What Causes Fainting?
Fainting: Is It Your Head or Your Heart?
Passing out isn't always a neurological issue. And although it's fairly common, fainting shouldn't be ignored. Here's what you need to know.
By Charlotte Libov
Medically Reviewed by Pat F. Bass III, MD, MPH
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Fainting, also called passing out, can be frightening. But it’s actually a common occurrence. An estimated one in three people will faint at some point in their lives.
The person usually regains consciousness within a few minutes, recovers quickly, and returns to normal. But sometimes fainting can signal a serious medical condition, even heart disease, so getting to the reason behind the incident is important.
The Signs and Causes of Fainting
Fainting occurs when one’s blood pressure suddenly drops, resulting in a decrease of blood flow to the brain.
A number of things can affect blood pressure, from abrupt changes in posture (like going from lying down to standing), dehydration, and certain medications. Feeling faint can include dizziness, lightheadedness, and nausea. One’s field of vision may even “black out.” This loss of consciousness triggers a loss in muscle control. That’s what causes the person to fall to the ground.
One of the most common types of fainting is caused by a sort of crossed signal between the brain and the vagus nerve, a large nerve that runs from the brain to the stomach. When this nerve is overstimulated, a person may faint. In such cases, you can usually figure out the reason — maybe you were standing for a long time, fainted at the sight of blood, or due to some kind of emotional distress, trauma, or pain.
Some people faint because they’ve suddenly constricted their carotid artery (the artery in the neck) by turning their head abruptly or wearing a too-tight collar. Straining to make a bowel movement or even urinating can sometimes cause fainting, too.
Fainting can also occur in people who have hypoglycemia, or low blood sugar, most common among diabetics because of fluctuating insulin levels. Dehydration can also cause fainting, particularly in the elderly. Certain types of medications, including
diuretics, heart medications, psychiatric drugs, antihistamines, and narcotics, can also trigger a fainting episode, as can alcohol.
When Fainting Means Heart Disease
Although less common, certain serious heart problems can cause fainting. The most common heart-related causes for fainting are problems with the heart’s electrical system, which can result in a too-slow heartbeat, known as bradycardia, or a too-fast heartbeat, known as tachycardia, according to Harvey Kramer, MD, a cardiologist at Danbury Hospital in Connecticut and an assistant professor of medicine at Yale University School of Medicine. Other less common heart-related causes of fainting include an enlarged, weakened heart, a tear in the aorta, or damage to the heart muscle from a heart attack.
Fainting may also be a sign of a valve disorder. Your heart has four valves, all of which must open and shut properly to keep blood flowing smoothly going through your heart. If there’s a valve problem, your body, including your brain, may not receive enough oxygenated blood, and fainting can occur, Dr. Kramer says. The valve problem most commonly responsible for fainting is aortic stenosis, which is the stiffening of the aortic valve, your heart’s main vessel. The condition usually occurs with age.
What should you do when you faint? “The bottom line is that if your doctor cannot come up with a reason why you fainted, or if it isn’t a benign reason, for example you were overdoing it in the heat or you got queasy because you were having blood drawn, then you need to see a cardiologist so cardiac causes can be ruled out,” Kramer says.
Video: Good Question: Why Do Locked Knees Cause Fainting?
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