Tool Module: Anesthesics and
Analgesics
Pain is one of the primary manifestations of injuries and of acute and
chronic illnesses. Even though natural healing processes can alleviate
pain and eventually make it disappear, people have been trying since time
immemorial to relieve it by means of substances extracted from plants
(such as morphine) or, more recently, produced in the laboratory.
Growing scientific understanding of the receptors and neurotransmitters
involved in the body’s nociceptive (pain-sensing) pathways has led to the
development of new medications that prevent or diminish pain. These
molecules act by binding to specific receptors that they activate or block
at various locations along these pathways. These various kinds of
medications can thus modify the transmission of pain both in the
peripheral nervous system and in the spinal cord and the brain.
There are two main classes of medications that modulate the perception
of pain: anesthetics and analgesics.
Anesthetics are used to reduce activity of the nervous
system so as to prevent pain during surgical operations.
General anesthetics are compounds that act on the brain by lowering its
level of alertness until the individual loses consciousness. Because pain
is a conscious process, it disappears when the anesthetized brain thus
falls asleep. But this is an artificial form of sleep, very different from
natural sleep. The source of the pain is still there, and so are the
physiological reflexes that accompany it, but the individual is no longer
aware of them.
Local or regional anesthetics are used to put only the nerves in a
certain part of the body “to sleep”. The most familiar examples may be the
local anesthetic that your dentist uses to put the nerves in your teeth to
sleep so that no pain reaches your brain while he or she is drilling, and
the epidural anesthesia used to relieve the pain of childbirth.
Analgesics are a class of medications used to
alleviate or eliminate pain. They can be used to relieve post-operative
pain, but their most common application is to virtually any kind of
chronic or acute pain. Analgesics are also referred to as antipain
medications, or painkillers.
Unlike general anesthetics, analgesics do not cause loss of
consciousness. Each type of analgesic medication requires a specific
amount of time to take effect and has a specific duration of action. Some
analgesics take effect in just a few seconds, but their effects last for
only a few minutes. Others, such as morphine, take effect more slowly (in
about 15 minutes), but their effects are longer-lasting (3 to 4 hours).
Two of the most commonly used categories of analgesics are opiates and
nonsteroidal anti-inflammatory drugs (NSAIDs).
Opiates included natural opiates—substances extracted
from opium poppies, such as morphine—and their synthetic derivatives, such
as meperidine (Demerol) and methadone. Opiates have long been and remain
the type of medication most often used to relieve moderate to severe pain.
The higher the dosage, the more effective they are, but the more their
undesirable side effects become disabling and dangerous as well. The most
common of these side effects are constipation, sleepiness, and depressed
breathing.
Opiates act on one or more of the main opioid receptors and their
subtypes in the brain. Some of the undesirable side-effects of opiates are
associated with the stimulation of certain specific receptor subtypes.
Researchers are therefore trying to synthesize opiates that do not target
these subtypes, but only those receptors that produce the desired
analgesic effect.
Nonsteroidal anti-inflammatory drugs (NSAIDs) are so
designated to distinguish them from steroids (glucocorticoids), which also
have significant anti-inflammatory properties. NSAIDs have an analgesic
effect that relieves light to moderate pain. The two best known NSAIDs are
aspirin and ibuprofen.
Aspirin (acetylsalicylic acid) is the most popular
medication in the world. It was first marketed in 1899, by Bayer, a German
company. Aspirin acts by inhibiting cyclooxygenase, an enzyme that is
important for the synthesis of prostaglandins, which are derived from
arachidonic acid, a molecule released when body tissue is injured.
Prostaglandins depolarize nociceptive neurons, thus making them more
sensitive and causing people to be more sensitive to pain. By blocking the
production of the enzyme that manufactures these prostaglandins, aspirin
thus reduces the painful inflammation that they cause.
NSAIDs such as aspirin have a number of other effects, some beneficial,
others harmful. NSAIDs lower fever, slow blood coagulation, can cause
stomach ulcers, and have toxic effects on the kidneys and liver.
Paracetamol, also called
acetaminophen, is another analgesic (painkilling) and
antipyretic (fever-lowering) medication. It has been on the market since
1950, and its strength is comparable to that of aspirin. Unlike aspirin,
acetaminophen has practically no anti-inflammatory effect and no effect on
the clumping of platelets that results in blood clotting. Hence it cannot
be prescribed, as aspirin can, to reduce the risks of heart attacks and
strokes caused by clotting in blood vessels. Acetaminophen is the active
ingredient in numerous medications sold under various names (Tylenol or
Panadol in Canada and the United States, and Doliprane, Dafalgan, or
Efferalgan in Europe). It has no serious side effects when taken in the
recommended doses, but overdoses can be highly toxic to the liver.
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