Heart Attack

A heart attack occurs when the supply of blood and oxygen to an area of heart muscle is blocked, usually by a clot in a coronary artery. If treatment is not started quickly, the affected area of heart muscle begins to die. This injury to the heart muscle can lead to serious complications, and can even be fatal. Sudden death from heart attack is most often due to an arrhythmia (irregular heartbeat or rhythm) called ventricular fibrillation. If a person survives a heart attack, the injured area of the heart muscle is replaced by scar tissue. This weakens the pumping action of the heart and can lead to heart failure and other complications.
Effective treatments for heart attack are available that can decrease the chances of sudden death and long-term complications. To be most effective, these treatments must be given fast—within 1 hour of the start of heart attack symptoms. Acting fast can save your life and limit damage to your heart.
Heart With Muscle Damage and Blocked Artery

Figure A is an overview of the heart and coronary artery showing damage (dead heart muscle) caused by a heart attack. Figure B shows a cross-section of the coronary artery with plaque buildup and a blood clot.
A heart attack is a life-threatening event. Everyone should know the warning signs of a heart attack and how to get emergency help. Many people suffer permanent damage to their hearts or die because they do not get help immediately.
Each year, more than a million persons in the United States have a heart attack, and about half (515,000) of them die. About one-half of those who die do so within 1 hour of the start of symptoms and before reaching the hospital.
Both men and women have heart attacks.
Emergency personnel can often stop arrhythmias with emergency cardiopulmonary resuscitation (CPR), defibrillation (electrical shock), and prompt advanced cardiac life support procedures. If care is sought soon enough, blood flow in the blocked artery can be restored in time to prevent permanent damage to the heart. Most people, however, do not seek medical care for 2 hours or more after symptoms begin. Many people wait 12 hours or longer.

Symptoms and Diagnosis
The warning signs and symptoms of a heart attack can include:
Chest discomfort. Most heart attacks involve discomfort in the center of the chest that lasts for more than a few minutes or goes away and comes back. The discomfort can feel like uncomfortable pressure, squeezing, fullness, or pain. Heart attack pain can sometimes feel like indigestion or heartburn.
Discomfort in other areas of the upper body. Pain, discomfort, or numbness can occur in one or both arms, the back, neck, jaw, or stomach.
Shortness of breath. Difficulty in breathing often comes along with chest discomfort, but it may occur before chest discomfort.
Other symptoms. Examples include breaking out in a cold sweat, having nausea and vomiting, or feeling light-headed or dizzy.
Signs and symptoms vary from person to person. In fact, if you have a second heart attack, your symptoms may not be the same as for the first heart attack. Some people have no symptoms. This is called a "silent" heart attack.
The symptoms of angina (chest pain) can be similar to the symptoms of a heart attack. If you have angina and notice a change or a worsening of your symptoms, talk with your doctor right away.
Diagnosis of a heart attack may include the following tests:
EKG (electrocardiogram). This test is used to measure the rate and regularity of your heartbeat. A 12-lead EKG is used in diagnosing a heart attack.
Blood tests. When cells in the heart die, they release enzymes into the blood. These enzymes are called markers or biomarkers. Measuring the amount of these markers in the blood can show how much damage was done to your heart. These tests are often repeated at intervals to check for changes. The specific blood tests are:
Troponin test. This test checks the troponin levels in the blood. This blood test is considered the most accurate to see if a heart attack has occurred and how much damage it did to the heart.
CK or CK-MB test. These tests check for the amount of the different forms of creatine kinase in the blood.
Myoglobin test. This test checks for the presence of myoglobin in the blood. Myoglobin is released when the heart or other muscle is injured.
Nuclear heart scan. This test uses radioactive tracers (technetium or thallium) to outline heart chambers and major blood vessels leading to and from the heart. A nuclear heart scan shows any damage to your heart muscle.
Cardiac catheterization. A thin, flexible tube (catheter) is passed through an artery in the groin (upper thigh) or arm to reach the coronary arteries. Your doctor can use the catheter to determine pressure and blood flow in the heart's chambers, collect blood samples from the heart, and examine the arteries of the heart by x ray.
Coronary angiography. This test is usually performed along with cardiac catheterization. A dye that can be seen by using x ray is injected through the catheter into the coronary arteries. Your doctor can see the flow of blood through the heart and see where there are blockages.

Causes
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.
Coronary artery disease (CAD) is the most common underlying cause of a heart attack. CAD is the hardening and narrowing of the coronary arteries by the buildup of plaque in the inside walls (atherosclerosis). Over time, plaque buildup in the coronary arteries can:
Narrow the arteries so that less blood flows to the heart muscle
Block completely the arteries and the flow of blood
Cause blood clots to form and block the arteries
A less common cause of heart attacks is a severe spasm (tightening) of the coronary artery that cuts off blood flow to the heart. These spasms can occur in persons with or without CAD. Artery spasm can sometimes be caused by:
Taking certain drugs, such as cocaine
Emotional stress
Exposure to cold
Cigarette smoking

Reference:
National Heart, Lung, and Blood Institute, USA.