What is Segment Elevation Myocardial Infarction (STEMI)?

What You Need to Know about Heart Attacks

Heart attack
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ST Segment Elevation Myocardial Infarction (STEMI) is the name cardiologists currently use to describe a classic heart attack.

A “myocardial infarction,” or heart attack, means that a portion the heart muscle (or myocardium) has died, almost always because its blood flow has been interrupted. The phrase “ST segment elevation” is used because with large heart attacks, the “ST segment” portion of the electrocardiogram (ECG) tracing usually appears elevated above the baseline.

Elevation of the ST segment usually indicates that substantial heart muscle damage has occurred.

What Causes a STEMI?

A STEMI is the most severe of the three types of acute coronary syndrome (ACS). ACS occurs when an atherosclerotic plaque ruptures within a coronary artery, causing at least some obstruction of the artery. The obstruction is due to the blood clots that often form when a plaque ruptures. 

The heart muscle that is supplied by the occluded artery quickly begins to suffer from ischemia, or lack of oxygen. Myocardial ischemia often produces angina (chest discomfort) as its earliest symptom. However, if the clot is extensive enough, some of the heart muscle begins to die, and a myocardial infarction occurs.

In some people experiencing ACS, clots will form, dissolve, and re-form during a period of hours or days without actually causing heart muscle to die. However, the on-again off-again clotting may cause angina to come and go for no apparent reason.

(That is, the angina may occur while the victim is at perfect rest, or even while sleeping.) This type of ACS is called unstable angina. Unstable angina should be treated as a medical emergency, since it usually indicates that a plaque has ruptured, and that the artery may become completely occluded at any moment.

If the obstruction produced by the ACS is extensive enough to cause heart muscle to die, a myocardial infarction occurs. However, in many cases the obstruction, while serious, does not quite cause total occlusion of the coronary artery. In these cases, while cell death occurs, some of the heart muscle supplied by the artery survives. In these “partial” heart attacks, the ST segment on the ECG does not become elevated. So this type of ACS is referred to as a “non-ST-segment elevation myocardial infarction,” or NSTEMI.

Finally, the clot that forms during ACS can cause complete blockage of the coronary artery, in which case all the heart muscle supplied by the artery begins to die. This, the most severe type of an ACS, is called a STEMI. In many cases, a STEMI is preceded (by hours, days or weeks) by unstable angina and/or an NSTEMI.

With any type of ACS, the death of of at least some heart muscle either occurs immediately, or is likely to occur in the very near future. So ASC is always a medical emergency, and always requires immediate and aggressive medical care.

What are the Symptoms of STEMI?

Classically, a STEMI is accompanied by intense pain or pressure in or around the chest, which often radiates to the neck, jaw, shoulder or arm.

Profuse sweating and breathlessness are also common, as is a frightening sense of doom.

However, heart attack symptoms may be substantially less obvious and more subtle than this classic pattern. So anyone who is at risk for coronary artery disease should pay attention to any unusual symptoms arising from anywhere above the waist.

How is a STEMI Diagnosed?

In most cases the diagnosis of a STEMI can be made relatively quickly once the sufferer comes under medical care. Symptoms compatible with a heart attack, accompanied by the classic pattern of ST segment elevation on the ECG, are usually enough for a doctor to initiate treatment.

 Cardiac enzymes are invariably elevated during a STEMI, but the diagnosis can usually be made — and treatment initiated — before the lab results are available.

What are the Consequences of a STEMI?

An acute heart attack, in addition to the pain and distress it causes, can produce sudden death from ventricular fibrillation, or can lead to acute heart failure. After a heart attack has run its course, the heart muscle may be left with a substantial amount of permanent damage. Chronic heart failure can ensue, as well as an increased risk of dangerous cardiac arrhythmias.

To prevent such acute and long-term problems, it is critical for anyone who thinks they might be having a heart attack to call 911 immediately.

How is a STEMI Treated?

Treatment must be initiated the moment a STEMI is diagnosed, in order to minimize heart muscle destruction. In addition to administering several drugs to attempt to stabilize the heart muscle — including oxygen, morphine, beta blockers and a statin — steps are taken immediately to open up the blocked artery.

Rapidly unblocking the coronary artery is critical; every minute counts. Unless the artery is opened within three hours of the occlusion, at least some permanent damage to the heart muscle will result. Much of the damage can be prevented for up to six hours, and some of the damage can be prevented for up to 12 hours. But the longer it takes to open the artery, the more damage is done. So every minute counts.

There are two general approaches to opening up an occluded coronary artery - the use of thrombolytic therapy (“clot-busting” drugs), or immediate angioplasty and stenting. But whichever method is chosen, the important thing is to get the artery opened rapidly.

Once the acute phase of treatment is over and the blocked artery is opened, there is still a lot that has to be done to stabilize the heart, and to reduce the odds of another heart attack. Read more about what should happen after you have survived a heart attack.


Goodman SG, Menon V, Cannon CP, et al. Acute ST-segment Elevation Myocardial Infarction: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines (8th Edition). Chest 2008; 133:708S.

O'Gara PT, Kushner FG, Ascheim DD, et al. 2013 ACCF/AHA Guideline for the Management of ST-elevation Myocardial Infarction: Executive Summary: a Report of the American College of Cardiology Foundation/American Heart Association Task Force on Practice Guidelines. Circulation 2013; 127:529.

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