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Clinical Cardiology |

Use of Low-Molecular-Weight Heparins in the Management of Acute Coronary Artery Syndromes and Percutaneous Coronary Intervention FREE

Graham C. Wong, MD; Robert P. Giugliano, MD, SM; Elliott M. Antman, MD
[+] Author Affiliations

Author Affiliations: TIMI Study Group, Brigham and Women's Hospital, Boston, Mass.


Clinical Cardiology Section Editor: Michael S. Lauer, MD, Contributing Editor.


JAMA. 2003;289(3):331-342. doi:10.1001/jama.289.3.331.
Text Size: A A A
Published online

Context Low-molecular-weight heparins (LMWHs) possess several potential pharmacological advantages over unfractionated heparin as an antithrombotic agent.

Objective To systematically summarize the clinical data on the efficacy and safety of LMWHs compared with unfractionated heparin across the spectrum of acute coronary syndromes (ACSs), and as an adjunct to percutaneous coronary intervention (PCI).

Data Sources We searched MEDLINE for articles from 1990 to 2002 using the index terms heparin, enoxaparin, dalteparin, nadroparin, tinzaparin, low molecular weight heparin, myocardial infarction, unstable angina, coronary angiography, coronary angioplasty, thrombolytic therapy, reperfusion, and drug therapy, combination. Additional data sources included bibliographies of articles identified on MEDLINE, inquiry of experts and pharmaceutical companies, and data presented at recent national and international cardiology conferences.

Study Selection We selected for review randomized trials comparing LMWHs against either unfractionated heparin or placebo for treatment of ACS, as well as trials and registries examining clinical outcomes, pharmacokinetics, and/or phamacodynamics of LMWHs in the setting of PCI. Of 39 studies identified, 31 fulfilled criteria for analysis.

Data Extraction Data quality was determined by publication in the peer-reviewed literature or presentation at an official cardiology society–sponsored meeting.

Data Synthesis The LMWHs are recommended by the American Heart Association and the American College of Cardiology for treatment of unstable angina/non–ST-elevation myocardial infarction. Clinical trials have demonstrated similar safety with LMWHs compared with unfractionated heparin in the setting of PCI and in conjunction with glycoprotein IIb/IIIa inhibitors. Finally, LMWHs show promise as an antithrombotic agent for the treatment of ST-elevation myocardial infarction.

Conclusions The LMWHs could potentially replace unfractionated heparin as the antithrombotic agent of choice across the spectrum of ACSs. In addition, they show promise as a safe and efficacious antithrombotic agent for PCI. However, further study is warranted to define the benefit of LMWHs in certain high-risk subgroups before their use can be universally recommended.

Figures in this Article

The inciting event in an acute coronary syndrome (ACS) typically involves disruption of a vulnerable atherosclerotic plaque with superimposed thrombosis, leading to varying degrees of occlusion of the culprit artery.13 ST-elevation myocardial infarction (STEMI) is usually associated with complete thrombotic occlusion of the culprit artery,4 while nonocclusive thrombus is the typical finding associated with unstable angina/non–ST-elevation myocardial infarction (UA/NSTEMI).5,6 Tissue factor exposed following plaque rupture leads to activation of the coagulation cascade and generation of factor Xa (Figure 1).7 Thrombin is formed, which leads to fibrin deposition, platelet activation, and ultimately the formation of a stable clot.8

Given the central role of thrombin in the pathogenesis of ACSs, an antithrombotic agent is an important element in therapy, regardless of whether ST elevation is present. However, there are important limitations associated with unfractionated heparin (currently the most commonly used antithrombotic agent), which has prompted a search for alternative compounds.9 One promising class of agents is the low-molecular-weight heparins (LMWHs), which offer potential advantages in terms of clinical efficacy, safety, and ease of use in the setting of ACS.

Figure 1. Differential Effects of Unfractionated Heparin and Low-Molecular-Weight Heparin
Graphic Jump Location
In acute coronary syndromes, thrombosis is typically initiated via exposure of tissue factor at the site of a disrupted plaque. The complex of tissue factor and Factor VIIa activates Factor X to Xa. Factor Xa subsequently participates in the prothrombinase complex (Factor Xa, Factor Va, Ca++, and a phospholipid membrane, usually from an activated platelet). The tissue factor/VIIa complex also activates Factor IX to IXa, which helps maintain the thrombotic process. Both unfractionated heparin (UFH) and low-molecular-weight heparins (LMWHs) act by complexing with antithrombin III. LMWHs have a relatively greater inhibitory effect on Factor Xa than on factor IIa (thrombin), such that the anti-Xa:IIa inhibitory ratio is greater than 1. This produces greater upstream inhibition of the coagulation cascade and relatively greater reduction of thrombin generation than when unfractionated heparin is administered. Unfractionated heparin has an anti-Xa:IIa inhibitory ratio of 1. While it is theoretically possible to achieve similar anti-Xa activity with unfractionated heparin as with LMWH, because of poor bioavailability very high doses of unfractionated heparin are required that would result in unacceptably high anti-IIa levels and an increased risk of bleeding. Both unfractionated heparin and LMWHs also inhibit coagulation through release of tissue factor pathway inhibitor (TFPI) from endothelial cells. However, unlike unfractionated heparin, LMWHs do not cause TFPI depletion, which has been associated with a paradoxical prothrombotic state during anticoagulant therapy in the setting of acute coronary syndromes.

Studies for this review were identified using MEDLINE searches, reviewing reference lists, and conferring with experts and pharmaceutical companies. The medical subject headings used were heparin, enoxaparin, dalteparin, nadroparin, tinzaparin, low molecular weight heparin, myocardial infarction, unstable angina, coronary angioplasty, thrombolytic therapy, reperfusion,, and drug therapy, combination. In addition, relevant abstracts from the annual meetings of the American Heart Association (AHA), American College of Cardiology (ACC), and European Society of Cardiology were reviewed. We selected for review randomized clinical trials that compared a LMWH with either unfractionated heparin or placebo for the treatment of STEMI and UA/NSTEMI. We also included nonrandomized trials and registries of LMWH that examined clinical outcomes, and/or pharmacokinetics and phamacodynamics in the setting of percutaneous coronary intervention (PCI). Data quality was determined by publication in a peer-reviewed journal or presentation at an official cardiology society–sponsored meeting. Of 39 studies identified, 31 fulfilled criteria for analysis.

Understanding of the pharmacological characteristics of LMWHs has evolved since previous reviews.10,11 The LMWHs act more proximally on the coagulation cascade than unfractionated heparin and more effectively inhibit thrombin generation (Figure 1). Increasing evidence also suggests that LMWHs may differ from unfractionated heparin in other thrombin-dependent and thrombin-independent mechanisms (Table 1). These differences may explain the different pharmacological profiles of LMWH and unfractionated heparin in the setting of ACSs and PCI.

Table Graphic Jump LocationTable 1. Limitations of Unfractionated Heparin

Although anti-Xa monitoring is not commonly performed nor recommended in routine clinical practice,12,13 it has been suggested that monitoring of LMWH activity may be of benefit in certain high-risk patient subgroups in which the optimal dose of LMWH has not been established.14 These include patients at weight extremes, patients with renal insufficiency (prolongation of anti-Xa activity),15 and patients who are pregnant (lower than normal anti-Xa activity for a given LMWH).16

Non–ST-Elevation ACSs (UA/NSTEMI)

The LMWHs have been compared with unfractionated heparin for the treatment of UA/NSTEMI in 4 large, prospective randomized trials.1720 Several meta-analyses comparing unfractionated heparin with LMWH have already demonstrated the superiority of LMWH over placebo2123 and the superiority of enoxaparin over unfractionated heparin21,22 (Figure 2).

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Figure 2. Effects of Low-Molecular-Weight Heparins in Unstable Angina on the Combined End Point of Death, Myocardial Infarction, and Recurrent Ischemia With or Without Revascularization
Graphic Jump Location
ESSENCE indicates Efficacy and Safety of Subcutaneous Enoxaparin in Non-Q-Wave Coronary Events; FRAXIS, Fraxiparine in Ischaemic Syndrome; FRIC, Fragmin in Unstable Coronary Artery Disease; LMWH, low-molecular-weight heparin; TIMI 11B, Thrombolysis in Myocardial Infarction; and UFH, unfractionated heparin. Reproduced with permission.57

Key differences in clinical trial design, patient selection, and characteristics of the preparations make comparisons among LMWHs difficult. Only 1 trial has compared 2 LMWH preparations in the setting of UA/NSTEMI. The Enoxaparin Versus Tinzaparin (EVET) trial randomized 438 patients presenting with UA/NSTEMI to either enoxaparin (subcutaneous injection of 1 mg/kg twice daily; n = 220) or tinzaparin (subcutaneous injection of 175 IU/kg daily; n = 318).24 The composite primary end point (death, reinfarction, or recurrent angina) was lower with enoxaparin compared with tinzaparin at day 7 (12.3% vs 21.1%; P = .02). This difference, which persisted to 30 days, was driven almost entirely by a reduction in recurrent angina (at 7 days: 11.8% vs 19.3% [P = .03]; at 30 days: 17.3% vs 26.1% [P = .02]). Major bleeding was uncommon and not statistically different between the enoxaparin and tinzaparin groups (3.6% vs 3.2%).

Combination of LMWH and Glycoprotein IIb/IIIa Inhibitors for the Initial Medical Management of ACSs

The combination of enoxaparin and tirofiban resulted in a trend toward a greater proportion of patients achieving more than 70% inhibition of platelet aggregation compared with the combination of unfractionated heparin and tirofiban in the Antithrombotic Combination Using Tirofiban and Enoxaparin (ACUTE I) trial (84% vs 65%),25 suggesting a possible synergistic effect on platelet inhibition from LMWH and glycoprotein IIb/IIIa inhibitors. This combination has since been evaluated in 5 trials of initial medical therapy for UA/NSTEMI (Table 2 and Table 3). The major finding was that major hemorrhage occurred rarely in the LMWH and glycoprotein IIb/IIIa inhibitor arms (0.3%-1.8%). These rates compare favorably with the overall 2.4% rate of major bleeding calculated in a meta-analysis of glycoprotein IIb/IIIa inhibitors and unfractionated heparin for the treatment of ACSs.26 Furthermore, no increase in bleeding was noted among UA/NSTEMI patients who proceeded to PCI. Although not powered to detect differences in clinical events, rates of ischemic events in these trials were noted to be similar between the LMWH and unfractionated heparin groups.

Table Graphic Jump LocationTable 2. Characteristics of Trials of Low-Molecular-Weight Heparin and Glycoprotein IIb/IIIa Inhibitor Combination for the Treatment of Unstable Angina/Non–ST-Elevation MI
Table Graphic Jump LocationTable 3. Clinical Efficacy and Major Non-CABG Bleeding in Trials Combining Low-Molecular-Weight Heparin and Glycoprotein IIb/IIIa Inhibitor Combination for the Treatment of Unstable Angina/Non−ST-Elevation MI

The combination of tirofiban and enoxaparin was studied in 1224 patients presenting with STEMI in the Treatment of Enoxaparin and Tirofiban in Acute Myocardial Infarction (TETAMI) trial. Patients ineligible for fibrinolysis were randomized in a 2 × 2 fashion to receive either enoxaparin (intravenous 30-mg bolus and subcutaneous injection of 1 mg/kg twice daily) or unfractionated heparin (intravenous 70 U/kg bolus and 15 U/kg per hour infusion) with or without tirofiban (intravenous 10-µg/kg bolus and 0.1-µg/kg per minute infusion) for 2 to 8 days.27 There were no differences noted in the primary efficacy end point between either enoxaparin and unfractionated heparin monotherapy groups (15.4% vs 17.3%) or between enoxaparin and unfractionated heparin combination groups (16.1% vs 17.2%). Major bleeding was rare and not statistically different among all 4 groups.

Duration of Therapy of LMWH in UA/NSTEMI

Evidence suggests that the prothrombotic state associated with an ACS may persist for several months following the index event,2830 providing a biologically plausible rationale for prolonged antithrombotic therapy in this setting. The LMWHs are attractive as long-term antithrombotic agents by virtue of their safety profile and ease of use, prompting several trials to evaluate their use in the outpatient setting following an acute coronary event.1820,31

Although the clinical benefit of inhospital LMWH therapy is maintained up to 1 year following the index event32 (hazard ratio, 0.88; 95% confidence interval, 0.81-0.97; P = .008 for the combined end point of death, MI, or urgent revascularization), no additional benefit from use of LMWH beyond hospital discharge has been convincingly demonstrated among UA/NSTEMI patients.17,18,31,33 In addition, an increase in bleeding with prolonged LMWH treatment has been noted.17

Prolonged treatment with subcutaneous injections of dalteparin in Fragmin and Fast Revascularization During Instability in Coronary Artery Disease (FRISC II) did significantly lower the overall risk of death, MI, and revascularization at 30 days compared with placebo (3.1% vs 5.9%; P = .002).33 However, this benefit was not sustained at 6 months. Because this difference was observed only in patients randomized to the conservative arm of FRISC II,34 the investigators suggested that prolonged antithrombotic therapy may offer benefit as a medical bridge to PCI in patients for whom invasive procedures are delayed.

Benefit of LMWHs in High-Risk Subgroups

A statistically significant benefit of prolonged dalteparin treatment compared with placebo was demonstrated in the subgroup of patients in FRISC and FRISC II who presented with elevated baseline troponin-T levels.35,36 Moreover, an analysis of Efficacy and Safety of Subcutaneous Enoxaparin in Unstable Angina and Non–Q-Wave MI/Thrombolysis in MI (ESSENCE/TIMI 11B) demonstrated that compared with unfractionated heparin, enoxaparin blunted the increase in event rates associated with higher baseline risk,37 and among those who underwent invasive therapy.38 However, the effects of early mechanical revascularization may minimize any early differences attributable to prolonged antithrombotic therapy.34

ST-Elevation Myocardial Infarction

Necessity for Adjunctive Antithrombotic Therapy in Thrombolytic Regimens. The effectiveness of pharmacological reperfusion for STEMI depends on the balance between fibrinolytic and prothrombotic activity. Fibrinolysis enhances the prothrombotic state associated with STEMI by promoting the generation and release of thrombin following successful clot lysis,3941 and activating the coagulation cascade, leading to further platelet activation.4244 Therefore, there is sound theoretical rationale to support the use of concurrent antithrombotic therapy to enhance the process of fibrinolysis in STEMI.

Results of Randomized Clinical Trials of LMWH in STEMI. To date, 8 phase 2 trials and 2 large exploratory clinical trials have been completed that evaluate LMWH with fibrinolytic therapy in STEMI (Table 4 and Table 5). Infarct-related arterial patency following fibrinolysis has been evaluated in 3 trials with enoxaparin (Acute MI-Streptokinase [AMI-SK],45 second trial of Heparin and Aspirin Reperfusion Therapy [HART II46], and Enoxaparin Antithrombin Therapy for ST-Elevation Thrombolysis in MI [ENTIRE-TIMI 2347]) and 1 trial with dalteparin (second trial of Biochemical Markers in ACSs [BIOMACS II48]). Use of adjunctive LMWH resulted in improved late coronary artery patency rates (3%-16% absolute improvement in TIMI 2 flow4547 and TIMI 3 flow49) and a tendency toward higher TIMI 3 flow rates (1%-17% absolute improvement4549) compared with unfractionated heparin (Table 5). Adjunctive LMWH may also be associated with improved tissue level perfusion following fibrinolysis assessed using ST-segment resolution and other noninvasive composite measurements of reperfusion seen with enoxaparin in the AMI-SK45 and Accelerated Streptokinase and Enoxaparin (ASENOX50) trials, respectively. Rates of other significant clinical events such late infarct-related arterial reocclusion45,46,49 and recurrent ischemia47,48,51 were reduced with LMWH compared with unfractionated heparin. Finally, similar bleeding rates were found with LMWH compared with unfractionated heparin in the majority of completed trials (Table 5).

Table Graphic Jump LocationTable 4. Characteristics of Trials Using LMWH in ST-Elevation Myocardial Infarction
Table Graphic Jump LocationTable 5. Clinical Efficacy and Major Bleeding in Trials of LMWH in ST-Elevation Myocardial Infarction

However, preliminary results presented at the AHA 2002 Scientific Sessions from the third Assessment of the Safety and Efficacy of a New Thrombolytic PLUS (ASSENT 3 PLUS) underscore the need for continued evaluation of the safety of LMWH as an adjunct to fibrinolysis.52 Among 1639 patients with STEMI receiving tenecteplase and either enoxaparin or unfractionated heparin in a prehospital setting, higher rates of both major bleeding (4.0% vs 2.8%) and intracranial hemorrhage (2.2% vs 1.0%; P = .05) were seen in the enoxaparin group compared with the unfractionated heparin group. There was a significant interaction between patient age and risk of bleeding because almost all cases of intracerebral hemorrhage were confined to patients older than 75 years.

This safety concern of enoxaparin among elderly patients will be addressed in the ongoing (ExTRACT-TIMI 25) trial. This double-blind, parallel group trial is randomizing approximately 21 000 fibrinolysis-eligible STEMI patients in a 1:1 fashion to receive either adjunctive unfractionated heparin (intravenous 60-IU/kg bolus and 12-IU/kg infusion per hour) or enoxaparin (intravenous 30-mg bolus and a subcutaneous injection of 1 mg/kg twice daily). Of note, patients older than 75 years will not receive the intravenous bolus of enoxaparin and will receive only 75% of the subcutaneous injection dose. The primary end point is a composite of all-cause mortality and nonfatal reinfarction within 30 days of randomization.

Low-Molecular-Weight Heparin and PCI

The issue of LMWH treatment in combination with PCI is becoming increasingly relevant given the results of several studies demonstrating the superiority of an invasive approach for the management of both UA/NSTEMI34,53 and STEMI.54 Compared with the large amount of literature evaluating LMWHs for the medical management of UA/NSTEMI, there are fewer data evaluating LMWHs in the invasive setting. Accordingly, while the ACC, AHA, European Society of Cardiology, and the American College of Chest Physicians have all acknowledged the safety and potential pharmacological advantages of LMWH over unfractionated heparin,5557 none of these organizations have endorsed the use of LMWH as the preferred antithrombotic agent in the invasive management of UA/NSTEMI. Nevertheless, data suggest that LMWH monotherapy is safe and efficacious in the setting of elective and urgent PCI (Table 6 and Table 7). In addition, several trials have also demonstrated the safety and efficacy using LMWH in combination with a glycoprotein IIb/IIIa inhibitor (Table 8 and Table 9).

Table Graphic Jump LocationTable 6. Characteristics of Trials Using Low-Molecular-Weight Heparin Alone in PCI*
Table Graphic Jump LocationTable 7. Major Bleeding and Event Rates of Trials Using Low-Molecular-Weight Heparin Alone in PCI*
Table Graphic Jump LocationTable 8. Characteristics of Trials of Low-Molecular-Weight Heparin in Combination With Glycoprotein IIb/IIIa Inhibitors in PCI*
Table Graphic Jump LocationTable 9. Clinical Efficacy and Major Bleeding in Trials of LMWH in Combination With Glycoprotein IIb/IIIa Inhibitors in PCI*

Unfractionated heparin retains several attractive properties compared with LMWH in the invasive setting, including low cost, complete reversibility with protamine, and the availability of an easily interpretable and standardized point-of-care test (activated clotting time).58 In contrast, protamine is able to fully reverse the effect of LMWH on thrombin, but only 60% of its anti-Xa activity.59 It remains controversial whether routine monitoring of anticoagulation is needed with LMWH in the catheterization laboratory.12,13,60 Moreover, the optimal level of anticoagulation for LMWH during invasive cardiac procedures has not been prospectively defined.

Since LMWH does not appreciably affect either the activated partial thromboplastin time or activated clotting time, clinical trials have focused on the anti-Xa activity as a surrogate to assess the level of anticoagulation. Although now commercially available, routine monitoring of anticoagulation with LMWH has not been universally adopted. The currently accepted therapeutic activity of LMWH in PCI (anti-Xa, >0.5-1.8 IU/mL) is based primarily on clinical data from the deep venous thrombosis literature60 and dose-ranging studies in UA/NSTEMI.61 In TIMI 11A, enoxaparin administered as an intravenous 30-mg bolus followed by a subcutaneous injection of 1 mg/kg twice daily generated trough anti-Xa activity of 0.5 IU/mL and peak activity of 1.1 IU/mL, with a low rate (2.7%) of major hemorrhage among patients undergoing PCI.61 Several trials have demonstrated that reliable levels of anti-Xa activity consistently greater than the 0.5 IU/mL threshold may be obtained using various doses of LMWH when administered several days or immediately before PCI (Table 10).

Table Graphic Jump LocationTable 10. Anticoagulant Profiles of LMWH in Elective and Urgent PCI

From the available data, 2 strategies have emerged guiding the use of LMWH (with or without concomitant glycoprotein IIb/IIIa inhibition) for PCI. First, unfractionated heparin has been substituted for LMWH prior to angiography and coronary intervention if the last LMWH dose was given more than 8 to 12 hours before the procedure.34,38,62,63 Second, a number of pilot trials have evaluated the use of LMWH as the sole anticoagulant throughout angiography and intervention without changing to unfractionated heparin.25,6473 However, these 2 strategies have not been directly compared. With the limited data available, both approaches appear to be safe and efficacious, with bleeding and clinical event rates similar to historical controls (Table 7 and Table 9). Ongoing large trials (ie, A to Z,74 SYNERGY75) will yield additional information on the safety and efficacy of enoxaparin and glycoprotein IIb/IIIa inhibition in the PCI setting.

Current Recommendations and Observations From Clinical Trials

Unstable Angina/Non–ST-Elevation MI. Based on the available evidence, the 2002 ACC and AHA guidelines list the use of either LMWH or unfractionated heparin as a class I recommendation for the treatment of UA/NSTEMI (level of evidence, A).57 In addition, use of enoxaparin is preferred over unfractionated heparin (class IIA; level of evidence, A), except if coronary artery bypass graft surgery is anticipated within 24 hours.57

ST-Elevation MI. Completed trials of adjunctive LMWH compared with either placebo or unfractionated heparin in the setting of STEMI have demonstrated lower rates of late ischemic events and improved rates of late coronary artery patency. The increased bleeding seen with prehospital enoxaparin in ASSENT-3 PLUS requires further study and possible dose modification in elderly patients, although it appears LMWHs are safe among patients younger than 75 years. ExTRACT-TIMI 25, ADVANCE-MI, and FINESSE will be able to provide more definitive data regarding the utility of enoxaparin in modern reperfusion regimens in all age groups.

Percutaneous Coronary Intervention

Although an expert consensus statement has been published with recommendations of LMWH in the setting of PCI,60 the most recent guidelines from the ACC and AHA do not discuss the use of LMWH as the sole anticoagulant in the setting of PCI.76 Moreover, the optimal level of anticoagulation has not been prospectively validated, although similar efficacy and safety outcomes with LMWH compared with unfractionated heparin (with and without concomitant glycoprotein IIb/IIIa inhibitors) have been shown with a target anti-Xa level of higher than 0.5 IU/mL.

Low-molecular-weight heparin may be used as the sole anticoagulant during PCI. Dalteparin should be given at a dose of 120 IU/kg subcutaneously twice daily provided PCI is performed within 8 hours.34 If PCI is to be performed more than 8 to 12 hours following the last subcutaneous injection dose, patients should be given an additional 60-IU/kg intravenous bolus of dalteparin.70 Enoxaparin may be given subcutaneously at a dose of 1 mg/kg twice daily without the need for further supplementation if PCI is performed within 8 hours.38,67,68 An additional 0.3-mg/kg intravenous bolus should be given if PCI is performed between 8 and 12 hours after the last subcutaneous dose.71,72 Subcutaneous enoxaparin should be reduced for patients with severe renal insufficiency (creatinine clearance <30 mL/min [<0.501 mL/s]). A single 0.5-mg/kg intravenous dose of enoxaparin without further subcutaneous dosing may be safe and efficacious in the setting of moderate-to-severe renal failure.73 Alternatively, unfractionated heparin may be substituted for enoxaparin if PCI is to be performed more than 8 to 12 hours following the last subcutaneous dose of enoxaparin.38

Low-molecular-weight heparin can also be used in conjunction with glycoprotein IIb/IIIa inhibitors. Data from several clinical trials suggest that the combination of enoxaparin with various glycoprotein IIb/IIIa inhibitors66,68,69,77 and dalteparin with abciximab70 are safe in the setting of PCI. More definitive information will be forthcoming about the combination of enoxaparin and glycoprotein IIb/IIIa inhibitors during PCI.74,75

The use of LMWH has been established as a first-line choice in the treatment of UA/NSTEMI, and increasing evidence suggests that it may supplant unfractionated heparin as an anticoagulant in the setting of PCI and STEMI. However, more clinical data regarding the safety of these agents in elderly patients and in combination with potent platelet inhibitors such as thienopyridines and glycoprotein IIb/IIIa inhibitors are needed before their routine adoption in the setting of STEMI and PCI.

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Figures

Figure 1. Differential Effects of Unfractionated Heparin and Low-Molecular-Weight Heparin
Graphic Jump Location
In acute coronary syndromes, thrombosis is typically initiated via exposure of tissue factor at the site of a disrupted plaque. The complex of tissue factor and Factor VIIa activates Factor X to Xa. Factor Xa subsequently participates in the prothrombinase complex (Factor Xa, Factor Va, Ca++, and a phospholipid membrane, usually from an activated platelet). The tissue factor/VIIa complex also activates Factor IX to IXa, which helps maintain the thrombotic process. Both unfractionated heparin (UFH) and low-molecular-weight heparins (LMWHs) act by complexing with antithrombin III. LMWHs have a relatively greater inhibitory effect on Factor Xa than on factor IIa (thrombin), such that the anti-Xa:IIa inhibitory ratio is greater than 1. This produces greater upstream inhibition of the coagulation cascade and relatively greater reduction of thrombin generation than when unfractionated heparin is administered. Unfractionated heparin has an anti-Xa:IIa inhibitory ratio of 1. While it is theoretically possible to achieve similar anti-Xa activity with unfractionated heparin as with LMWH, because of poor bioavailability very high doses of unfractionated heparin are required that would result in unacceptably high anti-IIa levels and an increased risk of bleeding. Both unfractionated heparin and LMWHs also inhibit coagulation through release of tissue factor pathway inhibitor (TFPI) from endothelial cells. However, unlike unfractionated heparin, LMWHs do not cause TFPI depletion, which has been associated with a paradoxical prothrombotic state during anticoagulant therapy in the setting of acute coronary syndromes.
Graphic Jump Location
Figure 2. Effects of Low-Molecular-Weight Heparins in Unstable Angina on the Combined End Point of Death, Myocardial Infarction, and Recurrent Ischemia With or Without Revascularization
Graphic Jump Location
ESSENCE indicates Efficacy and Safety of Subcutaneous Enoxaparin in Non-Q-Wave Coronary Events; FRAXIS, Fraxiparine in Ischaemic Syndrome; FRIC, Fragmin in Unstable Coronary Artery Disease; LMWH, low-molecular-weight heparin; TIMI 11B, Thrombolysis in Myocardial Infarction; and UFH, unfractionated heparin. Reproduced with permission.57

Tables

Table Graphic Jump LocationTable 1. Limitations of Unfractionated Heparin
Table Graphic Jump LocationTable 2. Characteristics of Trials of Low-Molecular-Weight Heparin and Glycoprotein IIb/IIIa Inhibitor Combination for the Treatment of Unstable Angina/Non–ST-Elevation MI
Table Graphic Jump LocationTable 3. Clinical Efficacy and Major Non-CABG Bleeding in Trials Combining Low-Molecular-Weight Heparin and Glycoprotein IIb/IIIa Inhibitor Combination for the Treatment of Unstable Angina/Non−ST-Elevation MI
Table Graphic Jump LocationTable 4. Characteristics of Trials Using LMWH in ST-Elevation Myocardial Infarction
Table Graphic Jump LocationTable 5. Clinical Efficacy and Major Bleeding in Trials of LMWH in ST-Elevation Myocardial Infarction
Table Graphic Jump LocationTable 6. Characteristics of Trials Using Low-Molecular-Weight Heparin Alone in PCI*
Table Graphic Jump LocationTable 7. Major Bleeding and Event Rates of Trials Using Low-Molecular-Weight Heparin Alone in PCI*
Table Graphic Jump LocationTable 8. Characteristics of Trials of Low-Molecular-Weight Heparin in Combination With Glycoprotein IIb/IIIa Inhibitors in PCI*
Table Graphic Jump LocationTable 9. Clinical Efficacy and Major Bleeding in Trials of LMWH in Combination With Glycoprotein IIb/IIIa Inhibitors in PCI*
Table Graphic Jump LocationTable 10. Anticoagulant Profiles of LMWH in Elective and Urgent PCI

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