A new, precise definition of acute myocardial infarction
ABSTRACTSeveral international cardiovascular societies have revised their diagnostic criteria for acute myocardial infarction (MI) (J Am Coll Cardiol 2007; 50:2173–2188). The cornerstone of diagnosis remains a high level of clinical suspicion, serial electrocardiograms, and troponin levels. This article reviews the new definition and the appropriate clinical tools necessary to diagnose acute MI accurately.
KEY POINTS
- The clinical presentation of acute MI varies considerably from patient to patient. Therefore, one must consider the symptoms, serial electrocardiographic findings, and serial biomarker results in concert.
- Troponin I or T is now the preferred biomarker of myocardial necrosis. Still, troponin can be elevated in many conditions other than ischemic heart disease.
- Electrocardiographic signs of acute ischemia have been precisely defined, but electrocardiography can give false-positive or false-negative results in a number of conditions.
- MI is now categorized into five types depending on cause.
CLINICAL CLASSIFICATION OF ACUTE MI
The new classification scheme of the different types of MI is shown in Table 4.
The new classification scheme does not include myocardial necrosis from mechanical manipulation of the heart during open heart surgery, from cardioversion, or from toxic drugs.
As clinicians are aware, it is not unusual to see elevated biomarker levels in a host of conditions unrelated to acute myocardial ischemia or MI. The new classification of acute MI is most helpful in this regard. It will likely be even more helpful in guiding treatment and management when new ultrasensitive troponin assays are widely introduced into clinical practice.
The new classification also negotiates the controversy regarding elevated biomarker levels following percutaneous coronary intervention. In brief, elevation of biomarkers is not entirely avoidable even with a successful percutaneous coronary intervention, and furthermore, there is no scientific cutoff for biomarker elevations. So, by arbitrary convention, the troponin level must rise to more than three times the 99th percentile upper reference limit to make the diagnosis of type 4a MI. A separate type 4b MI is ascribed to angiographic or autopsy-proven stent thrombosis.
The new guidelines also suggest that troponin values be more than five times the 99th percentile of the normal reference range during the first 72 hours following coronary artery bypass graft surgery (CABG) when considering a CABG-related MI (type 5). Whenever new pathologic Q waves appear in territories other than those identified before the procedure, MI should be considered, especially if associated with elevated biomarkers, new wall-motion abnormalities, or hemodynamic instability.
Thus, the diagnosis of acute MI now has widely accepted global criteria that distinguish various types of acute MI that occur under multiple circumstances. It is expected that describing the type of acute MI according to the new criteria will further enhance our understanding of the event, its proper management, and its prognosis.
