Don't try to diagnose yourself; seek immediate medical attention if you lose consciousness.
Fainting can be alarming, and it should be. While often the cause of fainting is something minor, fainting also can be a sign of a serious underlying medical concern. The problem is that you can't evaluate yourself, and you should let a physician determine if fainting is worrisome or not.
Fainting usually is caused by a temporary drop in blood pressure. During that brief drop, the brain does not get the blood flow that it needs—and you lose consciousness. One serious cause of this drop in blood pressure is bleeding, such as in the stomach or intestines, or from a rupture of the body's main artery, the aorta.
Several different heart problems also can temporarily lower blood pressure. One is heart block, in which the heart beats too slowly to pump enough blood. A very fast or irregular heart rhythm can cause the heart to pump blood less efficiently. Abnormalities of a heart valve, particularly a stiffening of the aortic valve, also can cause a temporary loss in pressure.
All of these heart problems often produce symptoms such as palpitations (a feeling like your heart is skipping a beat or racing), shortness of breath, or chest tightness. If you have any of these symptoms, it is urgent that you get immediate medical attention.
Another serious cause of a sudden loss of consciousness is a seizure, which is an abnormality of the brain, not related to blood pressure. Some seizures produce dramatic shaking movements and loss of consciousness for longer than most fainting spells. However, other seizures can be more subtle and hard to recognize as seizures.
Less serious causes
Sometimes fainting is caused by stimulation of the vagus nerve, which can briefly lower both heart rate and blood pressure. The condition is called vasovagal syncope (SIN-cope-ee). It can occur if you strain while having a bowel movement (or, for men, while passing urine), have blood drawn, get an injection, hear bad news, or even laugh too hard. These kinds of fainting episodes commonly affect young people but can occur in older adults. Just before a person faints from vasovagal syncope, he or she often feels nauseated or breaks out in a cold sweat.
You may lose consciousness for just a moment if your blood pressure drops when you stand, a condition called orthostatic hypotension. Gravity temporarily pulls blood down into the veins of your legs and feet. This reduces the amount of blood that returns to the heart and which thereafter can be pumped to your brain. Medications, especially blood pressure drugs, often cause orthostatic hypotension. So can dehydration, thyroid disorders, and neurological conditions such as Parkinson's disease.
The bottom line is that you need medical evaluation if you faint—or if you feel repeatedly as if you are about to faint. Note carefully any symptoms you remember before or after you pass out. Ask anyone who may have seen you faint describe to you what they saw. All of this information will help the doctor help you.
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