A heart attack occurs when a blockage in the coronary arteries those that supply the heart itself—shuts off the ?ow of oxygen- rich blood to heart muscle tissue. Without oxygen and nutrients, the heart muscle will begin to die. Prompt medical attention can restore blood ?ow and limit the extent of damage, but dead tissue cannot be restored. The lack of blood supply, called ischemia, can weaken your heart or stop it altogether. If there is a prolonged decrease in blood sup- ply, tissue dies, so this is an urgent matter. The severity of the heart attack depends on how much tissue is damaged and where in your heart the damage occurs.
Several different mechanisms can cause a heart attack:
• Atherosclerosis, in which the walls of the arteries thicken and accumulate fatty deposits called plaque, can narrow or block one or more arteries supplying a section of heart muscle.
• A blood clot can form within the artery and stick to the walls of the narrowed coronary artery, already thickened with plaque, and stop the blood ?ow.
• A blood clot also can form in the coronary artery itself, as a result of atherosclerotic plaque that cracks open, emptying its choles- terol and other components into the bloodstream.

• A coronary artery can temporarily spasm, narrowing the artery and restricting or stopping blood flow. These spasms most commonly occur in a blocked artery but may occur in a normal one.
The most common mechanism begins when a fracture develops within atherosclerotic plaque, exposing the inside of the plaque. This causes platelets to stick to the site of the rupture, triggering a cascade of events resulting in the formation of a blood clot that blocks the artery. This explains why aspirin, which helps reduce stickiness of platelets, is effective in reducing the risk of heart attack.
Every year in the United States, about 1.2 million people have heart attacks, and more than 40 percent of those people die before they reach a hospital. As scary as these numbers may sound, they are substantially lower than the ?gures of 25 years ago. Today, many Americans are doing a better job of reducing their own risk of heart attack. Doctors have made major advances in treatment, so that a person who gets medical help quickly is much more likely to survive a heart attack. A heart attack survivor has a much better chance of getting fully rehabilitated than ever before. The survival rates for men after a heart attack have improved in recent years, but this has not yet occurred for women. See also chapter 16, Women and Heart Disease.