Thrombolytic therapy in acute myocardial infarction. A perspective

Heparin as adjunctive therapy to coronary thrombolysis in acute myocardial infarction

For many years anticoagulation has played a role in the prevention and management of thromboembolic complications associated with acute myocardial infarction. However, the role of heparin therapy after pharmacologic thrombolysis in myocardial infarction remains controversial. Debate continues regarding the necessity of heparin treatment after thrombolytic therapy as well as the mode by which it is administered. The purpose of this review is to summarize the findings of clinical trials designed to evaluate the effectiveness and safety of heparin as an adjuvant agent to thrombolytic therapy in acute myocardial infarction. Data regarding the clinical effectiveness of heparin are presented. Information and recommendations regarding the optimal dose, route of administration, timing of initiation, and duration of heparin treatment are provided.

Thrombolysis in thromboembolic diseases

Thrombolysis in many manifestations of thromboembolic disease offers a valuable alternative to surgery. However, as thrombolysis is always associated with a bleeding hazard (though low) one should always weigh the risks against the expected benefits when the decision for or against this therapeutic option is made. Furthermore, in selecting the appropriate thrombolytic agent, one should be led by the urgency of reperfusion to maintain organ function. If one decides on an aggressive, high-dose, brief-duration regimen, reperfusion may be achieved more rapidly but may be incomplete in the majority of cases. On the other hand, by selecting an intermediate- or long-duration, low-dose regimen, reperfusion may happen too late to improve the patient's prognosis. Above all, one should keep in mind that the hazard of serious bleeding constantly increases with duration of thrombolysis. No matter which strategy is regarded as the best to resolve a clot in a particular patient with a particular type of thromboembolic disease, thrombolysis should be accompanied by high doses of i.v. heparin. Finally, if bleeding occurs in spite of all precautions taken, the new generation of fibrin-specific thrombolytic agents offers the advantage of short half-lives. In addition--in contrast to streptokinase--the hemostatic defect that they cause may be rapidly reversed by the infusion of antagonist drugs such as aprotinin, tranexamic acid, or epsilon-aminocaproic acid. This adds to the clinical safety profile of these thrombolytic agents.

Recent clinical developments in thrombolysis in acute myocardial infarction

Intracoronary streptokinase can accomplish reperfusion in 70 to 75% of patients with acute myocardial infarction (AMI), and intravenous streptokinase in approximately 50% of those with prior documented coronary occlusion. The time constraints for accomplishing significant myocardial salvage have proved to be quite restrictive, however. Studies in which treatment has begun after an average of 4 hours of symptoms have not shown significant improvement in ventricular function. In contrast, those in which intervention has been applied earlier, particularly in less than 2 to 3 hours, have consistently shown benefit. The price for applying thrombolytic therapy includes the risk of severe bleeding (about 5%) but, fortunately, mortality as a result of bleeding has been rare (less than or equal to 0.5%). Reperfusion may be only transient or incomplete (and insufficient). An early reocclusion rate of about 15 to 20% has been commonly noted, in fact. Recently, major studies have pointed to a reduction in early mortality in patients treated early (within about 3 hours) after the onset of symptoms. Much interest is now being focused on developing safer, more effective thrombolytic agents such as tissue plasminogen activator and anisoylated plasminogen streptokinase activator complex (APSAC). Adjunctive therapy with coronary angioplasty is also being applied. In the judgement of many, reperfusion therapy may represent the greatest advance in the approach to AMI of the current decade.