Acute Myocardial Infarction (STEMI/NSTEMI)

What is a STEMI?

An ST-elevation myocardial infarction (STEMI) is a type of heart attack that is more serious and has a greater risk of serious complications and death. It gets its name from how it mainly affects the heart’s lower chambers and changes how electrical current travels through them.

Any heart attack is a life-threatening medical emergency that needs immediate care. If you or someone you’re with appears to be having a heart attack, do not hesitate to call 911 (or the appropriate number for local emergency services). Any delay in receiving care can lead to permanent damage to the heart muscle or death.

Why is it called a STEMI?

Myocardial infarction is the medical term for a heart attack. An infarction is a blockage of blood flow to the myocardium, the heart muscle. That blockage causes the heart muscle to die.

A STEMI is a myocardial infarction that causes a distinct pattern on an electrocardiogram (abbreviated either as ECG or EKG). This is a medical test that uses several sensors (usually 10) attached to your skin that can detect your heart’s electrical activity. That activity is then displayed as a wave pattern on a paper readout or a digital display. The different parts of the wave are labeled using letters, starting at P and ending at U.

To best understand ST-elevation, it helps to know about two specific wave sections:

• QRS complex: This is the large peak that appears on a heart wave. The ventricles cause this wave when they pump blood out of your heart.
• ST-segment: This is a short section immediately after the QRS complex. Normally, there shouldn’t be any electrical activity in that segment, causing it to be flat and back to baseline.

When there’s an elevation in the ST segment, that often means there’s a total blockage of one of the heart's main supply arteries. When that is happening during a heart attack, it can be a sign that the muscle of the ventricles is dying. That’s critical information for healthcare providers to know during a STEMI because it means the heart muscle is in the process of dying. That also means reopening that artery and restoring blood flow as soon as possible may prevent permanent damage, or at least limit the severity of the damage.

That’s dangerous because the ventricles are the chambers of your heart that pump blood to your lungs and body. If there’s too much damage to the muscle in the ventricles, your heart can’t pump enough blood to support your body. That’s why STEMIs are so dangerous and why restoring blood flow quickly is so critical.

Your heart muscle also can’t regrow or regenerate itself, so if the muscle goes without blood flow for too long, the damage to that muscle can be permanent. However, restoring blood flow quickly may keep the damage from being permanent or at least minimize the severity of the damage.

What happens before and during a STEMI heart attack?

Blockages in the arteries that supply blood to your heart muscle are what cause most heart attacks. Usually, the blockage happens because plaque, a fatty, waxy buildup accumulates on the inside of your arteries. A blood clot can form on the plaque deposits, rapidly closing the artery and interrupting blood flow to the heart muscle.

Once blood flow is partially or totally blocked, it causes ischemia (iss-key-me-uh). This is the medical term for how your cells and tissues start to die because they aren't getting blood flow. During a heart attack, the loss of blood flow causes the muscle in your ventricles to begin to die. If too much heart muscle sustains damage, your heart may not have the ability to supply enough blood to your body. That leads to a condition known as cardiogenic shock, which is often fatal. Also, ischemia of the heart muscle may also trigger a dangerous electrical rhythm known as ventricular tachycardia or ventricular fibrillation, which can lead to cardiac arrest (where your heart stops entirely) and sudden death.

Among heart attacks, STEMIs are typically more severe. Between 2.5% and 10% of people who have one die within 30 days.