Thursday, October 21, 2010


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

Mad Sci Network.

June Wingert
Associate Scientist
Lexicon Genetics