Intrathecal morphine. Double-blind evaluation of optimal dosage for analgesia after major lumbar spinal surgery.

STUDY DESIGN: A prospective, randomized, double-blind study.

OBJECTIVES: To evaluate the efficacy and safety of three different dosages of intrathecal morphine sulfate for postoperative analgesia after lumbar spinal fusion.

SUMMARY OF BACKGROUND DATA: Analgesia and respiratory depression after intrathecal morphine sulfate injection are dose related. The optimal dose to use for major spinal surgery is not known.

METHODS: Sixty patients undergoing posterolateral lumbar fusion with or without decompression were divided randomly into 3 groups of 20 patients each. Anesthesia, monitoring, and surgery were similar for all patients. Just before closing of the wound, morphine sulfate was injected into the dural sack under direct visualization. Patients in groups 1-3 received 0.2 mg, 0.3 mg, and 0.4 mg morphine, respectively. Routine analgesia, consisting of diclofenac, was prescribed to use if necessary. Measurements were made and compared between the groups at zero hours (on admission to the Intensive Care Unit), 6 hours, 12 hours, 18 hours, and 24 hours after surgery.

RESULTS: At zero hours and at 12 hours after surgery, pain levels were similar in groups 2 and 3, but were significantly higher in group 1 (P < 0.05). The respiratory rate was significantly lower in group 3 than in the other two groups (P < 0.05), and the arterial CO2 tension (PaCO2) was consistently higher in group 3. PaCO2 decreased in all three groups over the first 24 hours. Pruritus and nausea did not differ among the three groups.

CONCLUSIONS: For adult patients undergoing posterolateral lumbar fusion, 0.3 mg (0.004 mg/kg) is probably the optimal dose of intrathecal morphine to manage pain.