Antithrombotic therapy for acute myocardial infarction

Evolving Role of Low-Molecular-Weight Heparins in ST-Elevation Myocardial Infarction

OBJECTIVE:

To review the available literature on the efficacy and safety of low-molecular-weight heparin (LMWH) in the treatment of ST-elevation myocardial infarction (STEMI) in patients treated with fibrinolytic therapy or conservative medical management.

DATA SOURCES:

A MEDLINE search (1966–March 2004) using the key words myocardial infarction, STEMI, LMWH, enoxaparin, and dalteparin identified pertinent articles. The references of these articles were reviewed for additional pertinent references.

STUDY SELECTION AND DATA EXTRACTION:

All human trials of LMWH in STEMI were evaluated. All pertinent studies were included in the review.

DATA SYNTHESIS:

LMWH did not show a benefit in STEMI without fibrinolytic therapy. Enoxaparin is similar to intravenous unfractionated heparin (UFH) in combination with nonspecific fibrinolytic therapy with regard to invasive reperfusion markers and 30-day clinical outcomes. Enoxaparin decreases composite endpoints in combination with fibrin-specific fibrinolytic therapy compared with UFH, primarily through a reduction in the incidence of reinfarction at 30 days. Bleeding rates with LMWH in combination with fibrinolytic agents are not greater than those with UFH.

CONCLUSIONS:

Enoxaparin is a reasonable alternative to UFH in patients with STEMI treated with fibrin-specific fibrinolytic therapy. LMWH in patients managed with nonspecific fibrinolytic therapy or conservative medical treatment does not provide an advantage over standard management. Large clinical trials are ongoing which will provide more definitive recommendations.

Randomized comparison of direct thrombin inhibition versus heparin in conjunction with fibrinolytic therapy for acute myocardial infarction: results from the GUSTO-IIb Trial. Global Use of Strategies to Open Occluded Coronary Arteries in Acute Coronary Syndromes (GUSTO-IIb) Investigators

OBJECTIVES

We sought to show that hirudin might interact differently with streptokinase (SK) and tissue-type plasminogen activator (t-PA), which could reduce the incidence of death or reinfarction at 30 days.

BACKGROUND

In a large-scale trial of patients with acute coronary syndromes, hirudin provided modest benefit compared with heparin. However, the interaction with thrombolytic agents was not specifically assessed.

METHODS

Patients with symptoms of acute myocardial infarction and electrocardiographic ST segment elevation were treated with thrombolytic therapy and randomly assigned to receive hirudin or heparin.

RESULTS

A total of 2,274 patients received t-PA, and 1,015 received SK. Baseline characteristics were balanced by antithrombin assignment. Among SK-treated patients, death or reinfarction at 30 days occurred more often in those treated with adjunctive heparin (14.4%) rather than hirudin (8.6%, odds ratio [OR] 1.78, 95% confidence interval [CI] 1.20 to 2.66, p = 0.004). Among t-PA-treated patients, the rates were 10.9% with heparin and 10.3% with hirudin (OR 1.06, 95% CI 0.81 to 1.38, p = 0.68; for treatment heterogeneity: chi-square 4.20, degrees of freedom [df] 1, p = 0.04). After adjustment for baseline differences between thrombolytic groups, the rates were 9.1% for SK with hirudin, 10.3% for t-PA with hirudin, 10.5% for t-PA with heparin and 14.9% for SK with heparin (for treatment heterogeneity: chi-square 4.5, df 1, p = 0.03), suggesting that the beneficial treatment effect of hirudin was limited to the SK-treated patients.

CONCLUSIONS

Hirudin interacts favorably with SK but not t-PA, highlighting the importance of thrombin activity after SK therapy and the potential for simulating the effects of a more potent fibrinolytic agent through direct antithrombin therapy.

Heparin as an adjuvant to thrombolytic therapy in acute myocardial infarction

For the treatment of acute myocardial infarction, heparin has been a topic of continuing debate for the past four decades. After review of the available data, the American College of Cardiology/American Heart Association Guidelines for the Early Management of Patients with Acute Myocardial Infarction, published in 1990, recommended intravenous heparin administration together or immediately after thrombolytic therapy to maintain the activated partial thromboplastin time approximately 1.5 to 2.0 times the control value for 24 to 72 hours. Over the past five years, with the proven benefits or thrombolytic therapy and antiplatelet therapy, investigators have been in search of the ideal thrombolytic agent as well as the best adjunctive antithrombotic strategy. We review a number of angiographic patency trials as well as the major thrombolytic mortality reduction trials in which adjunctive heparin therapy was directly assessed. These trials established the need for intravenous heparin administration with tissue plasminogen activator, but, on the other hand, do not substantiate the need for either subcutaneous or intravenous heparin use with streptokinase. New data from a large scale trial emphasizes the importance of maintaining the aPTT in the 55-70 second range to prevent bleeding complications and optimize clinical outcomes.