Establishing a New Target Range for Unfractionated Heparin for Acute Coronary Syndromes

INTRODUCTION: The target activated partial thromboplastin time (aPTT) range of 1.5 to 2.5 times the control value or 45 to 75 seconds recommended by the ACC/AHA for patients receiving unfractionated heparin (UFH) for acute coronary syndromes (ACS) is vulnerable to variation in test reagents. Rather than use the standard target aPTT range, it has been recommended that each institution establish its own target aPTT range based upon anti-factor Xa heparin levels. As a quality assurance project, we evaluated our institution's therapeutic aPTT range by examining the correlation between aPTTs and anti-factor Xa heparin levels and established a new target aPTT range with a new thromboplastin reagent based upon the therapeutic anti-factor Xa heparin levels. METHODS: Sixty-two plasma samples from 26 consecutive patients receiving UFH for ACS were analyzed. Plasma aPTTs measured with a thromboplastin reagent and a new thromboplastin reagent and anti-factor Xa heparin levels were obtained on each plasma sample. Linear regression analysis was performed to establish a new target aPTT range from corresponding therapeutic anti-factor Xa heparin levels. RESULTS: Thirty-two percent of patients with our institution's target range aPTTs of 61 to 100 seconds had anti-factor Xa heparin levels below 0.35 to 0.7 U/mL while 68% of patients had therapeutic anti-factor Xa heparin levels (positive predictive value = 68%). When the same blood was tested with a new thromboplastin reagent lot, only 9% of patients with target range aPTTs had anti-factor Xa heparin levels below 0.35 U/mL while 91% of patients had therapeutic anti-factor Xa heparin levels (positive predictive value = 91%). The Pearson correlation coefficient ( r ) for the new thromboplastin reagent lot was 0.93. The target aPTT range established with the new thromboplastin reagent lot was 61 to 100 seconds. CONCLUSION: Monitoring aPTTs without standardizing the thromboplastin reagent may not adequately reflect therapeutic heparin levels. Despite apparently target aPTTs, patients treated with UFH may be under-anticoagulated. Our new anti-Xa-adjusted target aPTT range shows an increase in the positive predictive value of aPTTs. Large-scale clinical studies are needed to determine the optimal anti-factor Xa range for ACS patients treated with UFH, with further refinements if glycoprotein IIb/IIIa inhibitors are concomitantly used and to show a benefit in clinical outcomes for monitoring plasma heparin levels with anti-factor Xa heparin levels. Institutional standardization of the aPTT is necessary to ensure optimal patient care when changing thromboplastin reagents.     

Pharmacokinetic and pharmacodynamic characterization of a medium-molecular-weight heparin in comparison with UFH and LMWH

Despite the well-established medical use of heparins, the question arises whether the efficacy-safety ratio of the available heparins can still be improved. Therefore, a medium-molecular-weight heparin (MMWH), a new heparin with an average molecular weight of 10.5 kDa and a narrow molecular weight range (9.5 to 11.5 kDa) was developed. Its in vitro activities amount to 174.9 anti-factor Xa (aXa) U/mg and 170.0 antithrombin (aIIa) U/mg. In the presented randomized, double-blind, cross-over study in healthy volunteers, the pharmacokinetics and pharmacodynamics of MMWH are compared with those of an unfractionated heparin (UFH) and a low-molecular-weight heparin (LWMH; enoxaparin). After subcutaneous administration of 9000 aXa-U of either heparin in 16 volunteers, the prolongation of the activated partial thromboplastin time (aPTT), the aXa activity, and the aIIa activities were determined at 11 time points spread over 24 hours after injection. The ex vivo analysis revealed striking pharmacodynamic and pharmacokinetic differences between the three heparins. UFH had the lowest bioavailability regarding the aPTT, aXa, and aIIa activities. Enoxaparin exhibited only low aIIa activity but the highest aXa activity. Unlike UFH and enoxaparin, MMWH showed a high recovery of aIIa activity, which suggests that it combines the high potency to inhibit thrombin that characterizes UFH with the high bioavailability of the LMWHs. Consequently, substantially lower doses are needed to bring about effects comparable to those of UFH and LMWH.     

Relationship between ex vivo anti-proteinase (factor Xa and thrombin) assays and in vivo anticoagulant effect of very low molecular weight heparin, CY222

There is uncertainty as to which activities of unfractionated heparin (UFH) and low MW heparin are responsible for their anticoagulant and antithrombotic properties. We have sought to answer this question by examining plasma samples taken during a recently conducted dose-finding study of the low MW heparin, CY222, in haemodialysis for chronic renal failure. In this study, in vivo anticoagulant effect was assessed by measurement of plasma FPA levels. UFH was administered as a dose of 5000 iu bolus + 1,500 iu/h maintenance infusion, while the effects of three doses of CY222 were studied (10,000, 15,000 and 20,000 Institute Choay anti-factor Xa u bolus, all with 1,500 Institute Choay anti-factor Xa u/h maintenance infusion). Anti-factor Xa levels were determined by chromogenic substrate assay. Anti-thrombin levels were determined by chromogenic substrate assay and by quantitation of catalysed thrombin-inhibitor complexes (using autoradiography). Analysis of the results indicate that plasma fibrinopeptide A (FPA) levels correlate with anti-factor Xa (r = -0.45) and anti-thrombin (substrate) (r = -0.63) levels of UFH, but only with the anti-factor Xa levels (r = -0.41) of CY222. These results suggest that the anti-factor Xa assay is currently the most suitable assay for monitoring low MW heparins such as CY222 in humans.     

Partial depletion of tissue factor pathway inhibitor during subcutaneous administration of unfractionated heparin, but not with two low molecular weight heparins

Tissue factor pathway inhibitor (TFPI) is released to circulating blood after intravenous (i.v.) and subcutaneous (s.c.) injections of heparins, and may thus contribute to the antithrombotic effect of heparins. We have recently shown that total TFPI activity, plasma free TFPI antigen, and heparin releasable TFPI were partially depleted during repeated and continuous i.v. infusion of unfractionated heparin (UFH), but not during s.c. treatment with a low molecular weight heparin (LMWH). The difference may be attributed to a different mode of action or the different mode of administration. In the present randomized cross-over study, s.c. administration of therapeutic doses of UFH was compared with s.c. administration of two LMWHs. 12 healthy male volunteers were treated for 3 d with UFH, 250 U/kg twice daily, dalteparin, 200 U/kg once daily, and enoxaparin, 1.5 mg/kg once daily. Six participants were also treated with UFH, 300 U/kg once daily. On day 5 a single dose of either drug was given. Peak levels of total TFPI activity and free TFPI antigen were detected 1 h after injection, whereas maximal prolongation of activated partial thromboplastin time (APTT) and peak levels of anti-factor Xa activity and anti-factor IIa activity were detected after 4 h. On UFH administered twice daily, free TFPI antigen decreased by 44% from baseline level before the first injection on day 1 to pre-injection level on day 5. On UFH administered once daily, basal free TFPI antigen decreased by 50%, 56% and 27% on day 2, 3 and 5 respectively, compared with day 1. Minimal depletion of TFPI was detected during treatment with LMWHs. The study demonstrates the different modes of action of LMWHs and UFH and may help to explain the superior antithrombotic efficacy of LMWHs.     

Novel concatameric heparin-binding peptides reverse heparin and low-molecular-weight heparin anticoagulant activities in patient plasma in vitro and in rats in vivo

Patients given unfractionated heparin (UFH) or low-molecular-weight heparin (LMWH) for prophylaxis or treatment of thrombosis sometimes suffer serious bleeding. We showed previously that peptides containing 3 or more tandem repeats of heparin-binding consensus sequences have high affinity for LMWH and neutralize LMWH (enoxaparin) in vivo in rats and in vitro in citrate. We have now modified the (ARKKAAKA)(n) tandem repeat peptides by cyclization or by inclusion of hydrophobic tails or cysteines to promote multimerization. These peptides exhibit high-affinity binding to LMWH (dissociation constant [K(d)], approximately 50 nM), similar potencies in neutralizing anti-Factor Xa activity of UFH and enoxaparin added to normal plasma in vitro, and efficacy equivalent to or greater than protamine. Peptide (ARKKAAKA)(3)VLVLVLVL was most effective in all plasmas from enoxaparin-treated patients, and was 4- to 20-fold more effective than protamine. Several other peptide structures were effective in some patients' plasmas. All high-affinity peptides reversed inhibition of thrombin-induced clot formation by UFH. These peptides (1 mg/300 g rat) neutralized 1 U/mL anti-Factor Xa activity of enoxaparin in rats within 1 to 2 minutes. Direct blood pressure and heart rate measurements showed little or no hemodynamic effect. These heparin-binding peptides, singly or in combination, are potential candidates for clinical reversal of UFH and LMWH in humans.     

Antithrombotic potencies of heparins in relation to their antifactor Xa and antithrombin activities: an experimental study in two models of thrombosis in the rabbit

Our purpose was to determine the relative contribution of the antifactor Xa and antithrombin activities of heparin to its antithrombotic potency. The antithrombotic activities of unfractionated heparin (UH), two low molecular weight heparins (LMWH, CY 216 and CY 222) with increasing anti-factor Xa/antithrombin ratio and a synthetic pentasaccharide (PS) with high affinity to antithrombin III and no antithrombin activity were evaluated. In the Wessler-thromboplastin model, the most potent antithrombotic agent, on a weight basis, was UH followed by CY 216, CY 222 and the PS which was 40 times less potent than UH. On an antithrombin unit basis, the antithrombotic potencies of UH, CY 216 and of CY 222 were equivalent. Thus, in this model, the antithrombotic effect results from the catalytic action of UH or LMWH on thrombin inhibition. In the Wessler-serum model, on a weight basis, the antithrombotic effectiveness of UH was unchanged, those of CY 216 and CY 222 were doubled, and that of the PS was increased 10 times. On an anti-factor Xa unit basis, CY 216 was as effective as UH, and PS as effective as CY 222. On an antithrombin unit basis, CY 216 and CY 222 were equivalent and more potent than UH. Thus, in this model, the antifactor Xa activity of heparin becomes important for its antithrombotic property. After a single subcutaneous injection of 1000 antifactor Xa U/kg, the antithrombotic effects of UH were maintained for more than 14 h in the two models. After injection of the same dose of CY 216 significant antithrombotic effects were observed only for 9 h, in the Wessler-thromboplastin model but for 18 h in the Wessler-serum model. At that time, no detectable antithrombin activity was measurable in the plasma while 0.11 units of antifactor Xa activity/ml was detected. Thus, the relative contribution of the anti-factor Xa and antithrombin activities to the antithrombotic effect of a LMWH differs according to the nature of the thrombogenic stimulus.     

A rationally designed heparin, M118, has anticoagulant activity similar to unfractionated heparin and different from Lovenox in a cell-based model of thrombin generation

Unfractionated heparin (UFH) enhances antithrombin (AT) inhibition of thrombin (IIa) and factor Xa (FXa). Low molecular weight heparins (LMWH) primarily enhance AT inhibition of FXa. M118 is a LMWH produced from UFH and retains its ability to promote both FXa and IIa inhibition. We tested the hypothesis that M118 has anticoagulant activities similar to UFH in an in vitro model of coagulation. Platelet IIa generation was assessed in a cell-based model that mimics aspects of coagulation in vivo. Inhibition of IIa generation as a function of concentration was steeper for UFH than Lovenox. The effect of M118 closely paralleled that of UFH. By contrast, M118 did not prolong the aPTT to as great a degree as UFH, though both prolonged the aPTT more than did Lovenox. Our data suggest that the ability to inhibit platelet surface IIa generation correlates with the therapeutic level of heparins and confirms similarities between the anticoagulant properties of M118 and UFH. Fred Spencer, MD served as a guest editor.     

How and when to monitor a patient treated with low molecular weight heparin.

Low molecular weight heparins (LMWHs) are as efficient as unfractionated heparin (UFH) for prevention and treatment of thromboembolism. There is no evidence that monitoring the dose improves the clinical efficacy. In contrast, any overdosage increases the risk of hemorrhage. Because renal function plays a significant role in the elimination of LMWH, curative treatment should be monitored with an anti-factor Xa assay in patients presenting renal insufficiency, in the elderly, and in patients presenting an increased hemorrhagic risk. It is advisable to sample the patient at peak activity (3 to 5 hours after the subcutaneous [sc] administration) and to target the mean anti-factor Xa activity that was found efficient and safe in the clinical trial. This target is different for each LMWH and each dose regimen.     

The anticoagulant capacity of plasmatic unfractionated heparin decreases at 23 degrees C.

Unfractionated heparin (UFH) and low-molecular-weight heparin (LMWH) are important clinical anticoagulants. As polynegative molecules they are potential triggers of the contact phase of coagulation. An incubation temperature lower than the physiological 37 degrees C favours intrinsic haemostasis activation by the polynegative molecule SiO2. The efficiency of UFH and LMWH after a plasmatic preincubation at 37 or at 23 degrees C is therefore studied. Samples (150 mul) of unfrozen pooled normal plasma supplemented with 0, 0.01, 0.1, or 1 IU/ml heparin or dalteparin in 5-ml polystyrole tubes were incubated for 10-70 min at 37 or at 23 degrees C. The extrinsic coagulation activity assay (EXCA) was then performed. Preincubation at 37 degrees C of 0.1 IU/ml plasmatic UFH does not result in any thrombin generation in EXCA-1, whereas preincubation at 23 degrees C results in a thrombin generation of about 0.1 IU/ml thrombin. Plasmatic UFH (0.01 IU/ml) at 23 degrees C acts nearly half as efficiently as 0.01 IU/ml plasmatic LMWH. Polynegatively charged niches particularly in the larger UFH molecule might trigger the contact system of haemostasis, especially at 23 degrees C. In contrast, the anticoagulant capacity of LMWH does not change significantly with temperature.     

The activated clotting time (ACT) can be used to monitor enoxaparin and dalteparin after intravenous administration

BACKGROUND: The use of low-molecular weight heparin (LMWH) during percutaneous coronary intervention (PCI) has been limited by the presumed inability to monitor its anticoagulant effect using bedside assays. OBJECTIVES: This study was designed to compare the dose-response of enoxaparin, dalteparin and unfractionated heparin (UFH) on the activated clotting time (ACT), and to determine whether the ACT or aPTT can be used to monitor intravenous (IV) low molecular weight heparin (LMWH). METHODS: A total of 130 patients undergoing cardiac catheterization were assigned to intravenous enoxaparin 0.5 mg/kg, dalteparin 50 international units/kg or UFH 50 units/kg. Of the 130 patients, 46 (35%) underwent PCI, all of whom received a glycoprotein (GP) IIb/IIIa inhibitor. We measured ACT, activated partial thromboplastin time (aPTT) and plasma anti-Xa levels after serial sampling. RESULTS: Both enoxaparin and dalteparin induced a significant rise in the ACT and aPTT, with an ACT dose-response approximately one-half the magnitude of that obtained using UFH. The time course of changes in the ACT and aPTT after administration of enoxaparin and dalteparin was virtually identical, with a return to baseline at approximately 2 hours. The enoxaparin and dalteparin-treated patients successfully underwent PCI with no major hemorrhagic complications. CONCLUSIONS: The ACT is equally sensitive to IV enoxaparin and dalteparin. These data support an ACT-guided strategy for intravenously administered LMWH during PCI. Additional studies with larger patient populations may be indicated to determine the ideal target ACT for LMWH in PCI.     

Use of a fixed activated partial thromboplastin time ratio to establish a therapeutic range for unfractionated heparin

BACKGROUND: The commonly recommended therapeutic range for patients receiving unfractionated heparin of 1.5 to 2.5 times the control activated partial thromboplastin time (aPTT) is not universally applicable. It has been suggested that the therapeutic range for each aPTT reagent should be based on plasma heparin levels. We sought to identify an aPTT ratio that corresponds to therapeutic anti--factor Xa heparin levels for combinations of several reagents and coagulometers that are commonly used. METHODS: Citrated plasma was collected from 126 unselected patients receiving unfractionated heparin. Four automated coagulometers and 6 commercial aPTT reagents were used to measure the aPTT. Plasma anti--factor Xa levels were measured by means of a commercially available assay. The relationship between the aPTT results and anti-factor Xa heparin levels for each reagent-coagulometer combination was determined by linear regression analysis, and the aPTT results corresponding to therapeutic anti--factor Xa heparin levels were calculated. RESULTS: For all reagent-coagulometer combinations studied, an aPTT ratio of 1.5 resulted in anti--factor Xa heparin levels considerably below the lower limit of the therapeutic range. When the aPTT was performed on any of the coagulometers assessed with the use of Actin (Dade Diagnostics, Aguada, Puerto Rico) and IL Test (Instrumentation Laboratories, Fisher Scientific, Unionville, Ontario) reagents, aPTT ratios necessary to achieve therapeutic anti--factor Xa heparin levels approximated 2.0 to 3.5. CONCLUSION: For laboratories that cannot perform heparin levels, the use of less responsive reagents and any of the coagulometers studied, along with target aPTT ratio between 2.0 and 3.5, appears to be a reasonable alternative.     

The relative importance of thrombin inhibition and factor Xa inhibition to the antithrombotic effects of heparin

The relative importance of antithrombin and anti-factor Xa activities of heparin fractions required to achieve optimal antithrombotic effects is unknown. To study this, we measured the effects of standard heparin, an octasaccharide heparin fraction (anti-factor Xa activity only), and dermatan sulfate (antithrombin activity only) on the prevention of thrombosis and related this to their anticoagulant effects in vivo in rabbits. Thrombosis was measured as the incorporation of 125I- fibrinogen into tissue thromboplastin-induced thrombi using a Wessler- type model. Ex vivo changes in thrombin clotting time (TCT) were used as an index of antithrombin activity, and a chromogenic anti-factor Xa assay was used to measure anti-factor Xa activity. In addition, the ability of the three sulfated polysaccharides to simultaneously inhibit the generation of thrombin activity and to enhance the inactivation of the factor Xa added to initiate thrombin generation in plasma was determined. Standard heparin, in a dose of 10 anti-factor Xa U/kg, inhibited thrombus formation by 90%, prolonged the TCT by two seconds, and resulted in an anti-factor Xa level of 0.32 U/mL. The octasaccharide heparin fraction, in a dose of 10 anti-factor Xa U/kg, inhibited thrombus formation by 41%, had no effect on the TCT, and resulted in an anti-factor Xa level of 0.28 U/mL. Higher doses of the octasaccharide resulted in a further increase in the anti-factor Xa levels but had no further effect on thrombus formation. Dermatan sulfate, in a dose of 500 micrograms/kg, inhibited thrombus formation by 95%, but had no affect on the TCT. These results indicate that the antithrombotic effect achieved by inhibiting factor Xa is limited and that better antithrombotic effects are achieved by heparin or heparin- like substances capable of influencing the inactivation and/or the generation of thrombin.     

Effects of pre-analytical variables on the anti-activated factor X chromogenic assay when monitoring unfractionated heparin and low molecular weight heparin anticoagulation.

The purpose of this study was to determine whether the anti-activated factor X (anti-FXa) assay is less affected by pre-analytical variables in monitoring patients on unfractionated heparin (UFH) and low molecular weight heparin (LMWH) than the activated partial thromboplastin time (aPTT). Forty-six subjects receiving either enoxaparin (LMWH) or UFH were randomly selected. Each study subject had six vacutainer tubes (3.8% sodium citrate, 3.2% sodium citrate) drawn by an atraumatic venipuncture. One tube from each set had a blood to anticoagulant ratio of 9: 1. The other tube had an intentional "short-draw" of approximately 6: 1 blood to anticoagulant ratio. All specimens had an aPTT and a chromogenic anti-FXa assay performed on each specimen regardless of heparin type. The aPTT assay mean with the 3.8% sodium citrate tube short-draw tube was statistically different from the other aPTT assays (P = 0.06). However, all six of the mean anti-FXa assays for the UFH and LMWH heparin subjects were not statistically or clinically different (analysis of variance, P = 0.9878 for UFH and P = 0.9060 for LMWH). The intentional short-draw tube did not affect the anti-FXa assay regardless of the anticoagulant. The anti-FXa assay appears to be a better method for monitoring heparin subjects than the aPTT due to the lack of effect of pre-analytical variables.     

Determination of the levels of unfractionated and low-molecular-weight heparins in plasma: their effect on thrombin-mediated feedback reactions in vivo. Preliminary results after subcutaneous injection.

We define a standard independent unit (SIU) of heparin as that amount that, in plasma containing 1 mumol of ATIII, raises the (pseudo-)first-order breakdown constant of factor Xa by 1 min-1. These units measure all material with a high affinity for ATIII (HAM); only material above the critical chain length of 17 monosaccharide units (above critical chain length material; ACLM) catalyzes the inactivation of thrombin. An SIU of ACLM is therefore analogously defined as the amount that, in plasma containing 1 mumol of ATIII, will raise the (pseudo-)first-order breakdown constant of thrombin by 1 min-1. Of any given heparin preparation one can determine the specific HAM and ACLM activities in terms of SIU/mg. On the basis of the factor Xa and thrombin breakdown constants found in a plasma sample one can then determine the levels of HAM and ACLM. Preliminary experiments were carried out in plasma samples obtained after subcutaneous injection of unfractionated heparin (UFH) and of two types of low-molecular-weight heparin (LMWH). About three times more of UFH activity than of LMWH activity has to be injected to obtain the same levels of ACLM in the plasma. Only with the LMWHs significant amounts of BCLM are found, which rises higher and persists longer than the ACLM. We determined the course of thrombin generation in platelet-rich plasma (PRP) and in platelet-poor plasma (PPP), as well as in the PPP factor Xa generation curve and the course of prothrombin conversion. The observed inhibitions correlated much better with the levels of ACLM than with those of below critical chain length material. The difference between UFH and LMWHs can therefore not be explained in terms of antithrombin and anti-factor-Xa activity. The essential difference between UFH and LMWH appears in the feedback effect of thrombin in PRP, where thrombin generation is both inhibited and retarded by LMWH, while it is only retarded but hardly inhibited by UFH.     

The effects of unfractionated heparin, low molecular weight heparin and danaparoid on the thromboelastogram (TEG): an in-vitro comparison of standard and heparinase-modified TEGs with conventional coagulation assays.

To investigate the effects of unfractionated heparin (UFH), low molecular weight heparin (LMWH) and danaparoid (DPD) added to whole blood in vitro on standard and heparinase-modified thromboelastogram (TEG) parameters compared with conventional assays of coagulation. The effects of UFH, LMWH and DPD on standard TEG parameters were compared with the prothrombin time, activated partial thromboplastin time, thrombin time and anti-activated factor X (anti-FXa) activity, at concentrations of these anticoagulants ranging from 0.025 to 1 U/ml. In the second part of the study, the effects of very low concentrations (0.005-0.05 U/ml) of UFH, LMWH and DPD on the difference between standard and heparinase-modified TEG parameters were compared with the prothrombin time, activated partial thromboplastin time, thrombin time and anti-FXa activity. Standard TEG parameters were outside the reference range at lower concentrations of UFH, LMWH and DPD than most conventional coagulation assays were able to detect. Only anti-FXa activity was more sensitive to the presence of these anticoagulants than the standard TEG alone. The lowest concentration of UFH, LMWH and DPD used in this study (0.005 U/ml) caused significant differences between the standard and heparinase-modified alpha-angles of the TEG. In addition, the difference between standard and heparinase-modified TEG parameters distinguished between low concentrations (0.005-0.05 U/ml) of UFH with greater sensitivity than anti-FXa activity, but were less sensitive to LMWH and DPD. The standard TEG is more sensitive to UFH, LMWH and DPD than most conventional coagulation tests, with the exception of anti-FXa activity. Calculation of the difference between standard and heparinase-modified TEG parameters greatly increases the sensitivity of the assay for the effects of these anticoagulants, and is more sensitive to very low quantities of UFH than anti-FXa activity.     

急性冠脉综合征应用低分子肝素与普通肝素后血清浓度变化的比较

目的：了解低分子肝素与普通肝素皮下注射后药代动力学变化及临床的关系。方法：急性冠脉综合征患者29例，其中14例患者接受肝素钠6250U，15例患者接受低分子肝素4100U（速避凝0.4ml)腹部皮下注射，注射前及注射后30min,1、2、4、8、12h测血浆肝素浓度（Hep),Xa因子浓度，凝血酶原时间（PT）、凝血酶时间（TT）、部分凝血酶原时间（APTT），以及及纤维蛋白原（FIB）。结果：肝素皮下注射后30min即可见到血浆中肝素浓度上升，作用高峰时间为4h,8h时基本恢复正常（P＜0．01），血浆肝素浓度从小于0．01U／ml上升至4h的0．11U/ml（平均）。同时PT、TT、APTT也同步上长升。肝素与PT、TT、APTT的相关系数分别为0．49、0．723、0．708（P＜0．01），低分子肝素皮下注射后，血浆Xa因子浓度上升较为平稳，1h与8h时的Xa因子浓度无明显差异，12h时血浆中仍有一定浓度的Xa因子，于用药前相比P＜0．05）。但PT、TT、APTT基本不变，与血浆Xa因子无明显关系，结论：肝素皮下注射后，其药代动力学变化比较快，对凝血因子影响因素，但不稳定，低分子肝素皮下注射后，血浆中Xa因子的变化较为稳定，对凝血因子无明显影响。在皮下注射时应首选低分子肝素。     

Haemorrhagic effect of enoxaparin, a low molecular weight heparin. Comparison with unfractionated heparin in humans.

The haemorrhagic effects of unfractionated heparin (UFH) and the low molecular weight heparin (LMWH) enoxaparin were investigated and compared in the gastric mucosa (haemorrhage induced by biopsy) and skin (haemorrhage induced by Simplate) of 12 healthy volunteers. Administration of UFH and LMWH (given in a dose of 75 anti-Xa U/kg intravenously) increased median gastric bleeding time (3.5 min) and geometric mean blood loss (11.5 microliters) to 19 min (p = 0.00003) and 54.1 microliters (p = 0.0021) after UFH and to 13 min (p = 0.008) and 29.0 microliters (p = 0.275) after LMWH. Median skin bleeding time (4.25 min) increased to 6.0 min after UFH (p = 0.003) and to 6.75 min after LMWH (p = 0.0008). Mean heparin activity in plasma was 20% higher after LMWH than after UFH. The calculated gastric bleeding time to heparin activity ratio was significantly lower for LMWH than for UFH (p < 0.05).     

The effect of platelet factor 4 (PF4) on assays of plasma heparin

Platelet factor 4 (PF4) is a potent antiheparin in vitro. In view of the large amount of PF4 secreted from platelet alpha-granules during routine blood collection and processing techniques, the potential significance of this release was investigated using three measurements of heparin activity: the activated partial thromboplastin time (aPTT), the thrombin time, and factor Xa inactivation using the chromogenic substrate S2222 for assay of factor Xa. The results demonstrate that purified PF4 neutralizes heparin activity when added in increasing amounts to normal platelet-poor plasma containing a fixed concentration of commercial porcine gut mucosal heparin. This effect was seen when assaying heparin activity by all three methods. In addition, when heparin was added in increasing concentrations to pooled plasma samples that were collected from normal volunteers, there was neutralization of heparin activity in blood samples collected by routine citrate anticoagulation (CIT60) in comparison to blood samples collected simultaneously with platelet secretion inhibiting agents added to the anticoagulant (CIT+). This effect was seen when assaying heparin by the aPTT and thrombin time. These data confirm that both purified and secreted PF4 have significant antiheparin activity when heparin is added in vitro to normal plasma. Neutralization of circulating heparin by PF4 secreted during blood collection from anticoagulated patients could result in underestimation of the actual in vivo heparin concentration. In order to evaluate the significance of this effect, purified PF4 was added to plasma collected from heparinized patients and again PF4 neutralized heparin activity. This was seen, however, only when heparin activity was measured by the thrombin time or Xa inactivation assays. There was minimal shortening of the aPTT when PF4 was added in final concentrations up to 1000 ng/ml. When blood samples were simultaneously collected from anticoagulated patients by both routine and special collection methods, these results were confirmed. There was a significant difference between heparin activities measured in the CIT+ (secreted PF4 58 ng/ml) and CIT60 (secreted PF4 1074 ng/ml) plasma samples by both thrombin time and Xa inactivation. There was no difference, however, in the aPT when both types of plasma samples were simultaneously collected and assayed for each anticoagulated patient. This suggests that there may be circulating heparin fractions which can prolong the aPTT but which do not interact with PF4.(ABSTRACT TRUNCATED AT 400 WORDS)     

Haemorrhagic Effect of Enoxaparin,a Low Molecular Weight Heparin: Comparison with Unfractionated Heparin in Humans

The haemorrhagic effects of unfractionated heparin (UFH) and the low molecular weight heparin (LMWH) enoxaparin were investigated and compared in the gastric mucosa (haemorrhage induced by biopsy) and skin (haemorrhage induced by Simplate) of 12 healthy volunteers. Administration of UFH and LMWH (given in a dose of 75 anti-Xa U/kg intravenously) increased median gastric bleeding time (3.5 min) and geometric mean blood loss (11.5 l) to 19 min (p = 0.00003) and 54.1 l (p = 0.0021) after UFH and to 13 min (p = 0.008) and 29.0 l (p = 0.275) after LMWH. Median skin bleeding time (4.25 min) increased to 6.0 min after UFH (p = 0.003) and to 6.75 min after LMWH (p = 0.0008). Mean heparin activity in plasma was 20% higher after LMWH than after UFH. The calculated gastric bleeding time to heparin activity ratio was significantly lower for LMWH than for UFH (0.05).     

Therapie der venösen Thromboembolie

The main objectives in the treatment of venous thromboembolism are to prevent clot extension and pulmonary embolism, to reduce mortality and to prevent recurrent thromboembolic events as well as postthrombotic disorders. Initial and effective anticoagulation with heparin, preferably with low molecular weight heparin (LMWH), or with fondaparinux is the most important measure. Unfractioned heparin (UFH) is as effective as LMWH, but requires coagulation-monitoring and is associated with a higher risk of heparin-induced thrombocytopenia. In patients with renal insufficiency direct determination of anti-factor Xa activity and dose adjustment is recommended, since drug accumulation can occur over time. In those patients UFH instead of LMWH might be favored. Long-term treatment should be administered with vitamin K-antagonists (INR-target range 2–3) for a duration of 3 to 6 months. In case of recurrent venous thromboembolism, indefinite therapy is recommended. Additional treatment with compression stockings is reasonable. Patients who do not require hospital treatment for other conditions, who have a low bleeding risk, no excessive venous congestion and no symptomatic pulmonary embolism can safely be treated at home. In most cases bed rest is not necessary. Thrombolysis or surgical thrombectomy is seldomly indicated in severe thromboembolism.