McKenna's Pharmacology for Nursing, 2e

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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.

KEY POINTS

■■ 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