Doctors are rethinking how heart attacks are defined and diagnosed.
Every few years, an international task force revises the definition of heart attack to reflect new knowledge and diagnostic techniques. In late 2012, the task force updated the previous definition to specify six different types of heart attacks, how each one occurs, and how each one can be diagnosed.
The new definitions identify the different kinds of heart attack (doctors use the term myocardial infarction) by their causes, which can range from a clot that blocks blood flow, to stress on the entire body from problems such as infections and low blood counts, to complications that occur during a procedure such as bypass surgery. The distinctions are important, because the various types of heart attacks may be treated entirely differently.
"Having a universally accepted language about the causes of heart attack is important to clinical studies of drugs and other approaches, which may benefit one type of heart attack, but not others," explains Dr. James Januzzi, Jr., a cardiologist at Harvard-affiliated Massachusetts General Hospital and a member of the Joint ESC/ACCF/AHA/WHF Task Force for the Universal Definition of Myocardial Infarction. This task force is comprised of members of the European Society of Cardiology, the American College of Cardiology Foundation, the American Heart Association, and the World Heart Federation.
Anatomy of a heart attack
Blockage can occur in any of your coronary arteries. Two common sites are the right coronary artery (A) and the left anterior descending artery (B). When blockages occur in these locations, adjoining areas of the heart are damaged (shown shaded).
Diagnosing heart attack today
Although many people think of a heart attack as an event that causes disabling chest pain, it is not always so straightforward. Because everyone's symptoms can be different, diagnosing a heart attack requires the combination of a doctor's judgment, signs and symptoms, and test results. "No one method is ideal on its own, but when they are used together, we can reach a conclusion that is highly accurate," says Dr. Januzzi.
To understand the significance of the possible causes of chest pain, it is important to determine what is actually going on inside the body.
In a heart attack, the amount of blood reaching heart muscle cells is inadequate to keep them alive. Usually, something has stopped blood flow through an artery that nourishes the heart (a coronary artery). Most often, this occurs when a plaque of atherosclerosis ruptures, spilling its cholesterol-rich contents into the center of the artery and triggering a blood clot. Sometimes, it is caused by a spasm of the artery that narrows the interior of the artery temporarily, preventing blood flow. As a result, cells in the area of the heart muscle fed by the artery are injured. If blood flow is restored within an hour or two, some of the injured cells may recover. If not, they begin to die from lack of oxygen. If the damage is permanent and extensive, the heart may no longer be able to contract and pump well. When this happens, the person may develop heart failure, or even die.
Because a heart attack can cause a variety of symptoms, from crushing chest pain to nausea to a feeling of anxiety, doctors must rely on more than symptoms to diagnose one. That's why experts are working to redefine exactly what qualifies as a heart attack.
The different kinds of heart attack
Not all heart attacks are the same. The task force that redefined the diagnosis of heart attack also identified six different types, as follows.
Type 1, the most common situation: A heart attack occurring from a blood clot or other blockage in blood flow through an artery in the heart. The person usually has coronary artery disease. This is an oxygen supply problem, due to reduced blood flow.
Type 2: A heart attack occurring when the heart needs more oxygen than it can get. This type of heart attack is an oxygen demand problem, resulting from higher need for blood flow.
Type 3: A fatal heart attack that causes death before the diagnosis can be confirmed with blood tests.
Type 4a: A heart attack that occurs during angioplasty, a procedure in which a blocked blood vessel is opened with a balloon on the end of a flexible tube inserted in the artery.
Type 4b: A heart attack that occurs when a clot blocks blood flow through a stent—a small metal mesh tube that has been inserted in the artery during angioplasty to keep the artery open.
Type 5: A heart attack that occurs during coronary artery bypass surgery.
Making the diagnosis
Traditionally, doctors have identified a heart attack through changes seen on an electrocardiogram (ECG), a test that reveals the heart's electrical activity as it contracts and relaxes. But because changes of a heart attack on an ECG can be subtle, and can develop over hours after the heart attack starts, doctors often looked for changes on two or more ECGs taken a few hours apart. Unfortunately, this can delay diagnosis and treatment.
For this reason, the task force recommends performing a blood test to measure the level of troponin, a protein found in abundance in the heart muscle. An injured heart releases large quantities of troponin into the bloodstream. Having more than a certain level of troponin in the blood is a sign that you've likely had a heart attack.
Here are the latest guidelines for diagnosing a heart attack, published late last year in the journal Circulation: a troponin level that is substantially higher than normal plus one or more of the following:
symptoms of heart attack, such as chest pain or pain radiating down the left arm
worrisome changes on an ECG or imaging test
identification of a clot in a coronary artery
What do these new heart attack categories and diagnosis guidelines mean to people who suffer a heart attack? Plenty, says Dr. Januzzi.
"We are laying the groundwork for heart attack care to be standardized. This will allow you to get the same state-of-the-art treatment at your local hospital or at a hospital anywhere in the world as you will at a Harvard-affiliated hospital," he says.
Troponins: Little proteins with great potential
Troponins—protein molecules released by damaged muscle—can help doctors diagnose a heart attack. They can also be used to help predict if a heart attack will happen.
"Highly sensitive troponin tests enable us to detect very low concentrations of the protein in people who have no symptoms of heart attack—for example, someone with coronary artery disease who is having a routine doctor visit. Studies have shown that elevated troponin levels can help predict the risk of a future heart attack," says Dr. Januzzi.
Researchers are still trying to figure out who should have a troponin blood test, but they feel the test has tremendous potential. "It will give us the ability to identify heart attacks earlier, or even before they happen. Can you imagine the number of lives that might be saved?" asks Dr. Januzzi.