The major cause of unstable angina and heart attack is coronary artery disease (CAD). Coronary artery disease occurs when plaque builds up over years inside your coronary arteries and reduces blood flow to the heart muscle. In many people, coronary artery disease begins in adolescence and gradually develops over a lifetime.

High cholesterol, high blood pressure, and smoking damage your arteries and contribute to plaque buildup. The process of plaque buildup in the arteries is called atherosclerosis. See pictures of atherosclerosis and how high blood pressure damages arteries.

Plaques are deposits of cholesterol, calcium, and other substances that are covered by a fibrous cap. If a sudden surge in blood pressure occurs, if the artery suddenly constricts, or if other factors such as inflammation are present, the fibrous cap can tear or rupture. The body tries to repair the tear, much as it might stop bleeding from a cut on the skin, by forming a blood clot over it. The blood clot can completely block blood flow through the coronary artery to the heart muscle and cause a heart attack. See a picture of how plaque causes a heart attack.

Newly formed plaques are most at risk for rupture. The fibrous caps on newly formed plaques tend to be more unstable and more prone to tearing than the thicker fibrous caps on plaques that have been present for a long period of time.

But plaque is not always the cause of a heart attack. In rare cases, the coronary artery spasms and contracts, obstructing blood flow and causing chest pain. If severe, the spasm can completely block blood flow and cause a heart attack. Most of the time in these cases, atherosclerosis is also involved, although sometimes the arteries are clear. Cocaine, cold weather, emotional stress, and other factors can cause these spasms. But in many other cases, it is not known what triggers the spasm.

A blood clot that forms over a ruptured plaque may not completely block the artery but may block blood flow enough to cause unstable angina. Unstable angina is a sign that a heart attack may soon follow, because the blood clot can quickly grow and block the artery. If the blood clot dissolves, and an immediate heart attack is avoided, the body will try over time to repair the tear on the surface of the plaque. But this newly repaired plaque can also be very unstable. It is more likely to rupture again, putting you at even greater risk of a heart attack.

Heart attack triggers

In most cases, there are no clear reasons why heart attacks occur when they do. But sometimes your body releases adrenaline and other hormones into the bloodstream in response to intense emotions such as anger, fear, and the "fight or flight" impulse. Heavy physical exercise, emotional stress, lack of sleep, and overeating can also trigger this response. Adrenaline increases blood pressure and heart rate and can cause coronary arteries to constrict, which may cause an unstable plaque to rupture.

Nicotine, which is found in tobacco products, and cocaine can cause similar responses.