Diarrhea from almost any cause can be controlled with the opioid analgesics, but if diarrhea is associated with infection such use must not substitute for appropriate chemotherapy. Crude opium preparations (eg, paregoric) were used in the past to control diarrhea, but now synthetic surrogates with more selective gastrointestinal effects and few or no CNS effects, eg, diphenoxylate or loperamide, are used. Several preparations are available specifically for this purpose (see Chapter 62).
E. Shivering
Although all opioid agonists have some propensity to reduce shivering, meperidine is reported to have the most pronounced anti-shivering properties. Meperidine apparently blocks shivering mainly through an action on subtypes of the α2 adrenoceptor.
F. Applications in Anesthesia
The opioids are frequently used as premedicant drugs before anesthesia and surgery because of their sedative, anxiolytic, and analgesic properties. They are also used intraoperatively as a part of induction, maintenance, and preparation for postoperative analgesia. Opioids are most commonly used in cardiovascular surgery and other types of high-risk surgery in which a primary goal is to minimize cardiovascular depression. In such situations, mechanical respiratory assistance must be provided.
Because of their direct action on the neurons of the superficial dorsal horn of the spine, opioids can also be used as regional analgesics, by administration into the epidural or subarachnoid spaces of the spinal column. A number of studies have demonstrated that long-lasting analgesia with minimal adverse effects can be achieved by epidural administration of 3–5 mg of morphine, followed by slow infusion through a catheter placed in the epidural space. It was initially assumed that the epidural application of opioids might selectively produce analgesia without impairment of motor, autonomic, or sensory functions other than pain. However, respiratory depression can occur after the drug is injected into the epidural space and may require reversal with naloxone. Effects such as pruritus and nausea and vomiting are common after epidural and subarachnoid administration of opioids and may also be reversed with naloxone. The use of intrathecal and epidural opioid is common practice for postoperative analgesia and can reduce the amount of systemic opioids, thereby reducing other opioid-related side effects such as sedation or constipation. In rare cases, chronic pain management specialists may elect to implant surgically a programmable infusion pump connected to a spinal catheter for continuous infusion of opioids or other analgesic compounds in chronic or cancer pain management.
G. Alternative Routes of Administration