Obstetric Pain: Special Considerations for Opioid Analgesia
Epidurals
22. M.T., a 28-year-old woman who has been in labor for 10 hours, is experiencing strong, erratic contractions at 5- to 15-minute intervals. Her cervix is minimally dilated, suggesting that delivery is still several hours away. Her pain is severe and, if it continues, may compromise her ability to assist in the labor and delivery process. A single, 50 mcg epidural dose of fentanyl is ordered for M.T. by anesthesia. What guidelines are necessary for the safe use of epidural opioids?
Fentanyl and morphine are frequently administered epidurally during labor and during lower extremity surgical procedures (e.g., cesarean delivery, total joint replacements) because they have limited systemic effects and long durations of analgesic action.88 Epidural analgesia has also been used in thoracic surgery.89 A single dose of morphine 2 to 10 mg given epidurally may provide analgesia for >12 hours.90 Respiratory depression can occur 1 to 3 hours after the administration of epidural morphine and can be readily reversed by naloxone. Facial or generalized pruritus also can occur hours after the administration of epidural morphine. If pruritus does occur, it can be controlled with low doses of IM or IV naloxone 0.02 to 0.08 mg without reversing the analgesic effect of the epidural morphine. Frequent monitoring of vital signs is essential after the administration of epidural opioids. Always be vigilant for delayed signs of toxicity, because their onset occurs up to several hours after the administration of the opioid.
Besides morphine, fentanyl (Sublimaze), sufentanil (Sufenta), and buprenorphine (Buprenex) also have been used epidurally. The duration of analgesia of epidurally administered opioids depends less on the serum half-life of the opioid than on the lipid solubility of the drug. The more lipophilic the opioid, the shorter its duration because these agents rapidly diffuse out of the epidural space. In contrast, the more hydrophilic opioids remain in the epidural space longer. For example, methadone, which is highly lipophilic, has a very short duration of activity when administered epidurally despite a long serum half-life of 24 hours.91,92 For M.T., fentanyl is a good choice because it has a fairly long duration of analgesia but will not produce significant systemic effects that would compromise the fetus through transplacental migration. Also, the duration of fentanyl is sufficiently short that it will not unnecessarily delay maternal postpartum recovery. A single 50 mcg dose of epidural fentanyl was given to M.T. through an epidural catheter and complete analgesia was achieved.
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