MadSci Network: Medicine

Re: what are the top ten ways why people faint

Date: Thu Feb 17 15:28:16 2005
Posted By: June M. Wingert , RM (NRM), Associate Scientist
Area of science: Medicine
ID: 1106696897.Me


Below you will find the top ten reasons for fainting.

Fainting is a symptom of an inadequate supply of oxygen and other 
nutrients to the brain, usually caused by a temporary decrease in blood 
flow. Blood flow to the brain can decrease whenever the body cannot 
quickly compensate for a fall in blood pressure.

1. Fainting may occur if the heart cannot pump enough blood to maintain a 
normal blood pressure. For example, an abnormal heart rhythm or a heart 
valve disorder may impair the heart's pumping ability. People with such 
disorders may feel fine when resting. However, they feel faint or actually 
faint when exercising because the heart cannot pump enough blood to meet 
the body's increased demand for oxygen. This type of fainting is called 
exertional or effort syncope. People with these disorders may also faint 
after exercising. During exercise, the increase in heart rate may enable 
the heart to pump enough blood to maintain adequate blood pressure, 
although just barely. When exercise stops, the heart rate (and the amount 
of blood pumped) begins to decrease. However, the blood vessels in 
muscles, which dilate (widen) during exercise to move more blood to and 
from the muscles, remain dilated. (The arterioles in muscles remain 
dilated to help supply oxygen and nutrients to muscle tissue, and the 
veins remain dilated to remove metabolic waste products produced during 
exercise.) The decrease in the amount of blood pumped out combined with 
dilation of the arterioles and veins causes blood pressure to fall, and 
fainting results.
2. An abnormality of the heart called hypertrophic cardiomyopathy (see 
Cardiomyopathy: Hypertrophic Cardiomyopathy) can also cause fainting that 
usually occurs during exercise. This disorder may occur in younger people 
as well as older people, particularly those who have high blood pressure. 
If untreated, it can lead to death.
3. Fainting may occur if blood volume is too low. An obvious cause of low 
blood volume is bleeding. Another cause is dehydration, which may be due 
to diarrhea, excessive sweating, inadequate intake of fluids, or excessive 
urination (which is a common symptom of untreated diabetes (see Diabetes 
Mellitus) or Addison's disease (see Adrenal Gland Disorders: Addison's 
Disease)). In older people, the use of diuretics is a common cause of 
dehydration, particularly during warm weather or during an illness when 
obtaining or drinking enough fluids may be difficult. (Diuretics help the 
kidneys eliminate salt and water by increasing urine formation and thus 
decrease fluid volume in the body.)
4. Fainting may occur if the vagus nerve, which supplies the neck, chest, 
and intestine, is stimulated. When stimulated, the vagus nerve slows the 
heart. Such stimulation also causes nausea and cool, clammy skin. This 
type of fainting is called vasovagal (vasomotor) syncope. The vagus nerve 
is stimulated by pain (such as intestinal cramps), fear, other distress 
(such as that due to the sight of blood), vomiting, a large bowel 
movement, and urination. Fainting during or immediately after urination is 
called micturition syncope. Rarely, vigorous swallowing causes fainting 
due to stimulation of the vagus nerve.
5. Fainting may also occur if straining reduces the amount of blood 
flowing back to the heart. Fainting due to coughing (cough syncope) 
usually results from such straining. Fainting after urination (micturition 
syncope) or after a bowel movement is partly due to straining (in addition 
to stimulation of the vagus nerve). Older men who must strain to empty 
their bladder because of a large prostate gland are particularly 
susceptible. Fainting when lifting weights (weight lifter's syncope) 
results from the strain of trying to lift or push heavy weights without 
breathing adequately during the exercise.
6. Fainting that occurs when a person sits or stands up too quickly is 
called orthostatic (postural) syncope. It is particularly common among 
older people. It is caused by orthostatic hypotension (see Low Blood 
Pressure: Orthostatic Hypotension). In orthostatic hypotension, the 
compensatory mechanisms, particularly the constriction of blood vessels 
and the increase in heart rate, do not adequately restore blood pressure 
when a person stands and gravity causes blood to pool in the leg veins. A 
related form of fainting, called parade ground syncope, occurs when people 
stand still for a long time on a hot day. If the leg muscles are not used, 
blood is not pumped back to the heart. As a result, blood pools in the leg 
veins, and blood pressure falls.
7. In older people, an excessive decrease in blood pressure after eating a 
meal (postprandial hypotension (see Low Blood Pressure: Postprandial 
Hypotension)) may cause fainting.
8. Fainting may result from very rapid breathing (overbreathing, or 
hyperventilation), which may be due to anxiety. This type of fainting is 
called hyperventilation syncope. Overbreathing removes large amounts of 
carbon dioxide from the body. The decreased level of carbon dioxide causes 
blood vessels in the brain to constrict, and the person may feel faint or 
actually faint.
9. Rarely, fainting results from a mild stroke in which blood flow to a 
part of the brain suddenly decreases. Fainting due to a stroke is more 
common among older people. Many other disorders, such as a deficiency of 
red blood cells (anemia), lung disorders, a decreased blood sugar level 
(hypoglycemia), and diabetes can cause fainting, especially if the 
compensatory mechanisms are also impaired.
10. Certain drugs may cause fainting. They include many of those used to 
treat high blood pressure, angina, and heart failure. Doses of these drugs 
must be carefully adjusted to prevent blood pressure from decreasing too 

Check out the following sites for more information on fainting

Thanks for taking the time to send in a question to the Mad Sci Network.

June Wingert
Associate Scientist
Lexicon Genetics

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