Low dose intrathecal morphine effects on post-hysterectomy pain: a randomized placebo-controlled study

Background: Spinal anesthesia with different doses of intrathecal morphine has been shown to relieve post-operative pain. We studied in a prospective randomized, double-blind fashion the effects of morphine 0, 100, 200, or 300 mg added to intrathecal bupivacaine on first post-operative 24 h patient-controlled analgesia morphine (PCA-morphine) consumption after abdominal hysterectomy under general anesthesia. Methods: One hundred and forty-four American Society of Anesthesiologists I-II women were assigned to receive spinal anesthesia with 12 mg of hyperbaric bupivacaine combined with 100, 200, and 300 mg morphine or saline before standardized general anesthesia was induced. Low transverse incision abdominal hysterectomy was performed. Post-operative outcome measures were recorded at 1, 2, 4, 6, 12, and 24 h. Primary outcome was 24 h PCA-morphine. Secondary outcomes were pain by visual analogue scale (0-10), nausea, pruritus, sedation, and respiratory depression. Results: Intrathecal morphine reduced accumulated 24 h post-operative morphine consumption. Morphine 100 mg significantly reduced morphine consumption vs. placebo at 0-6 h, 6-12 h, and for the entire 0-24 h time interval post-operation. Morphine 200 mg further significantly reduced morphine consumption vs. morphine 100 mg at 0-6 h and for the entire 0-24 h post-operation. There was no further reduction of morphine consumption seen with morphine 300 mg. No serious side effects were seen. Emesis was similar in all groups, and pruritus was experienced only in the morphine groups. Conclusion: Intrathecal morphine supplementation to bupivacaine reduces first 24 h PCA-morphine consumption after abdominal hysterectomy under general anesthesia, and we found no benefit from increasing the dose over 200 mg.