What Causes Fainting After Exercise?

Feeling Faint After Exercise Could Signal a Heart Problem

fatigued woman after exercise
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A great workout that makes you sweat and gets your blood pumping should make you feel good when you're leaving the gym. While everyone responds differently to exercise -- and some people can be weak in the knees, for example, when leaving a cycling class -- one thing that shouldn't happen after a work-out is feeling faint or actually fainting.

In fact, fainting after exercise can be a sign that something is seriously wrong.

Common Causes of Fainting

Fainting (the medical term is "syncope") occurs when the brain isn't getting enough blood. Common causes are overheating, suddenly getting up after lying down for a while and feeling the effects of certain medications.

After exercise, your blood pressure can drop because as the heart slows down, it pumps less blood into vessels that are still dilated as a result of your exertion.

Serious Causes of Fainting

Fainting after exercise can also signal a serious a heart problem. Some of the possibilities include:

  • Aortic stenosis: Your heart's aortic valve is narrowed from blockage, keeping blood from pumping out of the heart.
  • Hypertrophic cardiomyopathy: The heart muscle has become unusually thick, making it harder to pump blood to the body.
  • Long Q-T syndrome: This is a rare but deadly disorder of heart rhythm seen most often in children and young adults.

Tests to See What's Wrong

If you faint (or just come close) after exercise, report it to your doctor immediately.

You may need to take one or more of these tests to figure out what's wrong:

  • Electrocardiogram (EKG or ECG) -- a test that measures electrical impulses in the heart
  • Stress test -- your heart rate is increased either by using a treadmill or by an injection, and its response is monitored
  • Echocardiogram -- ultrasound is used to "see" how your heart is functioning

    If these tests show no problems, a "tilt test" may be ordered. You are placed on a table, which is then tilted to decrease blood flow to the brain. Fainting during this test means you may have neurally mediated syncope (NMS), a relatively harmless condition that is the most common cause of fainting.

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    "Aortic Stenosis." MedlinePlus. 12 May 2008. National Institutes of Health. 14 Nov. 2008 <http://www.nlm.nih.gov/medlineplus/ency/article/000178.htm>.

    "Hypertrophic Cardiomyopathy." MedlinePlus. 15 May 2008. National Institutes of Health. 14 Nov. 2008 <http://www.nlm.nih.gov/medlineplus/ency/article/000192.htm>.

    "Long Q-T Syndrome." americanheart.org. 2008. American Heart Association. 14 Nov. 2008 <http://www.americanheart.org/presenter.jhtml?identifier=993>.

    "Syncope." americanheart.org. 2008. American Heart Association. 14 Nov. 2008 <>.

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