A heart attack occurs when the supply of oxygen-rich blood to the heart is disrupted, usually by a blood clot in one of the coronary arteries that supply the heart with blood. The heart is composed of a special type of muscle that never rests and therefore has high oxygen requirements. When the heart muscle is deprived of oxygen for even a brief period of time, the myocardial tissue begins to die (infarct). Medically, heart attacks are known as myocardial infarctions.

Coronary artery disease is the leading cause of heart attacks in the United States, accounting for more than half of all cardiovascular events in men and women under the age of 75. Atherosclerosis is the leading cause of coronary artery disease. Sometimes called "hardening of the arteries," atherosclerosis is characterized by fatty plaque deposits that gradually block arteries, causing them to lose their suppleness. A blood clot can form after such a plaque deposit ruptures.

Heart attacks can occur both with and without warning signs. Many people experience episodes of cardiac ischemia before a heart attack. Ischemia describes a lack of oxygen-rich blood. Ischemia may have no symptoms (silent ischemia) or it may be accompanied by a type of chest pain known as angina. In many cases, angina occurs at predictable times, usually during periods of activity when the heart's oxygen requirements are increased, such as after exercise. If the angina occurs at irregular or unpredictable times, and is not associated with exertion, it is known as unstable angina. This is a dangerous warning sign that a heart attack may be imminent.

Depending upon the severity of the attack and of the subsequent scarring, as well as how rapidly the person gets access to medical service, a heart attack can lead to:

The location of the damage in the heart muscle is also important. Different coronary arteries supply different areas of the heart, thus the severity of the damage depends upon which artery was blocked, the extent of the blockage and how much of the heart muscle depended on that blocked artery.

A heart attack is not the same thing as cardiac arrest, even though many people use the terms interchangeably. Cardiac arrest occurs when the heart actually stops beating and pumping blood. It is usually caused by an abnormal heart rhythm that causes the heart's main pumping chambers (e.g., ventricles) to quiver and contract irregularly (ventricular fibrillation). The term “massive heart attack” is also mistakenly used to describe cardiac arrest, but they are not the same thing. A heart attack may lead to cardiac arrest, but these are separate events.