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P A R T 4
Drugs acting on the central and peripheral nervous systems
incorporate emotional aspects of pain and response
to pain. At peripheral nerve sites, they may block the
release of neurotransmitters which are related to pain
and inflammation.
Pain perception
Many factors play a role in a person’s perception of
pain. Past experience has a big impact on how pain is
perceived. Having experienced pain in the past, a person
may fear the intensity it could reach and the overall
impact of that pain. Learned response to pain also
plays a large role. Children learn the accepted response
to painful stimuli when growing up. Some children are
taught to ignore pain and deal with it without showing
emotion. Some children learn that reacting to pain can
lead to much-wanted attention. The environmental
setting in which the pain occurs also has an influence
on perception and response to pain. A parent may not
be willing to admit pain when the children are present,
feeling that the role of the parent is to be strong. If you
cut your finger when you are alone, you may perceive
pain and react loudly. If you cut your finger when you
are surrounded by young children, you may show no
reaction and just go on with your activity. These varied
influences on pain perception and response often make
it very difficult to effectively evaluate and manage pain.
Pain management
Accurately assessing pain can lead to effective pain man-
agement. Because so many factors play a role in pain
perception and it is very subjective, assessment has to
depend on the person’s report of pain. Healthcare pro-
viders often use a scale system to evaluate an individual’s
pain. Individuals may be asked to rank their pain on a
scale from 0 to 10, with 0 being no pain and 10 being
the worst possible pain. Some pain scales use drawings
of faces and ask the person to pick the face that most
reflects the pain they feel. Numerous methods, both
non-pharmacological and pharmacological, may be used
to manage pain. Non-pharmacological treatments can
include warmth, massage, positioning, acupuncture or
meditation. Pharmacological methods often include the
use of non-steroidal anti-inflammatory drugs (NSAIDs)
or paracetamol (Chapter 16) for tissue-related pain or
atypical antipsychotics or other CNS depressants for
the treatment of neurogenic pain. These methods can
be used individually or in combination. The goal is to
achieve maximum pain relief.
One major method of pain management involves
the use of
opioids
. The opioids, or narcotics, were first
derived from the opium plant. Although most opioids
are now synthetically prepared, their chemical structure
resembles that of the original plant alkaloids. All drugs
in this class are similar, in that they occupy specific
opioid receptors in the CNS. Their actions in the body
are related to the stimulation of the various opioid recep-
tors that they occupy.
■■
When tissue is injured, various chemicals are released
and pain results.
■■
A-delta and C fibres carry pain impulses to the spinal
cord.
■■
According to the gate control theory of pain,
impulses travel from the spine to the cortex via tracts
that can be modulated along the way at specific gates.
These gates can be closed to block the transmission
of pain impulses by descending nerves from the upper
CNS, which relate to emotion, culture, placebo effect
and stress, and by large diameter sensory A fibres,
which are associated with touch.
■■
Endogenous endorphins and encephalins react with
opioid receptors to regulate the transmission of pain.
■■
Opioids are derived from the opium plant; they
bind to opioid receptors to relieve pain and promote
feelings of well-being or euphoria.
OPIOIDS
The opioid drugs used vary with the type of opioid recep-
tors with which they react. This accounts for a change in
pain relief, as well as a variation in the side effects that
can be anticipated. Four types of opioid receptors have
been identified: mu (
μ
), kappa (
κ
), beta (
β
) and sigma
(
σ
). The mu-receptors are primarily pain-blocking recep-
tors. Besides analgesia, mu-receptors also account for
respiratory depression, a feeling of euphoria, decreased
GI activity, pupil constriction and the development of
physical dependence. The kappa-receptors are associated
with some analgesia and with pupillary constriction,
sedation and dysphoria. The beta-receptors react with
encephalins in the periphery to modulate pain transmis-
sion. The sigma-receptors cause pupillary dilation and
may be responsible for the hallucinations, dysphoria and
psychoses that can occur with opioid use. The adminis-
tration of opioids requires specific considerations related
to age (Box 26.1).
O
pioid agonists
The
opioid agonists
(Table 26.1) are drugs that react
with the opioid receptors throughout the body to cause
analgesia, sedation or euphoria (Figure 26.2). Antici-
pated effects other than analgesia are mediated by the
types of opioid receptors affected by each drug. Because
of the potential for the development of physical depend-
ence while taking these drugs, the opioid agonists
are classified as controlled substances. The degree of
KEY POINTS
