Management of postoperative pain by continuous epidural infusion of analgesics

The anatomy and physiology of the epidural space and the mechanism of action, sites of action, and pharmacokinetics of analgesics administered by continuous epidural infusion are reviewed, and the efficacy, adverse effects, and postoperative indications for use of analgesics administered by this route are discussed. Narcotics selectively block pain conduction by occupying specific opiate receptors in the spinal cord. Local anesthetics provide analgesia by axonal membrane blockade; they also can produce nonselective sympathetic and somatic (sensory and motor) blockade in addition to analgesia. A narcotic-local anesthetic mixture should provide an additive analgesic effect, without an increase in the incidence of adverse effects. Comparative efficacy studies have shown that continuous epidural infusions of narcotics, local anesthetics, and narcotic-local anesthetic combinations, when used appropriately, may produce better analgesia than conventional bolus methods of pain relief. Continuous epidural infusions also offer a safety advantage over intermittent epidural injections because peak and trough levels of the analgesic agent are avoided. Adverse effects of epidurally administered narcotics include respiratory depression, pruritus, urinary retention, nausea and vomiting, and sedation. Adverse effects of epidurally administered local anesthetics include urinary retention, hypotension, numbness, motor weakness, tachyphylaxis, and, rarely, systemic toxicity. The cost of epidurally administered drugs is substantially higher than that for i.m. or i.v. narcotic analgesia, but this cost may be offset by other benefits such as a shorter hospital stay. Current studies suggest superior analgesia for the majority of surgical procedures with continuous epidural analgesia infusions compared with more traditional methods of providing analgesia.