What are dizziness and fainting?

Causes and Symptoms

In humans, several mechanisms have evolved by which adequate blood flow to organs
is maintained. Without a constant blood supply, the body’s tissues would die from
a lack of essential nutrients and oxygen. In particular, the brain and heart are
very sensitive to changes in their blood supply as they, more than any other
organs, must receive oxygen and nutrients at all times. If they do not, their
cells will die and cannot be replaced.

While the heart supplies most of the force needed to propel the blood throughout
the body, tissues rely on changes in the size of arteries to redirect blood flow
to where it is needed most. For example, after a large meal the blood vessels that
lead to the gastrointestinal tract enlarge (vasodilate) so that more blood can be
present to collect the nutrients from the meal. At the same time, the blood
vessels that supply muscles decrease in diameter (vasoconstrict) and effectively
shunt the blood toward the stomach and intestines. During exercise, the blood
vessels that supply the muscles dilate and the ones leading to the intestinal
tract vasoconstrict. This mechanism allows the cardiovascular system to supply the
most blood to the most active tissues.

The brain is somewhat special in that the body tries to maintain a nearly constant
blood flow to it. Located in the walls of the carotid arteries, which carry blood
to the brain, are specialized sensory cells that have the ability to detect
changes in blood
pressure. These cells are known as baroreceptors. If the
blood pressure going to the brain is too low, the baroreceptors send an impulse to
the brain, which in turn speeds up the heart rate and causes a generalized
vasoconstriction. This reflex response raises the body’s blood pressure,
reestablishing adequate blood flow to the brain. If the baroreceptors detect too
high a blood pressure, they send a signal to the brain, which in turn slows the
heart rate and causes the arteries of the body to dilate. These reflexes prevent
large fluctuations in blood flow to the brain and other tissues.

Most people have experienced a dizzy feeling or maybe even a fainting response
when they have stood up too quickly from a prone position. The ability of the
baroreceptors to maintain relatively constant arterial pressure is extremely
important when a person stands after having been lying down. Immediately upon
standing, the pressure in the carotid arteries falls, and a reduction of this
pressure can cause dizziness or even fainting. Fortunately, the falling pressure
at the baroreceptors elicits an immediate reflex, resulting in a more rapid heart
rate and vasoconstriction, minimizing the decrease in blood flow to the brain.

Blood pressure is not the only factor that is essential in maintaining tissue
viability. The accumulation of waste products and a lack of essential nutrients
and gases can also have a profound effect on how much blood flows through a
particular tissue and how quickly. In a region of the carotid arteries near the
baroreceptors are chemoreceptors. Chemoreceptors detect the concentration of the
essential gas oxygen and the concentration of the gaseous waste product
carbon
dioxide. When carbon dioxide concentrations increase and
oxygen concentrations decrease, the chemoreceptors stimulate regions in the brain
to increase the heart rate and blood pressure in an attempt to supply the tissues
with more oxygen and flush away the excess carbon dioxide. If the chemoreceptors
detect high levels of oxygen and low levels of carbon dioxide, an impulse is
transmitted to the brain, which in turn slows the heart rate and decreases the
blood pressure.

Normally, most of the blood flow to the brain is controlled by the baroreceptor and chemoreceptor reflexes. However, the brain has a backup system. If blood flow decreases enough to cause a deficiency of nutrients and oxygen and an accumulation of waste products, special nerve cells respond directly to the lack of adequate energy sources and become strongly excited. When this occurs, the heart is stimulated and blood pressure rises.

Dizziness is a sensation of light-headedness often accompanied by a sensation of
spinning (vertigo). Occasionally, a person experiencing dizziness will
feel nauseated and may even vomit. Most attacks of dizziness are harmless,
resulting from a brief reduction in blood flow to the brain. There are several
causes of dizziness, and each alters blood flow to the brain for a slightly
different reason.

A person rising rapidly from a sitting or lying position may become dizzy. This is
known as postural hypotension, which is caused by a relatively slow reflexive
response to the reduced blood pressure in the arteries providing blood to the
brain. Rising requires increased blood pressure to supply the brain with adequate
amounts of blood. Postural hypotension is more common in the elderly and in
individuals prescribed antihypertensive medicines (drugs used
to lower high blood pressure).

If the patient experiences vertigo with dizziness, the condition is usually caused
by a disorder of the inner ear equilibrium system. Two
disorders of the inner ear that can cause dizziness are labyrinthitis
and Ménière’s
disease. Labyrinthitis, inflammation of the fluid-filled
canals of the inner ear, is usually caused by a virus. Since these canals are
involved in maintaining equilibrium, when they become infected and inflamed, one
experiences the symptom of dizziness. Ménière’s disease is a degenerative disorder
of the ear in which the patient experiences not only dizziness but also
progressive hearing loss.

Some brain-stem disorders also cause dizziness. The brain stem houses the
vestibulocochlear nerve, which transmits messages from the ear to several other
parts of the nervous system. Any disorder that alters the functions of this nerve
will result in dizziness and vertigo. Meningitis (inflammation of the
coverings of the brain and spinal cord), brain tumors,
and blood-flow deficiency disorders such as atherosclerosis may affect the function of the
vestibulocochlear nerve.

Syncope (fainting) is often preceded by dizziness. Syncope
is the temporary loss of consciousness as a result of an inadequate blood flow to
the brain. In addition to losing consciousness, the patient may be pale and
sweaty. The most common cause of syncope is a vasovagal attack, in which an
overstimulation of the vagus nerve slows the heart. Often
vasovagal syncope results from severe pain, stress, or fear. For example, people
may faint when hearing bad news or at the sight of blood. More commonly,
individuals who have received a painful injury will faint. Rarely, vasovagal
syncope may be caused by prolonged coughing, straining to defecate or urinate,
pregnancy, or forcing expiration. Standing still for long
periods of time or standing up rapidly after lying or sitting can cause fainting.
With the exception of vasovagal syncope, all the causes of syncope are
attributable to inadequate blood returning to the heart. If blood pools in the
lower extremities, there is a reduced amount available for the heart to pump to
the brain. In vasovagal syncope and some disorders of heart rhythm such as
Adams-Stokes syndrome, it is the heart itself that does not force enough blood
toward the brain.

Treatment and Therapy

Short periods of dizziness usually subside after a few minutes. Deep breathing and
rest will usually help relieve the symptom. Prolonged episodes of dizziness and
vertigo should be brought to the attention of a physician.

Recovery from fainting likewise will occur when adequate blood flow to the brain is reestablished. This happens within minutes because falling to the ground places the head at the same level as the heart and helps return the blood from the legs. If a person does not regain consciousness within a few minutes, a physician or emergency medical team should be notified.

The most common cause of syncope is decreased cerebral blood flow resulting from
the limitation of cardiac output. When the heart rate falls below its normal
seventy-five beats per minute to approximately thirty-five beats per minute, the
patient usually becomes dizzy and faints. Although slow heart rates can occur in
any age group, they are most often found in elderly people who have other heart
conditions. Drug-induced syncope can also occur. Drugs for congestive heart
failure (digoxin) or antihypertensive medications that slow the heart rate
(propranolol, metoprolol) may reduce blood flow to the brain sufficiently to cause
dizziness and fainting.

Exertional syncope occurs when individuals perform some physical activity to which
they are not accustomed. These physical efforts demand more work from the
cardiovascular system, and in patients with some obstruction of the arteries which
leave the heart, the cardiovascular system is overstressed. This defect, combined
with the vasodilation in the blood vessels that provide blood to the working
muscles, reduces the amount of blood available for use by the brain. If the person
also hyperventilates during exercise, he or she will effectively
reduce the amount of carbon dioxide in the blood and rid the cardiovascular system
of this normal stimulus for increasing heart rate and blood flow to the brain.
Some persons also hold their breath during periods of high exertion. For example,
people attempting to lift something very heavy often take a deep breath just prior
to exerting and then hold their breath when they lift the object. This practice,
known as the Valsalva maneuver, increases the pressure within the chest cavity,
which in turn reduces the amount of blood returning to the heart. A decrease in
blood returning to the heart (venous return) causes a decrease in the availability
of blood to be pumped out of the heart and reduces cardiac output. The reduction
in cardiac output decreases the amount of blood flowing to the brain and initiates
a fainting response. It is interesting to note that humans also use the Valsalva
maneuver when defecating or urinating, particularly when they strain. These acts
can also lead to exertional syncope.

For a physician to diagnose and treat dizziness and fainting accurately, he or she
must take an accurate medical history, paying particular attention to
cardiovascular and neurological problems. In addition to experiencing episodes of
dizziness and fainting, patients often have a weak pulse, low blood pressure
(hypotension), sweating, and shallow breathing. Heart rate and blood pressure are
monitored while the patient assumes different positions. The clinician also
listens to the heart and carotid arteries to determine whether there are any
problems with these tissues, such as a heart valve problem or atherosclerosis of
the carotid arteries. An electrocardiogram (ECG or EKG) can
detect abnormal heart rates and rhythms that may reduce cardiac output. Laboratory
tests are used to determine whether the patient has low blood sugar
(hypoglycemia), too little blood volume (hypovolemia), too
few red blood cells (anemia), or abnormal blood gases
suggesting a lung disorder. Finally, if the physician suspects a neurological
problem such as a seizure disorder, he or she may run an electroencephalogram (EEG) to record brain activity.

Treatment for any of these underlying disorders may cure the dizziness and
fainting episodes. In patients with postural hypotension, merely being aware of
the condition will allow them to change their behavior to lessen the chances of
becoming dizzy and fainting. These patients should not make any sudden changes in
posture that could precipitate an attack. Often, this means simply slowing down
their movements and learning to assume a horizontal position if they feel dizzy.
Patients also can learn to contract their leg muscles and not hold their breath
when rising. This increases the amount of blood available for the heart to pump
toward the brain. If these techniques do not provide an adequate solution for
postural hypotension, then a physician can prescribe drugs to increase blood
pressure.

Heart rhythm disturbances (arrhythmias) that cause an abnormally
fast or slow heart rate can be corrected with drug therapy such as quinidine or
disopyramide (if the rate is too rapid) or a pacemaker (if the rate is too slow).
It is interesting to note that even too fast a heart rate can cause dizziness and
fainting. In patients with this type of arrhythmia, the heart beats at such a
rapid rate that it cannot efficiently fill with blood before the next contraction.
Therefore, less blood is pumped with each beat.

Other treatments for dizziness and fainting may include correcting the levels of
certain blood elements. Patients with hypoglycemia often feel dizzy. The brain and
spinal cord require glucose as their energy source. In fact, the brain and spinal
cord have a very limited ability to utilize other substrates such as fat or
protein for energy. Because of this, patients often feel light-headed when there
are inadequate levels of glucose in the blood. Patients can correct this condition
by eating more frequent meals, and if necessary, physicians can administer drugs
such as epinephrine or glucagon. These agents liberate glucose from storage sites
in the liver.

Individuals with a low blood volume are often dehydrated and upon becoming
rehydrated no longer have dizziness or fainting episodes. If dehydration
is not corrected and becomes worse, the patient can go into shock, a state of
inadequate blood flow to tissues that will result in death if left untreated. In
addition to being dizzy or fainting, the patient is often cold to the touch and
has a rapid heart rate, low blood pressure, bluish skin, and rapid breathing.
These patients are treated by emergency medical personnel, who keep the individual
warm, elevate the legs, and infuse fluid into a vein. Drugs may be used to help
bring blood pressure back to normal. The cause of the shock should be identified
and corrected.

Perspective and Prospects

As humans evolved, they assumed an upright posture. This is advantageous because
it allows for the use of the front limbs for other things besides locomotion.
Unlike most four-legged animals, however, humans have their brains above their
hearts and must continually force blood upwards to reach this vital tissue. This
adaptation to the upright posture is a continuing physiological problem because
the cardiovascular system must counteract the forces of gravity to provide the
brain with blood. If this does not occur, the individual becomes dizzy and
faints.

Another significant problem that humans face is adaptation to brain blood flow
during exercise. The amount of blood flowing to a tissue is usually proportional
to the metabolic demand of the tissue. At rest, various organs throughout the body
receive a certain amount of the cardiac output. For example, blood flow to
abdominal organs such as the spleen and the kidneys requires about 43 percent of
the total blood volume. The total flow to the brain is estimated to be 13 percent,
and the skin and skeletal muscles require 21 percent and 9 percent, respectively.
Other areas such as the gastrointestinal tract and heart receive the remaining 14
percent. During exercise, the skeletal muscles may receive up to 80 percent of the
cardiac output while the rest of the organs are perfused at a much reduced
rate.

Most data indicates that the brain receives only 3 percent of the total cardiac
output during heavy exercise. Even though there is a large change in the
redistribution of cardiac output, physiologists do not know the absolute amount of
blood reaching the brain or the mechanism for the change in the perfusion
rate.

With strenuous aerobic exercise such as jogging, there is an increase in cardiac
output. During strenuous anaerobic exercise such as weight lifting, however, there
may be a decrease in cardiac output, attributable to the Valsalva maneuver.
Therefore, it has been difficult to predict accurately, using available
techniques, the volume of blood reaching this critical tissue.

Bibliography

Babikian, Viken K.,
and Lawrence R. Wechsler, eds. Transcranial Doppler
Ultrasonography. 2nd ed. Boston: Butterworth-Heinemann, 1999.
Print.

Brandt, Thomas.
Vertigo: Its Multisensory Syndromes. 2nd ed. New York:
Springer, 2003. Print.

Furman, Joseph M., and
Stephen P. Cass. Vestibular Disorders: A Case-Study
Approach. 3rd ed. New York: Oxford UP, 2010. Print.

Geelen, G., and J. E.
Greenleaf. “Orthostasis: Exercise and Exercise Training.” Exercise
and Sport Sciences Reviews 21 (1993): 201–30. Print.

Guyton, Arthur C.
Human Physiology and Mechanisms of Disease. 6th ed.
Philadelphia: Saunders, 1997. Print.

Leikin, Jerrold B.,
and Martin S. Lipsky, eds. American Medical Association Complete
Medical Encyclopedia. New York: Random House Reference, 2003.
Print.