1. Central nervous system effects—The principal effects of opioid analgesics with affinity for μ receptors are on the CNS; the more important ones include analgesia, euphoria, sedation, and respiratory depression. With repeated use, a high degree of tolerance occurs to all of these effects (Table 31–3).
a. Analgesia—Pain consists of both sensory and affective (emotional) components. Opioid analgesics are unique in that they can reduce both aspects of the pain experience. In contrast, nonsteroidal anti-inflammatory analgesic drugs, eg, ibuprofen, have no significant effect on the emotional aspects of pain.
b.
Euphoria—Typically, patients or intravenous drug users who receive intravenous morphine experience a pleasant floating sensation with lessened anxiety and distress. However, dysphoria, an unpleasant state characterized by restlessness and malaise, may also occur.
c.
Sedation—Drowsiness and clouding of mentation are common effects of opioids. There is little or no amnesia. Sleep is induced by opioids more frequently in the elderly than in young,
TABLE 31–3 Degrees of tolerance that may develop to some of the effects of the opioids.
healthy individuals. Ordinarily, the patient can be easily aroused from this sleep. However, the combination of morphine with other central depressant drugs such as the sedative-hypnotics may result in very deep sleep. Marked sedation occurs more frequently with compounds closely related to the phenanthrene derivatives and less frequently with the synthetic agents such as meperidine and fentanyl. In standard analgesic doses, morphine (a phenanthrene) disrupts normal rapid eye movement (REM) and non-REM sleep patterns. This disrupting effect is probably characteristic of all opioids. In contrast to humans, a number of other species (cats, horses, cows, pigs) may manifest excitation rather than sedation when given opioids. These paradoxical effects are at least partially dose-dependent.
d.
Respiratory depression—All of the opioid analgesics can produce significant respiratory depression by inhibiting brainstem respiratory mechanisms. Alveolar Pco2 may increase, but the most reliable indicator of this depression is a depressed response to a carbon dioxide challenge. The respiratory depression is dose-related and is influenced significantly by the degree of sensory input occurring at the time. For example, it is possible to partially overcome opioid-induced respiratory depression by a variety of stimuli. When strongly painful stimuli that have prevented the depressant action of a large dose of an opioid are relieved, respiratory depression may suddenly become marked. A small to moderate decrease in respiratory function, as measured by Paco2 elevation, may be well tolerated in the patient without prior respiratory impairment. However, in individuals with increased intra-cranial pressure, asthma, chronic obstructive pulmonary disease, or cor pulmonale, this decrease in respiratory function may not be tolerated. Opioid-induced respiratory depression remains one of the most difficult clinical challenges in the treatment of severe pain. Ongoing research to overcome this problem is focused on μ-receptor pharmacology and serotonin signaling pathways in the brainstem respiratory control centers.
e.
Cough suppression—Suppression of the cough reflex is a well-recognized action of opioids. Codeine in particular has been used to advantage in persons suffering from pathologic cough. However, cough suppression by opioids may allow accumulation of secretions and thus lead to airway obstruction and atelectasis.
f.
Miosis—Constriction of the pupils is seen with virtually all opioid agonists. Miosis is a pharmacologic action to which little or no tolerance develops, even in highly tolerant addicts (Table 31–3); thus, it is valuable in the diagnosis of opioid overdose. This action, which can be blocked by opioid antagonists, is mediated by parasympathetic pathways, which, in turn, can be blocked by atropine.