New Definition for Myocardial Infarction

Representatives of four major worldwide organizations dedicated to treating heart disease (the American College of Cardiology, the American Heart Association, the European Sociey of Cardiology, and the World Heart Federation), have agreed to new criteria for diagnosing myocardial infarctions (heart attacks). These new criteria will be published officially next month in the Journal of the American College of Cardiology and in Circulation.

A myocardial infarction occurs when the blood supply to part of the heart muscle is blocked, usually by the sudden formation of a blood clot within a coronary artery, leading to death of part of the heart muscle. Classically, a "heart attack" is diagnosed when any two of these three things occur: typical symptoms (chest pain, shortness of breath), typical changes on the electrocardiogram (ECG), and elevation in the blood of certain biomarkers (usually, the enzyme known as MB-CK) that are released by damaged heart cells.

What's changed in the new definition of myocardial infarction is that the preferred biomarker is now troponin. Elevations in troponin levels above the 99th percentile of the upper reference limit, when accompanied by either symptoms compatible with heart attack or by ECG changes, will now clinch the diagnosis.

Troponin is far more specific for heart cell damage than older biomarkers, and any elevation in troponin levels is considered to indicate heart muscle damage. Troponin is also more "sensitive;" it is released earlier and more reliably than older biomarkers. The bottom line: fewer cases where actual heart cell damage has occurred will be "missed."

What's the significance of this new definition?

1) Under the new definition, many patients with chest pain who previously would be told a heart attack had been "ruled out" will now be diagnosed as actually having a heart attack. Accordingly, because they indeed do have unstable coronary artery disease, they will get more appropriate and more aggressive therapy than they would have received in the past, and presumably their long-term outcomes will be improved. This, in fact, is a chief reason for the new definition.

2) The number of patients who are diagnosed with heart attack will increase by at least 25% under this new definition, according to some estimates. Officially, the incidence of heart attacks will rise. Therefore, in a year or two expect breathless news stories about the alarming new "epidemic" of heart attacks. Any time we get better at making earlier diagnoses of any disease - whether coronary artery disease, infectious disease, or cancer - the reported incidence of that disease will go up. This is something we need to remember whenever experts call for new federal regulations (which seems to be the main job of experts), based on the increasing incidence of some disease or other.

3) Because these "new" heart attack patients will have a milder form of heart attack, the acute mortality rate reported for heart attacks will go down. It will be very tempting for some to use this entirely predictable improvement in acute outcomes to justify the expensive technologies or procedures that will be said to have produced it. Careful analysis will actually be required to tease out the reasons for improvements in clinical outcomes. We already know of one - a new definition of heart attack.