2+/-6.3 seconds 2 hours after dosing (P<0.01). No significant changes in vital signs, physical examination, ECGs, or clinical laboratory values were observed. Neither 30000 IU heparin alone nor 10.5 g SNAC alone altered the hemostatic parameters. Emesis was associated with 10.5 g SNAC. A taste-masked preparation of SNAC 2.25 g was administered orally with heparin 30000 to 150000 IU. Both aPTT and anti-factor Xa increased with escalating doses of heparin. This preparation was well tolerated. Conclusions-Heparin, administered orally in combination with the delivery agent SNAC, produces significant elevations in 4 indexes of anticoagulant effect in healthy human volunteers. These results establish the feasibility of oral delivery of anticoagulant doses of heparin in humans and may have broader implications for the absorption of macromolecules.

Anticoagulant and antiprotease activities of a heparinoid sulfated glucoside-bearing polymer

We studied anticoagulant and antiprotease activities of the poly(glucosyloxyethyl methacrylate) sulfate [poly(GEMA)-sulfate] in plasma and purified enzyme systems in order to evaluate the anticoagulant behavior of a heparin-like sulfated glucoside-bearing polymer. As a result, we found that poly(GEMA)-sulfate can inhibit some coagulation proteases, although its antiprotease behavior differed from those of heparin and dextran sulfate. Poly(GEMA)-sulfate could not enhance antithrombin activity; therefore, we did not observe any significant inhibition of Factor Xa via antithrombin. However, we found that poly(GEMA)-sulfate was able to inhibit thrombin through the activation of heparin cofactor II. In addition, poly(GEMA)-sulfate was able to inhibit Tenase. In our previous research. we found that the anticoagulant activity of poly(GEMA)-sulfate is due primarily to the formation of an insoluble complex with fibrinogen. This paper showed that the antiprotease activities of poly(GEMA)-sulfate contribute to some extent to its anticoagulant activity.

Concurrence of anaphylaxis and acute heparin-induced thrombocytopenia in a patient with heparin-induced antibodies

We report the occurrence of acute heparin-induced thrombocytopenia in a patient with anaphylaxis that began immediately after an intravenous bolus dose of unfractionated heparin. This case report is the first to document the concurrence of these 2 reactions to heparin. An abrupt fall in platelet count was documented immediately after the anaphylactic response. Study results for antibodies characteristic of heparin-induced thrombocytopenia were positive in 2 assays: serotonin release assay and heparin platelet factor 4 enzyme-linked immunosorbent assay. The patient's antibody was exclusively immunoglobulin G. Any explanation for the relationship between the antibody response observed and the histamine release remains speculative.

Hirudin causes more bleeding than heparin in a rabbit ear bleeding model

This study was undertaken to determine the appropriateness of the current practice of using the activated partial thromboplastin time (APTT) to select hirudin doses. A rabbit bleeding ear model was used to compare the effects of various doses of heparin and hirudin on the relationship between the APTT and bleeding. In addition, the effects of these agents on the thrombin clotting time (TCT) and factor Xa clotting time also were examined. Both heparin and hirudin produced a concentration-dependent increase in bleeding. When bleeding was plotted as a function of APTT ratio, even a small increase in APTT ratio within the therapeutic range of 1.5 to 2.5 resulted in a marked increase in bleeding with hirudin but not with heparin. The TCT was more responsive than the APTT or factor Xa clotting time to increases in hirudin-induced bleeding. In this model, hirudin produces more bleeding than heparin when the agents are used in doses that increase the APTT ratio to the same extent. These studies highlight the pitfalls of extrapolating from experience with heparin when choosing a test to monitor new antithrombotics. Our findings also suggest that the TCT may be more responsive than the APTT for monitoring hirudin therapy.

Influence of Sulfation on Platelet Aggregation and Activation with Differentially Sulfated Hyaluronic Acids

A number of sulfated hyaluronic acid derivatives (HyalS(2.5), HyalS(3), and HyalS(4)) were prepared by sulfation of the -OH groups present on hyaluronic acid and were generically termed HyalS(x). The anticoagulant properties of this series of compounds has previously been shown to be good in terms of their whole blood clotting inhibition and factor Xa and thrombin inactivation. The purpose of the present study was to investigate whether the use of these compounds would be beneficial to patients who would normally be given heparin, and to perform some preliminary investigations into their effects on platelets. The three compounds were thus studied by investigating their ability to inhibit von Willebrand factor-dependent platelet agglutination in comparison with unfractionated heparin. Agglutination was determined turbidometrically after the addition of ristocetin to stirred formaldehyde-fixed platelets and was demonstrated to be dependent on the presence of sulfate groups on the polysaccharide chain and correlated with the degree of HyalS(x) sulfation. Interactions possibly important in low shear environments were investigated by measuring the pharmacological action of the HyalS(x) on spontaneous platelet activation and aggregate formation by flow cytometry. The data indicate that platelet activation is not correlated with the number of sulfate or hydroxyl groups on HyalS(x), suggesting that activation occurs not via electrostatic interactions or H bonding, but via some other mechanism. A differentiation between low and high glycosaminoglycan sulfation densities is observed with respect to platelet aggregation, which is correlated with the number of sulfated groups per disaccharide unit. The ability of HyalS(x) to inhibit platelet aggregation induced by ADP and thrombin was measured by aggregometry. HyalS(4) resisted thrombin stimulation to a similar extent as heparin. All Hyal derivatives, however, were better at inhibiting ADP-induced aggregation than was heparin. We conclude, therefore, that clinical use of HyalS(x) in place of heparin may be beneficial because ristocetin-dependent agglutination, and therefore resistance to platelet aggregation in high shear environments, in addition to resistance to stimulation by ADP, has been shown to be superior to heparin. Spontaneous platelet activation and aggregation are induced at an overall low level, even at high HyalS(x) concentrations, and are comparable with that of heparin.