A heart attack occurs when blood flow to a part of your heart is blocked for a long enough time that part of the heart muscle is damaged or dies. Your doctor calls this a myocardial infarction.

Most heart attacks are caused by a blood clot that blocks one of the coronary arteries. The coronary arteries bring blood and oxygen to the heart. If the blood flow is blocked, the heart is starved of oxygen and heart cells die.

A hard substance called plaque can build up in the walls of your coronary arteries. This plaque is made up of cholesterol and other cells. A heart attack can occur as a result of plaque buildup.

The plaque can develop cracks or tears. Blood platelets stick to these tears and form a blood clot. A heart attack can occur if this blood clot completely blocks oxygen-rich blood from flowing to the heart. This is the most common cause of heart attacks.

The slow buildup of plaque may almost block one of your coronary arteries. A heart attack may occur if not enough oxygen-rich blood can flow through this blockage. This is more likely to happen when your body is stressed (for example, by a serious illness).

The cause of heart attacks is not always known. Heart attacks may occur:

  • When you are resting or asleep
  • After a sudden increase in physical activity
  • When you are active outside in cold weather
  • After sudden, severe emotional or physical stress, including an illness

First Aid

If you think someone is having a heart attack:

  • Have the person sit down, rest, and try to keep calm.
  • Loosen any tight clothing.
  • Ask if the person takes any chest pain medication for a known heart condition, such as nitroglycerin, and help them take it.
  • If the pain does not go away promptly with rest or within 3 minutes of taking nitroglycerin, call for emergency medical help.
  • If the person is unconscious and unresponsive, call your local emergency number, then begin CPR.


  • Do NOT leave the person alone except to call for help, if necessary.
  • Do NOT allow the person to deny the symptoms and convince you not to call for emergency help.
  • Do NOT wait to see if the symptoms go away.
  • Do NOT give the person anything by mouth unless a heart medication (such as nitroglycerin) has been prescribed.

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