|MadSci Network: Medicine|
Greetings, Below you will find the top ten reasons for fainting. www.mc.vanderbilt.edu/reporter/?ID=2287 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. Causes 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 much. Check out the following sites for more information on fainting www.anxietynetwork.com/pdfear.html www.merck.com/mmhe/sec03/ch023/ch023b.html www.eyewitnessnewstv.com/global/story.asp?s=1230242 Thanks for taking the time to send in a question to the Mad Sci Network. June Wingert Associate Scientist Lexicon Genetics Texas
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