1. Opioid tolerance—Opioid tolerance is the phenomenon whereby repeated doses of opioids have a diminishing analgesic effect. Clinically, it has been described as an increasing opioid dose requirement to achieve the analgesia observed at the initiation of opioid administration. Although development of tolerance begins with the first dose of an opioid, tolerance may not become clinically manifest until after 2–3 weeks of frequent exposure to ordinary therapeutic doses. Nevertheless, perioperative and critical care use of ultrapotent opioid analgesics such as remifentanil have been shown to induce opioid tolerance within hours. Tolerance develops most readily when large doses are given at short intervals and is minimized by giving small amounts of drug with longer intervals between doses.
A high degree of tolerance may develop to the analgesic, sedating, and respiratory depressant effects of opioid agonists (Table 31–3). It is possible to produce respiratory arrest in a nontolerant person with a dose of 60 mg of morphine. However, in a patient who is opioid-dependent or requires escalating opioid administration to manage intractable cancer pain, doses such as 2000 mg of morphine taken over a 2or 3-hour period may not produce significant respiratory depression. Tolerance also develops to the antidiuretic, emetic, and hypotensive effects but not to the miotic, convulsant, and constipating actions. Following discontinuation of opioids, loss of tolerance to the sedating and respiratory effects of opioids is variable, and difficult to predict. However, tolerance to the emetic effects may persist for several months after withdrawal of the drug. Therefore, opioid tolerance differs by effect, drug, time, and the individual (genetic-epigenetic factors).
Tolerance also develops to analgesics with mixed receptor effects but to a lesser extent than to the agonists. Adverse effects such as hallucinations, sedation, hypothermia, and respiratory depression are reduced after repeated administration of the mixed receptor drugs. However, tolerance to the latter agents does not generally include cross-tolerance to the agonist opioids. It is also important to note that tolerance does not develop to the antagonist actions of the mixed agents or to those of the pure antagonists.
Cross-tolerance is an extremely important characteristic of the opioids, ie, patients tolerant to morphine often show a reduction in analgesic response to other agonist opioids. This is particularly true of those agents with primarily μ-receptor agonist activity. Morphine and its congeners exhibit cross-tolerance not only with respect to their analgesic actions but also to their euphoriant, sedative, and respiratory effects. However, the cross-tolerance existing among the μ-receptor agonists can often be partial or incomplete. This clinical observation has led to the concept of “opioid rotation,” which has been used for many years in the treatment of cancer pain. A patient who is experiencing decreasing effectiveness of one opioid analgesic regimen is “rotated” to a different opioid analgesic (eg, morphine to hydromorphone; hydromorphone to methadone) and typically experiences significantly improved analgesia at a reduced overall equivalent dosage. Another approach is to recouple opioid receptor function as described previously through the use of adjunctive nonopioid agents. NMDA-receptor antagonists (eg, ketamine) have shown promise in preventing or reversing opioid-induced tolerance in animals and humans. Use of ketamine is increasing because well-controlled studies have shown clinical efficacy in reducing postoperative pain and opioid requirements in opioid-tolerant patients. Agents that independently enhance μ-receptor recycling may also hold promise for improving analgesia in the opioid-tolerant patient.