Non-ST-Segment Myocardial Infarction Overview

A heart attack, also known as a myocardial infarction, occurs when the flow of blood to the heart muscle is blocked, depriving it of oxygen. It is one of several conditions referred to as acute coronary syndrome (ACS) in which blood flow to the heart is suddenly reduced or blocked.

Many people do not realize that there is more than one form of a heart attack. Some only involve a partial obstruction of blood flow. Heart specialists refer to this as a non-ST-segment elevation myocardial infarction (NSTEMI). NSTEMI is a milder form of heart attack and accounts for around two-thirds of all cases.

This article explains what NSTEMI is and how it differs from a “classic” heart attack. It also describes how NSTEMI is diagnosed and how treatment can vary based on the assessment of a person’s risk factors for a severe heart attack.

Types of Acute Coronary Syndrome

All forms of ACS are usually caused by the rupture of plaque in a coronary (heart) artery, leading to either a partial or complete obstruction. Depending on the severity of the obstruction, ACS can be classified into the following three types.

• Unstable angina is a type of ACS that causes chest pain and is usually caused by plaque buildup in the artery, which may lead to the partial rupture of the artery. Unlike stable angina, which occurs during exertion, unstable angina can occur anytime and is considered more serious. Unstable angina doesn’t cause permanent heart damage.
  
• ST-segment elevation myocardial infarction (STEMI) is considered a “classic” heart attack. It occurs when a ruptured plaque completely blocks a major coronary artery, resulting in extensive heart damage.
• NSTEMI is either the complete blockage of a minor coronary artery or the partial obstruction of a major coronary artery. Symptoms can be the same as STEMI, but the damage to the heart is far less extensive.
  

NSTEMI vs. STEMI

A diagnosis of NSTEMI is typically made when the person has symptoms of unstable angina. These include:

• Crushing pain or tightness in the chest that may spread to the shoulder or other parts of the upper body
• Chest pain that occurs when you’re not exerting yourself
• Shortness of breath
• Sweating
• Anxiety

Doctors can differentiate STEMI from NSTEMI using an electrocardiogram (ECG), which records the heart’s electrical activity.

In an ECG, there will typically be a flat line between heartbeats called the “ST segment.” During a classic heart attack, the ST segment will be raised. With NSTEMI, there will be is no evidence of ST-segment elevation.

Because NSTEMI causes damage to the heart muscle, it is still considered a heart attack. Even so, NSTEMI has more in common with unstable angina and usually has better outcomes.

NSTEMI rarely leads to STEMI because they tend to involve different blood vessels. NSTEMI is more likely in people with diffuse coronary disease. This form of heart disease involves a network of tiny blood vessels (called collateral vessels) that service the heart only occasionally. By contrast, STEMI will involve the major coronary arteries.

Even so, if NSTEMI involves the partial blockage of a major coronary artery, it can progress to STEMI within hours, weeks, or months if not properly treated.

Emergency Treatment

NSTEMI treatment is identical to that for unstable angina. If you have cardiac symptoms, the emergency medical team will start intensive treatment to stabilize the heart and prevent further damage to the heart muscle.

Stabilization will primarily focus on two things:

• Eliminating acute ischemia (insufficient blood flow)
• Stopping the formation of blood clots

Acute Ischemia

With acute ischemia, the heart does not get enough oxygen, resulting in cell death. Doctors can help eliminate this by using drugs called beta-blockers and statins.

The drugs work in different ways:

• Beta-blockers prevent damage caused by the excessive production of adrenaline. Adrenaline is released at the time of crisis; it causes blood vessels to narrow and heart pressure to rise. Beta-blockers reverse this effect and help restore circulation.
• Statins stabilize ruptured plaque and reduce arterial inflammation. This helps prevent further obstruction.

Beta-blockers and statin drugs will usually alleviate acute ischemia within minutes. Oxygen and morphine may be given to improve respiration and reduce pain.

Nitrates are also often used for patients who have continued ischemic pain.

Blood Clot Formation

During NSTEMI, the emergency medical team will provide medications to prevent the formation of blood clots around the site of the obstruction. Doing so prevents a "complete occlusion" in which a vessel is completely blocked.

This may involve medications like aspirin, Plavix (clopidogrel), and other drugs that thin the blood and prevent the clumping of blood cells called platelets. Other types of medication, such as enoxaparin, which slows blood clotting, may also be prescribed. This is an important part of the treatment protocol for NSTEMI.

The treatment does not include thrombolytic drugs (“clot busters”) used for STEMI. The drugs are avoided because they often do not help and can lead to a medical emergency known as cardiogenic shock. This occurs when the ventricles of the heart fail to pump adequate blood to the body, causing a dangerous drop in blood pressure.

After Stabilization

Once a person with NSTEMI is stabilized, the heart specialist, called a cardiologist, will assess whether further interventions are needed. Many cardiologists will use a TIMI (thrombosis in myocardial infarction) score to make the determination.

The TIMI score assesses whether the person has any of the following risk factors for a classic heart attack:

• Age 65 years or older
• Presence of at least three risk factors for coronary heart disease
• Prior coronary blockage of greater than 50%
• ST-segment deviation on the admission ECG
• At least two angina episodes in the past 24 hours
• Elevated cardiac enzymes
• Use of aspirin within the past seven days

If you have two risk factors or less (TIMI score 0-2), you may not need further intervention. If the score is higher, the cardiologist may want to take more aggressive steps.

This includes cardiac catheterization with angioplasty and stenting. This procedure involves the insertion of a flexible tube (called a catheter) through an artery in the arm or leg to the site of the obstruction. The end of the catheter is then inflated to widen the vessel, after which a narrow length of tubing (called a stent) is left behind to keep the vessel open.

A cardiac stress test may also be used to determine the need for aggressive treatments. The test measures blood pressure, blood oxygen, and the heart’s electrical activity while you are running on a treadmill or riding a stationary cycle.

Summary

Non-ST-segment elevated myocardial infarction (NSTEMI) is a type of heart attack in which a minor artery of the heart is completely blocked or a major artery of the heart is partially blocked. It is less serious than a “classic” heart attack, known as an ST-segment elevation myocardial infarction (STEMI).

NSTEMI can cause the same symptoms as STEMI but is generally less damaging to the heart. It is differentiated from STEMI with an electrocardiogram (ECG), which can tell if the electrical activity between heartbeats—called the ST segment—is either elevated or non-elevated.

The treatment of NSTEMI is focused on restoring blood circulation and preventing the formation of blood clots. This typically involves beta-blockers, statin drugs, and blood thinners like aspirin or Plavix (clopidogrel). After the individual is stabilized, the cardiologist will determine if further treatment is needed.

Correction - November 8, 2022: This article was updated to correct the description of unstable angina.