Postoperative Pain Control: Opioid Administration

Meperidine [Demerol], a mu opioid analgesic, is commonly used for postoperative pain control. Meperidine is commonly underdosed and administered too infrequently even by physicians aware of its pharmacokinetics (Marks and Sachar, 1973). The common postoperative meperidine order of 75 mg parenterally every 4 hours as needed often is inadequate for several reasons. Meperidine produces clinical analgesia for only 2.5–3.5 hours, and a dose of 75 mg every 4 hours is equivalent to only 5–7.5 mg of morphine. Therefore, to obtain postoperative analgesia equal to that from 10 mg of morphine sulfate every 4 hours, a clinician would have to use 100– 150 mg of meperidine every 3 hours. Because of its unique toxicity, meperidine is often contraindicated in patients with impaired renal function and those receiving antidepressants of the monamine oxidase inhibitor class (Wood and Cousins, 1989). Normeperidine (6–N–desmethylmeperidine) is a toxic meperidine metabolite excreted through the kidney ' In patients with normal renal function, nonneperidine has a half–life of 15 to 20 hours; this time is extended greatly in elderly individuals and patients with impaired renal function. Normeperidine is a cerebral irritant that can cause effects ranging from dysphoria and irritable mood to convulsions (Kaiko, Foley, Grabinski, Heidrich, Rogers, lnturissi, and Reidenberg, 1983; Szeto, Inturissi, Houde, Saal, Cheigh, and Reidenberg, 1977). These effects have been observed even in young, otherwise healthy patients given sufficiently high doses of normeperidine postoperatively. Therefore, meperidine should be reserved for very brief courses in otherwise healthy patients who have demonstrated an unusual reaction (e.g., local histamine release at the infusion site) or allergic response during treatment with other opioids such as morphine or hydromorphone.

Titration of opioids should be based on the patient's analgesic response and side effects. Remember that patients vary greatly in their analgesic dose requirements and responses to opioid analgesics. Relative potency estimates provide a rational basis for selecting the appropriate starting dose to initiate analgesic therapy, changing the route of administration (e.g., from parenteral to oral), or when switching to another opioid. Dosage conversion factors based on relative potency estimates may differ somewhat between individual patients. When estimating the initial postoperative dose of an opioid analgesic, a clinician should consider whether patients have been receiving opioid analgesics preoperatively. In such patients, supplemental postoperative doses should be adjusted above the preoperative baseline requirement unless the operation itself is likely to remove the painful stimulus.

An "as–needed" order for opioid administration can result in prolonged delays while the nurse unlocks the controlled substances cabinet and prepares the drug for administration and until the drug takes effect. These delays can be eliminated by administering analgesics on a regular time schedule initially. For example, if the patient is likely to have pain requiring opioid analgesics for 48 hours following surgery, morphine could be ordered every 4 hours by the clock (not "as needed") for 36 hours. Once the duration of analgesic action is determined for a patient, the dosage frequency should be adjusted to prevent pain from recurring. Depending on patient preferences, the orders may be written so that the patient can refuse an analgesic if not in pain or forego it if asleep. However, as in dosing with other drugs that require a steady blood level to remain effective, interruption of an around–the–clock dosage schedule during the hours of sleep may cause the patient to be suddenly awakened by intense pain as blood analgesic levels decline.

It may be acceptable late in the postoperative course to give the same drug every 4 hours as requested. Switching from an around–the–clock to an as–needed dosage schedule later in the patient's course is one way to provide pain relief while minimizing the risk of adverse effects as the patient's analgesic dose requirement diminishes. As part of this schedule, a patient's pain should be assessed at regular intervals to determine the efficacy of the drug intervention, the presence of side effects, or the need for dosage adjustment or supplemental doses for breakthrough pain. Effective use of opioid analgesics should facilitate routine postoperative activities–––e.g., coughing and deep breathing exercises, ambulation, and physical therapy (Alexander, Parikh, and Spence, 1973; Rawal and Sjostrand, 1986; Wasylak, Abbott, English, and Jeans, 1990). The opioid should be withheld if the patient is sedated when awake or whenever there is respiratory depression (usually fewer than 10 breaths per minute).

Opioids may be administered by a variety of routes; oral dosing is usually the most convenient and least expensive route of administration. It is appropriate as soon as the patient can tolerate oral intake and is the mainstay of pain management in the ambulatory surgical population.

Preoperative intravenous or epidural access may be appropriate for postoperative management of severe pain, even when the oral route is available. Relatively few side effects will occur ordinarily, providing that these modalities are carefully managed by clinicians with appropriate expertise. Using potent analgesics or invasive techniques postoperatively, at a time when a patient's level of consciousness and physical function are returning to normal, requires careful titration and patient assessment.

Drug dosage, frequency, side effects, and risks differ even more noticeably between the intravenous and epidural routes than between the oral and intravenous routes. Clinicians not familiar with epidural opioid doses and pharmacokinetics must review the literature carefully before using that route. In addition, side effects (e.g., confusion, respiratory depression, hypotension, urinary retention, or pfuritus) associated with opioids can be greater with intravenous and epidural administration and require ongoing assessment and monitoring. Other potential problems that dictate expert vigilance and followup during epidural analgesia include abscess development or anesthesia of a nerve root at the site of catheter tip. These routes of administration are best limited to specially trained staff who are knowledgeable and skilled in the management of patients receiving intravenous or epidural opioids, typically under the direction of an acute or postoperative pain treatment service.