Epidural analgesia.

The process of nociception, the anatomy of the epidural space, and the placement of the epidural catheter are reviewed, and the pharmacology and pharmacokinetics, analgesic efficacy, and potential adverse effects of epidurally administered narcotics and local anesthetics are discussed, as well as patient monitoring standards and solution preparation guidelines for these agents. The epidural space is located between the dura mater (the outer-most membrane surrounding the spinal cord) and the vertebral canal. The site of catheter placement is determined by the dermatomes corresponding to the site of desired analgesia. The primary factors that differentiate epidural narcotics are related to their pharmacokinetic profiles. Morphine, which is hydrophilic, has a slower onset of action and a longer duration of analgesia than lipophilic compounds such as fentanyl; morphine also results in less segmentalization (the degree to which analgesia is limited to discrete dermatomal segments corresponding to the level of the epidural narcotic injection) than is seen with lipophilic compounds. Studies have shown that epidural narcotics provide superior pain relief compared with systemic narcotics. Common adverse effects associated with therapeutic doses of intraspinal narcotics include itching, nausea and vomiting, urinary retention, and sedation; respiratory depression is uncommon after epidural administration of narcotics. The most bothersome adverse effect encountered with analgesic doses of local anesthetics is paresthesia. Solutions for epidural administration must be sterile and preservative free. Epidural administration of narcotics and local anesthetics seems to provide better pain relief than conventional methods but may be associated with more bothersome adverse effects.