McKenna's Pharmacology for Nursing, 2e

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P A R T 4  Drugs acting on the central and peripheral nervous systems

OPIOIDS Opioid agonists alfentanil codeine dextropropoxyphene fentanyl hydromorphone methadone

Opioid antagonists naloxone naltrexone

Triptans eletriptan

morphine oxycodone pethidine remifentanil tapentadol tramadol Opioid agonists–antagonists buprenorphine

naratriptan rizatriptan sumatriptan zolmitriptan

ANTIMIGRAINE AGENTS Ergot derivatives ergotamine

P ain , by definition, is a sensory and emotional experi- ence associated with actual or potential tissue damage. The perception of pain is part of the clinical pres- entation in many disorders and is one of the hardest sensations for people to cope with during the course of a disease or dysfunction. The drugs involved in the management of severe pain, whether acute or chronic, are discussed in this chapter. These agents all work in the central nervous system (CNS)—the brain and the spinal cord—to alter the way that pain impulses arriving from peripheral nerves are processed. These agents can change the perception and tolerance of pain. Two major types of drugs are considered here: the opioids—the opium derivatives that are used to treat many types of pain; and the antimigraine drugs, which are reserved for the treatment of migraine headache, a type of severe headache. Opioid antagonists, which are used to block the effects of the opioids in cases of overdose, are also discussed. PAIN Pain is described as an unpleasant sensation and emo- tional experience. In many ways it is a subjective experience. The physiological processes that cause pain are perceived and reacted to in different ways because of learned experiences, cultural differences and environmental stimuli. Pain occurs whenever tissues are damaged. The injury to cells releases many chemicals, including kinins and prostaglandins, which stimulate specific sensory nerves. Pain can be acute or chronic. Acute pain occurs in response to recent tissue damage or injury. This type of pain makes a person aware of an injury and should lead to measures to care for the injury and teaches the person to avoid similar situations that could cause this pain. Chronic pain is constant or inter- mittent pain that keeps occurring long past the time the injured area would be expected to heal. Chronic pain can cause a stress reaction, interrupt much-needed sleep and interfere with all of the activities of daily living. Pain can also be classified by location. “Where does it hurt?” is a common question in assessing pain. Sometimes the location of the pain is a direct indicator of where

the tissue damage has occurred. In some cases so-called referred pain occurs. A person experiencing pain from damage to the heart muscle may actually feel the pain in the neck or jaw. The sensation of pain is experienced in a different area of the body. Referred pain often follows predictable pathways, which helps healthcare providers figure out where the injury has occurred. Pain can be further classified by originating source as nociceptive, neuropathic or psychogenic. Nociceptive pain is caused by a direct stimulus to a pain receptor. Neuropathic pain is caused by nerve injury. Psychogenic pain is pain that is associated with emotional, psychological or behavioural stimuli. Pain impulse transmission and perception Two small-diameter sensory nerves, called the A-delta and C fibres , respectively, respond to stimulation by gen- erating nerve impulses that produce pain sensations. The A-delta fibres are small, myelinated fibres that respond quickly to acute pain. The C fibres are unmyelinated and are slow conducting. Pain impulses from the skin, subcutaneous tissues, muscles and deep visceral struc- tures are conducted to the dorsal, or posterior, horn of the spinal cord on these fibres. In the spinal cord, these nerves form synapses with spinal cord nerves that then send impulses to the brain (Figure 26.1). In addition, large-diameter sensory nerves enter the dorsal horn of the spinal cord. These so-called A fibres do not transmit pain impulses; instead, they transmit sensations associated with touch and temperature. The A fibres, which are larger and conduct impulses more rapidly than do the smaller fibres, can actually block the ability of the smaller fibres to transmit their signals to the secondary neurons in the spinal cord. The dorsal horn, therefore, can be both excitatory and inhibitory with regard to pain impulses that are transmitted from the periphery. The impulses reaching the dorsal horn are transmit- ted upwards towards the brain by a number of specific ascending nerve pathways. These pathways run from the spinal cord into the thalamus, where they form synapses with various nerve cells that transmit the information to the cerebral cortex, along the spinothalamic tracts .