The most common cause of a heart attack is sudden narrowing or blockage of a coronary artery, which blocks oxygen to the heart. This can happen when plaque in the coronary artery breaks and a blood clot forms in the artery.

Plaque is a buildup of cholesterol, white blood cells, calcium, and other substances in the walls of arteries. Over time, plaque may narrow the artery and the artery hardens (atherosclerosis). This condition is called coronary artery disease.

Plaques are covered by a fibrous cap. If a sudden surge in blood pressure occurs, if the artery suddenly constricts, or if something else such as inflammation is present, the fibrous cap can tear or rupture. The body tries to repair the tear-much as it does to stop the 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. This will cause a heart attack.

Sometimes a blood clot that forms over a ruptured plaque may not completely block the artery. But the clot may block blood flow enough to cause angina symptoms. These symptoms may happen with rest and may not go away with rest or nitroglycerin. These symptoms are an emergency, 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 again 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.