• Nonaddictive

• Minimal adverse effects

• Effective without altering the patient's state of awareness

• Does not cause tolerance

• Inexpensive

Pain medications may be divided into three categories:

• Nonnarcotic analgesics

• Opioid analgesics

• Adjuvant analgesics

Nonnarcotic analgesics. The nonnarcotic analgesics include aspirin, NSAIDs, and acetaminophen. Aspirin acts centrally and peripherally to block the transmission of pain impulses. Furthermore, it reduces fever and inflammation and inhibits synthesis of the prostaglandins that increase the sensitivity of nociceptors.

The NSAIDs exert their analgesic effects primarily through inhibition of cyclooxygenase, the rate-limiting enzyme for prostaglandin synthesis. Typical NSAIDs inhibit cyclooxygenase 1 (COX-1; constitutive) and cyclooxygenase 2 (COX-2; induced in areas of inflammation). More recently, medications specific for COX-2 (rofecoxib, Vioxx®) have been developed. The advantage of these agents is that they reduce pain, fever, and inflammation without the unwanted side effects accompanying COX-1 inhibition, particularly those leading to gastric ulcers. NSAIDs inhibit the inflammatory response by decreasing the sensitivity of blood vessels to bradykinin and histamine, reversing vasodilation, and reducing the release of inflammatory mediators from mast cells, basophils, and granulocytes.

Acetaminophen, another alternative to aspirin, is an effective analgesic and fever-reducing agent. However, this medication has no effect on inflammation.

Opioid analgesics. Medications with morphine-like actions are referred to as opioid or narcotic agents. Opioid drugs exert their effects through three major categories of opioid receptors: mu (m), kappa (k) and delta (D). Analgesia appears to involve m-receptors (largely at supraspinal sites) and k-receptors (principally within the spinal cord). Morphine produces analgesia through interaction with m-receptors. In fact, most clinically used opioids are relatively selective for m-receptors. Morphine can stimulate m2-receptors spi-nally or mi-receptors supraspinally. When given systemically, it acts predominantly through supraspinal m1-receptors. Other effects of m-receptor activation include respiratory depression; reduced gastrointestinal motility (leading to constipation); and feelings of well-being or euphoria.

Morphine may be administered orally, intravenously, or epidurally. An advantage of epidural administration is that it provides effective analgesia while minimizing the central depressant effects associated with systemic administration. The mechanism of action with the epidural route of administration involves opioid receptors on the cell bodies of first-order sensory neurons in the dorsal root ganglia as well as their axon terminals in the dorsal horn. Stimulation of these receptors inhibits release of substance P and interrupts transmission of the pain signal to the second-order sensory neuron.

Adjuvant analgesics. Adjuvant analgesics include medications such as antidepressants and antiseizure medications. The effectiveness of these agents may be due to the existence of nonendorphin synapses in the endogenous analgesic pathway. For example, the neurotransmitter serotonin has been shown to play a role in producing analgesia. Tricyclic antidepressant medications, such as imipramine, that block the removal of serotonin from the synapse suppress pain in some individuals. Certain antiseizure medications, such as carbamazepine and phenytoin, have specific analgesic effects that are effective under certain conditions. For example, these medications, which suppress spontaneous neuronal firing, are particularly effective in management of pain that occurs following nerve injury. Other agents, such as the corticosteroids, reduce pain by decreasing inflammation and the nociceptive stimuli responsible for the pain.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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