What causes Heart Attack ?
Coronary Heart Disease
                
A heart attack happens if the flow of oxygen-rich blood to a section of heart muscle suddenly becomes blocked and the heart can't get oxygen. Most heart attacks occur as a result of coronary heart disease (CHD).
                
CHD is a condition in which a waxy substance called plaque builds up inside of the coronary arteries. These arteries supply oxygen-rich blood to your heart.
                
When plaque builds up in the arteries, the condition is called atherosclerosis. The buildup of plaque occurs over many years.
                
Eventually, 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.
                
Coronary Artery Spasm
                
A less common cause of heart attack is a severe spasm (tightening) of a coronary artery. The spasm cuts off blood flow through the artery. Spasms can occur in coronary arteries that aren't affected by atherosclerosis.
                
What causes a coronary artery to spasm isn't always clear. A spasm may be related to:
                
Taking certain drugs, such as cocaine
                
Emotional stress or pain
                
Exposure to extreme cold
                
Cigarette smoking