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Harvard Commentaries
Harvard Commentaries
Reviewed by the Faculty of Harvard Medical School

Man to Man Man to Man

Fainting -- Harmless or a Red Flag?

October 23, 2014

By Harvey B. Simon M.D.

Harvard Medical School

Few things are more alarming than a sudden and complete loss of consciousness and control. It can happen to anyone, anywhere, and without warning. Often, you start out feeling a little nauseated and the room grows dim. You may feel clammy and sweaty. Then you wake up — on the floor. It's all over in a minute or two and you quickly get back to normal. But it's been frightening and embarrassing.

You fainted, blacked out or passed out. Doctors call it syncope (sin-Ko-pee). Syncope is not the same as falling, dizziness, lightheadedness, grogginess or confusion. And a fainting spell should not be ignored. In many cases, it's harmless, but in some people it can be a warning that serious trouble lies ahead.

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What's Happening to Me?

About 40% of Americans will faint at least once in their lifetimes. In most cases, the problem is a brief drop in blood flow to the brain.

Every time you stand up, gravity pulls nearly a quarter of your body's blood supply into your legs and lower body. Your heart and circulation must respond within seconds to keep your blood pressure high enough to pump blood up to your brain.

When things go well, your heart will speed up by 10 to 15 beats a minute, and your nervous system will tell your arteries to narrow down so you can meet the challenge. But if everything is not just right, your blood pressure may drop — and you may, too. Fainting can occur shortly after you get up or after you've been standing for a while. It's much less frequent when people are sitting or lying down, which is why fainting causes more concern under these circumstances.

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The Garden-Variety Fainting Spell

The most common type of fainting is what doctors call neurovascular or vasovagal syncope. The chief culprit is thevagus nerve, which influences the heart and blood vessels.

Neurovascular syncope occurs when the heart slows down instead of speeding up. The blood vessels widen instead of narrowing. Your blood pressure falls, which deprives the brain of its full supply of blood. But from the brain's point of view, the lapse of consciousness serves a protective function: By falling down, you restore blood flow to the brain long before any damage can be done.

Here are some of the many things that can trigger neurovascular fainting:

  • Strong emotion. It's why a marine faints during a blood test, why a medical student passes out during his first trip to the operating room or why a patient collapses when a doctor gives him bad news.
  • Pain, usually intense
  • Pressure on an internal abdominal organ, such as the bowel or bladder; examples include diarrhea caused by a virus and urinary obstruction due to an enlarged prostate.
  • A bowel movement or urination
  • Coughing
  • Dehydration. When the tank isn't full, it's harder to keep the pressure up.
  • Prolonged standing in one spot. The muscular activity of walking assists the circulation, but standing still has the opposite effect.
  • Medications, particularly drugs that are prescribed to lower the blood pressure and/or slow the heart rate. Nitrates (nitroglycerin and others)and beta blockers (Inderal and others )head the long list. If you faint, take a list of all your medications with you when you see your doctor.
  • Alcohol. It's not a medication, but it acts like a drug. Even small amounts of alcohol can increase the risk of syncope when any other trigger comes along.
  • Underlying illness, ranging from an acute infection, such as the flu, to chronic problems that can damage nerves, such as diabetes or Parkinson's disease.

Most people recover from neurovascular fainting within 30 seconds to a minute on their own. Doctors will suspect another cause if it takes more than five minutes for the person to come around.

If you see someone faint, you can help by making sure the individual is lying flat with his legs elevated.

If you feel faint, lie down, ideally with your leg spropped upon a chair, before you falld own. If you don't have space to lie down, sit down with your knees bent and your head down between your knees. And you may be able to ward off a fainting spell by crossing your legs and tensing your leg, abdominal and buttock muscles or by clenching your fists as tight as possible when warning symptoms first appear; both maneuvers help by boosting blood pressure.

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When Heart Problems Cause Fainting

Fainting due to heart disease — called cardiac syncope — is serious. Anyone who faints should notify a doctor. But people with heart disease or cardiac risk factors should get prompt medical attention after a fainting spell.

Cardiac syncope usually occurs in people who already know they have heart disease, particularly coronary artery disease. Most often, fainting results from an arrhythmia, a disorder of the heart's pumping rhythm. Patients may experience a slow, erratic or rapid pulse before they faint. Or they may complain of chest pressure or pain or shortness of breath. But arrhythmias can cause fainting without other symptoms, and the worst of them can be fatal. That's why your doctor will order tests, such as monitoring your heart rhythm for 24 hours or longer if he suspects an arrhythmia.

Another important cause of cardiac syncope is a blockage of blood flow across the aortic valve. This valve separates the heart's main pumping chamber from the circulation. The blockage could be due to an abnormal narrowing of the aortic valve(aortic stenosis) or an abnormal thickening of the heart muscle itself (hypertrophic cardiomyopathy). In either case, fainting is often triggered by physical exertion. A heart murmur is usually the first clue to these problems. A doctor will diagnose either condition with an echo cardiogram.

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Neurologic Reasons for Fainting

It's logical to blame the brain for a sudden loss of consciousness. But neurologic disorders, such as seizures or vascular blockages, are relatively uncommon culprits. And in most cases patients have other signs or symptoms pointing to nervous system disease.

Many patients who faint are given tests such as EEGs, ultrasounds, CT scans and MRIs. Unless there is clear evidence of a neurologic disorder, those expensive tests are rarely useful.

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Other Causes of Fainting

There are many less common causes of fainting than neurovascular syncope, heart disease and brain disorders.

  • People with diabetes may lose consciousness because of low blood sugar (hypoglycemia).
  • A migraine attack can cause fainting, usually in conjunction with a severe headache.
  • Pressure on the neck, perhaps caused by a tight collar, can cause fainting in rare cases.
  • Blood clots that travel to the lungs (pulmonary emboli) are uncommon but serious causes of syncope.
  • Psychiatric disorders such as panic attacks, generalized anxiety disorder, hysteria and even simple hyperventilation, usually triggered by anxiety, can cause fainting.

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Keep a Clear Head

If you faint, remember that it's most likely neurovascular syncope. But in case it's something more serious, take careful note of the circumstances surrounding your loss of consciousness. Ask anyone who saw you faint for additional details. Tell it all to your doctor, along with your medical history and current medications.

Young people in good health with a typical story for neurovascular fainting may not need to do more than review the situation with a physician. But most other people should have a careful physical exam and an ECG. And if there are any suggestions of heart disease, neurological abnormalities or other serious problems, you'll need other tests. So even if you wake up quickly and feel perfectly well after you've fainted, do the wise and upright thing by reporting to your doctor.

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Harvey B. Simon, M.D. is an Associate Professor of Medicine at Harvard Medical School and a member of the Health Sciences Technology Faculty at Massachusetts Institute of Technology. He is the founding editor of the Harvard Men's Health Watch newsletter and author of six consumer health books, including The Harvard Medical School Guide to Men's Health (Simon and Schuster, 2002) and The No Sweat Exercise Plan, Lose Weight, Get Healthy and Live Longer (McGraw-Hill, 2006). Dr. Simon practices at the Massachusetts General Hospital; he received the London Prize for Excellence in Teaching from Harvard and MIT.

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