Prospective evaluation of eligibility for thrombolytic therapy in acute myocardial infarction

Thrombolytic eligibility

The Fibrinolytic Therapy Trialists' Collaborative Group has demonstrated that patients without clear contraindications who present with ischaemic chest pain within 12 hours of the onset of symptoms and who have ST segment elevation or bundle branch block on their electrocardiogram (ECG), will benefit from thrombolytic therapy. Therefore the treatment of patients presenting with ischaemic chest pain is guided by the initial ECG. This paper addresses the question of thrombolytic eligibility in several subsets of patients who may benefit from treatment. It also explores the data which confirm the benefit for patients presenting with an inferior myocardial infarction and for those presenting from six-12 hours after the onset of symptoms. In conclusion, thrombolytic therapy should not be routinely withheld from diabetic or elderly patients, menstruating women and patients who have had cardiopulmonary resuscitation.

New antiplatelet agents

Aspirin is an established therapy for the management of acute myocardial infarction (AMI) and unstable angina. Secondary prevention with chronic aspirin therapy is also indicated for patients with stable angina. Aspirin inhibits cyclo-oxygenase-I, a key enzyme in the biosynthetic pathway leading to the production of thromboxane A2. It therefore inhibits only one of the many activation pathways leading to platelet aggregation. Other antiplatelet agents that have also been evaluated in clinical trials include ticlopidine and clopidogrel, which inhibit adenosine diphosphate-mediated platelet aggregation, but these agents are known to be effective against only one of the 90 known agonists that stimulate platelet aggregation. The final common pathway for platelet aggregation involves the glycoprotein IIb/IIIa receptor combining with fibrinogen. Several inhibitors of the glycoprotein IIb/IIIa receptor have been developed and have an important role as adjunctive therapy in angioplasty. Recent trials have been performed in patients with unstable angina, and trials of adjunctive therapy are currently underway in patients receiving thrombolysis for AMI, and for secondary prevention. These drugs have various different features, including specificity for blockade of the glycoprotein IIb/IIIa receptor, half life, duration of the haemostatic effect and potential for antigenicity. Recently concluded and ongoing trials of both intravenous and oral agents are expected to provide further support for the introduction of these agents into clinical management of patients with acute coronary syndromes.

Issues in extrapolating from clinical trials to clinical practice and outcomes

Extrapolation of clinical trial results to clinical practice requires consideration of whether the trial patients were representative of clinical practice, whether the trial therapy studied was optimal, whether the sample size was adequate, and the impact of adjunctive treatments. In thrombolytic trials in particular, regional variations in attitudes to coronary angiography may have affected outcomes. Clinical trial results need to be interpreted in the light of the cost effectiveness of the application. The assessment of clinical outcomes depends on interplay between the structure and process of care, patient factors and chance. Large standardised databases are necessary to assess clinical practice and outcomes.

Thrombolysis for acute myocardial infarction

Thrombolytic therapy has been a major advance in the management of acute myocardial infarction. Unfortunately, it continues to be underused or is administered later than is optimal. Thrombolytic therapy works by lysing infarct artery thrombi and achieving reperfusion, thereby reducing infarct size, preserving left ventricular function, and improving survival. The most effective thrombolytic regimens achieve angiographic epicardial infarct-artery patency in only approximately 50% of patients within 90 minutes. Bleeding requiring transfusion occurs in approximately 5% of patients and stroke in approximately 1.8% with these regimens, which include adjunctive aspirin and intravenous heparin. There are several ways in which reperfusion rates and thus patient outcomes might be improved, such as different dosing regimens of established agents; combinations of different agents; improved adjunctive therapy such as direct antithrombin agents, low-molecular-weight heparin, or glycoprotein IIb/IIIa receptor antagonists; or the development of novel thrombolytic agents with enhanced fibrin specificity, resistance to native inhibitors, or prolonged half-lives allowing bolus administration. All of these strategies are being tested in clinical trials. The best approach currently is to administer thrombolytic therapy as soon as possible to all patients without contraindications who present within 12 hours of symptom onset and have ST-segment elevation on the ECG or new-onset left bundle-branch block, unless an alternative reperfusion strategy is planned.

Use of reperfusion therapy for acute myocardial infarction in the United States: data from the National Registry of Myocardial Infarction 2

BACKGROUND

There is clear evidence that reperfusion therapy improves survival in selected patients with an acute myocardial infarction. However, several studies have suggested that many patients with an acute myocardial infarction do not receive this therapy. Whether this underutilization occurs in patients appropriate for such therapy remains unclear.

METHODS AND RESULTS

We examined the use of reperfusion therapy in patients with an acute myocardial infarction hospitalized at 1470 hospitals participating in the National Registry of Myocardial Infarction 2. We identified 84 663 patients who were eligible for reperfusion therapy as defined by diagnostic changes on the initial 12-lead ECG, presentation to the hospital within 6 hours from symptom onset, and no contraindications to thrombolytic therapy. Twenty-four percent of these eligible patients did not receive any form of reperfusion therapy (7.5% of all patients). When multivariate analyses were used, left bundle-branch block (odds ratio [OR]=0.22; 95% CI=0.20 to 0.24), lack of chest pain at presentation (OR=0.22; 95% CI=0.21 to 0.24), age >75 years (OR=0.40, 95% CI=0.36 to 0.43), female sex (OR=0.88, 95% CI=0.83 to 0.92), and various preexisting cardiovascular conditions were independent predictors that the patient would not receive reperfusion therapy.

CONCLUSIONS

Reperfusion therapy may be underutilized in the United States. Increased use of reperfusion therapy could potentially reduce the unnecessarily high mortality rates observed in women, the elderly, and other patient groups with the highest risk of death from an acute myocardial infarction.

Change in ST segment elevation 60 minutes after thrombolytic initiation predicts clinical outcome as accurately as later electrocardiographic changes

OBJECTIVE

To compare prospectively the prognostic accuracy of a 50% decrease in ST segment elevation on standard 12-lead electrocardiograms (ECGs) recorded at 60, 90, and 180 minutes after thrombolysis initiation in acute myocardial infarction.

DESIGN

Consecutive sample prospective cohort study.

SETTING

A single coronary care unit in the north of England.

PATIENTS

190 consecutive patients receiving thrombolysis for first acute myocardial infarction.

INTERVENTIONS

Thrombolysis at baseline.

MAIN OUTCOME MEASURES

Cardiac mortality and left ventricular size and function assessed 36 days later.

RESULTS

Failure of ST segment elevation to resolve by 50% in the single lead of maximum ST elevation or the sum ST elevation of all infarct related ECG leads at each of the times studied was associated with a significantly higher mortality, larger left ventricular volume, and lower ejection fraction. There was some variation according to infarct site with only the 60 minute ECG predicting mortality after inferior myocardial infarction and only in anterior myocardial infarction was persistent ST elevation associated with worse left ventricular function. The analysis of the lead of maximum ST elevation at 60 minutes from thrombolysis performed as well as later ECGs in receiver operating characteristic curves for predicting clinical outcome.

CONCLUSION

The standard 12-lead ECG at 60 minutes predicts clinical outcome as accurately as later ECGs after thrombolysis for first acute myocardial infarction.

Unmet therapeutic needs in the management of acute ischemia

Unstable angina and myocardial infarction (MI) continue to present a major challenge in clinical management. These acute ischemic coronary syndromes (AICS) are a spectrum of clinical presentations of the same pathophysiologic mechanism: thrombus formation superimposed on atherosclerotic plaque disruption or erosion. Current approaches to the management of AICS, which include both interventional and pharmacologic therapy, have been introduced to clinical practice during the past 20 years, and most of them have demonstrated efficacy in clinical studies. A common inadequacy of current therapies, however, is the lack of significant inhibition of platelet aggregation--the crucial event in the formation of coronary thrombi and the pathogenesis of AICS. The final common pathway to platelet aggregation is the activation of the platelet glycoprotein (GP) IIb-IIIa receptor, which allows the cross-linking of adjacent platelets by the adhesive plasma proteins fibrinogen and von Willebrand's factor. The emergence of the GP IIb-IIIa receptor as a potential treatment target has led to the development of several inhibitors of its function. The inhibitors most advanced in clinical development are the chimeric monoclonal antibody abciximab (ReoPro) and the cyclic peptide eptifibatide (INTEGRILIN). In phase III clinical trials, both abciximab and eptifibatide have been shown to reduce the incidence of cardiac events in patients at risk for abrupt vessel closure after coronary angioplasty. Inhibition of the GP IIb-IIIa receptor is the most promising novel approach to the treatment of unstable angina and MI, and it may soon be an indispensable component of the management of patients with AICS.

Interventional management of acute myocardial infarction (AMI)

The best way to limit infarct size and improve survival in patients with early heart attacks is to restore as quickly as possible patency in the infarct-related artery and blood flow to the threatened myocardium. The value of thrombolytic therapy and aspirin has been shown in large clinical trials. A regimen of accelerated recombinant tissue plasminogen activator is more effective than those using streptokinase. In older patients, there is a greater risk of haemorrhagic stroke; nevertheless, thrombolytic treatment saves more lives because the mortality of myocardial infarction (MI) is higher. Thrombolytic therapy fails to restore blood flow sufficiently rapidly or completely in nearly one-fifth of patients. Its efficacy, therefore, has been compared with immediate or direct angioplasty (PTCA). If it can be done promptly enough, PTCA is superior in preventing recurrent ischaemia and the combined outcome of death or non-fatal reinfarction, and is associated with a lesser risk of intracranial haemorrhage. It may also be cheaper because patients spend less time in hospital and fewer of them require late revascularisation. PTCA should be considered for patients with cardiogenic shock or for those in whom there is a contraindication to thrombolytic therapy. The benefits of prompt treatment have been reduced by excessive delay in reaching hospital and door-to-needle time. After fibrinolysis, coronary angiography and PTCA may be reserved for those with spontaneous angina or exercise-induced ischaemia.

Treatment of acute myocardial infarction today

The management of acute myocardial infarction (AMI) has undergone major changes in the last decade. Today clinical practice can be based on sound evidence derived from a large number of well-conducted, randomized, large-scale clinical trials. Because of this, Scientific Societies, such as the European Society of Cardiology, have recently produced evidence-based guidelines for the treatment of AMI. This article summarizes the up-to-date, evidence-based treatments for patients with AMI, and their limitations in terms of uncertainty and transferability to real populations.

Thrombolytic treatment for myocardial infarction: an examination of practice in 39 United Kingdom hospitals. Myocardial Infarction Audit Group

OBJECTIVE

To examine use of thrombolytic drugs for myocardial infarction and use of contraindications to treatment in the United Kingdom.

DESIGN

Observational study, based on a continuing audit.

SETTING

39 hospitals in the United Kingdom.

PATIENTS

30,029 patients admitted between November 1992 and June 1995 with suspected myocardial infarction.

RESULTS

Of 13,628 patients with a final diagnosis of definite myocardial infarction 10,316 (75.7%) were considered eligible for thrombolytic treatment on the basis of typical cardiographic changes or new left bundle branch block. Of these, 8139 (59.7%) were diagnosed at admission to hospital and 6991 (85.9%) were administered thrombolytic drugs; 14.1% were considered too late for treatment or had a clinical contraindication. In 2177 patients (16% of 13,628)-thrombolytic treatment was given in the absence of contraindications and after the diagnosis of infarction had been confirmed by further electrocardiographic evidence. A further 591 (4.3%) with a final diagnosis of definite infarction without typical cardiographic changes also received thrombolytic treatment as did 1018 patients without a final diagnosis of definite infarction. In total, 9459 of 13,628 patients (71.6%) received thrombolytic treatment. The range of use of treatment between hospitals for a final diagnosis of infarction was 49.1-85.4%. This variation reflected differences in the frequency with which a diagnosis of definite myocardial infarction was made at admission, and the subsequent use of clinical contraindications to thrombolytic treatment.

CONCLUSIONS

75.7% of patients with a final diagnosis of definite myocardial infarction were eligible for thrombolytic treatment on the basis of cardiographic changes. Differences between hospitals in the frequency with which a diagnosis of infarction was made on admission, and differences in subsequent use of thrombolytic drugs, results in wide variation in treatment rates. Differences in use of thrombolytic treatment mainly reflect different thresholds for the use of clinical contraindications relating to haemorrhagic risk.