Most heart attacks are caused by a blood clot that blocks one of the coronary arteries, the blood vessels that bring blood and oxygen to the heart muscle. When blood cannot reach part of your heart, that area starves for oxygen. If the blockage continues long enough, cells in the affected area die. The Most Common Cause Coronary heart disease (CHD)is the most common underlying cause of a heart attack. CHD, also called coronary artery disease, is the hardening and narrowing of the coronary arteries caused by the buildup of plaque inside the walls of the arteries. When plaque builds up in the arteries, the condition is called atherosclerosis (ath-er-o-skler-O-sis). The buildup of plaque occurs over many years. Over time, an area of plaque can rupture (break open) inside of an artery. This causes a blood clot to form on the plaque's surface. If the clot becomes large enough, it can mostly or completely block blood flow through a coronary artery. If the blockage isn't treated quickly, the portion of heart muscle fed by the artery begins to die. Healthy heart tissue is replaced with scar tissue. This heart damage may not be obvious, or it may cause severe or long-lasting problems. Other Causes Heart attack also can occur due to problems with the very small, microscopic blood vessels of the heart. This condition is called microvascular disease. It's believed to be more common in women than in men. A less common cause of heart attacks is a severe spasm or tightening of the coronary artery that cuts off blood flow to the heart. These spasms can occur in persons with or without coronary artery disease. What causes a coronary artery to spasm isn't always clear. A spasm may be related to emotional stress or pain, exposure to extreme cold, cigarette smoking, or by taking certain drugs like cocaine. Risk Factors You Cannot Change Certain factors make it more likely that you will develop coronary artery disease and have a heart attack. These risk factors include some things you cannot change. If you are a man over age 45 or a woman over age 55, you are at greater risk. Having a family history of early heart disease, diagnosed in a father or brother before age 55 or in a mother or sister before age 65, is another risk factor. You are also at risk if you have a personal history of angina or previous heart attack, or if you have had a heart procedure such as percutaneous coronary intervention (PCI) or coronary artery bypass surgery (CABG). Risk Factors You Can Change Importantly, there are many risk factors that you can change. These include - smoking  - being overweight or obese  - physical inactivity  - high blood pressure  - high blood cholesterol  - high blood sugar due to insulin resistance or diabetes   - an unhealthy diet (for example, a diet high in saturated fat, trans fat, cholesterol, and sodium). smoking being overweight or obese physical inactivity high blood pressure high blood cholesterol high blood sugar due to insulin resistance or diabetes an unhealthy diet (for example, a diet high in saturated fat, trans fat, cholesterol, and sodium). Metabolic Syndrome Some of these risk factorssuch as obesity, high blood pressure, and high blood sugartend to occur together. When they do, it's called metabolic syndrome. In general, a person with metabolic syndrome is twice as likely to develop heart disease and five times as likely to develop diabetes as someone without metabolic syndrome.