ST-Segment Elevation Myocardial Infarction

The Most Severe Type of Heart Attack

ST-segment elevation myocardial infarction (STEMI) is the term cardiologists use to describe a classic heart attack. It is one type of myocardial infarction in which a part of the heart muscle (myocardium) has died due to the obstruction of blood supply to the area.

The ST segment refers to the flat section of an electrocardiogram (ECG), in particular, the flat section that connects two distinct complexes on the tracing (the QRS complex and the T wave). When a person has the most severe type of heart attack, this segment will no longer be flat but will appear abnormally elevated.

Symptoms of ST-segment elevation myocardial infarction (STEMI)
Verywell / Laura Porter

Types and Severity

STEMI is one of three types of acute coronary syndrome (ACS). ACS occurs when a plaque ruptures from within a coronary artery, causing the partial or complete obstruction of that artery. The obstruction itself is caused when blood clots form around the area of the rupture.

When obstructed, the portion of the heart muscle serviced by that artery will quickly suffer from a lack of oxygen, called ischemia. Chest pains (angina) are often the first signs of this. If the obstruction is extensive enough, some of the heart muscle will begin to die, resulting in myocardial infarction.

ACS is categorized by the level of obstruction and the resulting damage to the heart muscle:

  • ST-segment elevation myocardial infarction (STEMI): If the complete obstruction of a coronary artery occurs, resulting in the death of heart muscle tissue, we refer to that as STEMI, the worst form of ACS.
  • Unstable angina: In some cases, the clots will form, dissolve, and re-form during a period of hours or days without causing a fixed obstruction. When this happens, the person may experience on-again-off-again angina even when resting. This type of ACS is called unstable angina.
  • Non-ST-segment elevation myocardial infarction (NSTEMI): This occurs when the obstruction doesn't completely stop the blood flow. While some cell death will occur, other parts of the muscle will survive. It may be called a "partial heart attack."

Regardless of how an ACS event is classified, it is still considered a medical emergency since unstable angina and NSTEMI are often early warning signs of a major heart attack.


STEMI will typically result in intense pain or pressure in or around the chest, often radiating to the neck, jaw, shoulder, or arm. Profuse sweating, breathlessness, and a profound sense of impending doom are also common.

At times, the signs may be far less obvious, manifesting with nonspecific or generalized symptoms such as:

  • Pain around the shoulder blades, arm, chest, jaw, left arm, or upper abdomen
  • A painful sensation described as having a "clenched fist in the chest"
  • Discomfort or tightness in the neck or arm
  • Indigestion or heartburn
  • Nausea and vomiting
  • Fatigue or sudden exhaustion
  • Shortness of breath
  • Dizziness or lightheadedness
  • Increased or irregular heart rate
  • Clammy skin 

As a general rule of thumb, anyone at significant risk of a heart attack should pay close attention to any unusual symptom arising from above the waist.


In most cases, the diagnosis of STEMI can be made quickly once the person is under medical care. A review of symptoms, accompanied by the evaluation of the ST segment on the ECG, is usually enough for a doctor to begin treatment. A review of cardiac enzymes may also help but usually arrives well after acute treatment is started.

It is important to stabilize the person as quickly as possible. In addition to pain and distress, STEMI can cause sudden death due to ventricular fibrillation (a serious disturbance of the heart rhythm) or acute heart failure (when the heart cannot pump enough blood to properly supply the body).

After a heart attack has run its course, the muscle itself may be left with substantial permanent damage. Chronic heart failure is a common consequence of this, as is the increased risk of dangerous cardiac arrhythmias (irregular heartbeats ).


Treatment must be started the moment STEMI is diagnosed. In addition to administering drugs to stabilize the heart muscle (including morphine, beta-blockers, and statin medications), efforts will be made to immediately reopen the blocked artery.

This requires speed. Unless the artery is opened within three hours of the blockage, at least some permanent damage can be expected. Generally speaking, much of the damage can be minimized if the artery is unblocked within the first six hours of an attack.

Up until 12 hours, some damage may be averted. After that, the longer it takes to unblock the artery, the more damage there will be. 

There are several approaches to reopening an arterial obstruction:

  • Thrombolytic therapy involves the use of clot-busting drugs.
  • Angioplasty is the medical term for the surgical repair/reopening of an artery.
  • Stenting involves the insertion of a mesh tube to reopen the artery.

Once the acute phase of treatment is over and the blocked artery is reopened, there is still a lot that has to be done to stabilize the heart, and to reduce the odds of another heart attack.

This usually involves an extensive period of recovery, including an exercise-based rehabilitation program, dietary changes, and the use of ​anticoagulants (blood thinners) and lipid control medications.

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  1. Vogel B, Claessen BE, Arnold SV, et al. ST-segment elevation myocardial infarction. Nat Rev Dis Primers. 2019;5(1):39. doi:10.1038/s41572-019-0090-3

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