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Heart Disease Health Center

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Fainting - Topic Overview

What is fainting?

Fainting is a sudden, brief loss of consciousness. When people faint, or pass out, they usually fall down. After they are lying down, most people will recover quickly.

The term doctors use for fainting is syncope (say "SING-kuh-pee").

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Fainting one time is usually nothing to worry about. But it is a good idea to see your doctor, because fainting could have a serious cause.

What causes fainting?

Fainting is caused by a drop in blood flow to the brain. After you lose consciousness and fall or lie down, more blood can flow to your brain so you wake up again.

The most common causes of fainting are usually not signs of a more serious illness. In these cases, you faint because of:

  • The vasovagal reflex, which causes the heart rate to slow and the blood vessels to widen, or dilate. As a result, blood pools in the lower body and less blood goes to the brain. This reflex can be triggered by many things, including stress, pain, fear, coughing, holding your breath, and urinating.
  • Orthostatic hypotension, or a sudden drop in blood pressure when you change position. This can happen if you stand up too fast, get dehydrated, or take certain medicines, such as ones for high blood pressure.

Fainting caused by the vasovagal reflex is often easy to predict. It happens to some people every time they have to get a shot or they see blood. Some people know they are going to faint because they have symptoms beforehand, such as feeling weak, nauseated, hot, or dizzy. After they wake up, they may feel confused, dizzy, or ill for a while.

Some causes of fainting can be serious. These include:

Sometimes the cause is unknown.

When is fainting the sign of a serious problem?

Fainting may be the sign of a serious problem if:

  • It happens often in a short period of time.
  • It happens during exercise or a vigorous activity.
  • It happens without warning or if it happens when you are already lying down. (When fainting is not serious, a person often knows it is about to happen and may vomit or feel hot or queasy.)
  • You are losing a lot of blood. This could include internal bleeding that you can't see.
  • You feel short of breath.
  • You have chest pain.
  • You feel like your heart is racing or beating unevenly (palpitations).
  • It happens along with numbness or tingling on one side of the face or body.
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