Clinical implications of the new definition of myocardial infarction
2003; BMJ; Volume: 90; Issue: 1 Linguagem: Inglês
10.1136/heart.90.1.99
ISSN2053-5864
Autores Tópico(s)Cardiac electrophysiology and arrhythmias
Resumoyocardial infarction (MI) can be considered from several perspectives: clinical, electrocardiographic, biochemical, pathological, epidemiological, and imaging.The diagnosis of MI has psychological, social, and legal implications.MI is often used as a major end point in clinical trials. WORLD HEALTH ORGANIZATION (WHO) DEFINITION cHistorically, there has been tacit agreement as to the meaning of the term ''myocardial infarction''.The World Health Organization definition, which has been widely used, requires the presence of two of the following three features: symptoms of myocardial ischaemia, elevation of cardiac marker (enzyme) concentrations in the blood, and a typical electrocardiographic pattern involving the development of Q waves or persistent T wave changes.w1 Using specific and highly sensitive immunoassays for myocardial proteins, such as cardiac troponins T and/or I, it is now possible to identify patients with small areas of myocardial necrosis.The emphasis on cardiac protein markers in the new American College of Cardiology/ European Society of Cardiology (ACC/ESC) definition of MI, published in September 2000, has simplified the classification of MI. 1 The new diagnostic criteria include a characteristic rise and fall in blood concentrations of cardiac troponins and/or creatine kinase (CK)-MB in the context of spontaneous ischaemic symptoms or coronary intervention (table 1). 1 If it is accepted that any myocardial necrosis caused by ischaemia constitutes MI, many patients who were formerly diagnosed as having unstable angina pectoris will be now diagnosed as having had a small MI.For example, in a review of data from the Hennepin County Medical Centre (Minnesota, USA), the incidence of MI increased by 37% when the new definition of MI was applied.w2 However, the specificity of the new tests will reduce the number of false positive diagnoses of MI.Under the WHO classification, which was expanded for the monitoring trends and determinants in cardiovascular disease (MONICA) epidemiological study, w3 patients could be classified as having definite or possible MI, prolonged angina pectoris, or ischaemic cardiac arrest.This system divided patients into six symptomatic categories, 27 electrocardiographic categories, and six enzymatic categories in 10 different combinations.The new ACC/ESC definition makes it possible to classify patients on the basis of seven clinical or pathological scenarios.The limitations of the new definition of MI include the lack of a definition of cardiac arrest, and the lack of an MI classification for patients who present with characteristic symptoms of MI but die within 4-6 hours of symptom onset, w4 a window in which cardiac markers, the ECG, and histological findings (which take some hours to develop) may be non-diagnostic.Cardiac arrest and sudden cardiac death have different meanings for clinicians, epidemiologists, biochemists, and pathologists.w5 However, such events were also difficult to classify under the WHO-MONICA definition.w3 We believe that new definitions of MI are needed for patients suffering myocardial necrosis following coronary artery bypass grafting (CABG), and for those with silent MIs, w6 aborted MIs, w7 and threatened MIs; the latter group includes those with a thrombotic occlusion of the infarct artery without cardiac marker elevation.Clinical features c Spontaneous ischaemic episode (usually) lasting.20 minutes c Coronary artery intervention Biochemistry c The preferred cardiac markers are troponin I or T because of their specificity c CK-MB has lower specificity than troponins T and I, but may be used c Myoglobin or CK-MB isoforms should be considered for rapid diagnosis c Total CK, aspartate transaminase (serum glutamate oxaloacetate transaminase) and LDH have low specificity and are less satisfactory c Elevation of troponin or CK-MB is defined as a value exceeding the 99th centile of a reference control group c Sampling of troponins or CK-MB should be done at presentation, at 6-9 hours, and at 12-24 hours.Electrocardiography c Electrocardiographic criteria are not specific enough to identify non-ST elevation MI c ST elevation MI is indicated by new ST elevation in at least two contiguous leads, measuring >0.2 mV in leads V1-V3, or >0.1 mV in all other leads c Established MI (in the absence of confounders) is indicated by any Q wave in leads V1-V3 or by Q waves of >1 mm for >30 ms in two other contiguous leads c Presumed new left bundle branch block may not be accompanied by ST segment deviation; the characteristic changes indicative of acute MI in patients with prior left bundle branch block require further definition Pathology c It takes 6 hours for myocyte necrosis to become evident on histopathology c The pathological identification of MI depends in part on the staging of the inflammatory cell infiltrate: acute = neutrophils; healing = mononuclear cells; healed = collagen without cellular infiltration c Infarcts are classified by size: microscopic (focal necrosis); small (,10% of the left ventricle); medium (10-30% of the left ventricle); large (.30% of the left ventricle) Imaging c Manifestations of MI include regional wall motion abnormalities on echocardiography, contrast angiography, radionuclide scanning or magnetic resonance imaging c These abnormalities may include evidence of ''infarct zone'' wall thinning, changes in tissue texture, and/or abnormalities in wall motion ACC, American College of Cardiology; CK, creatine kinase; ESC, European Society of Cardiology; MI, myocardial infarction
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