Effect of combining ultrdow-dose naloxone with morphine in intravenous patient-controlled analgesia: The cut-off ratio of naloxone to morphine for antniemesis after gynecologic surgery

Background/Purpose: Admixing an ultralow dose of naloxone with intravenous morphine patient-controlled analgesia (PCA) has been shown to decrease postoperative nausea. However, the cut-off ratio of the naloxonemorphine admixture for antiemetic effects has not been investigated. The purpose of this study was to investigate the cut-off ratio of naloxone-morphine admixture in PCA for antiemesis after gynecologic surgery. Methods: This double-blind study enrolled 120 female patients who were scheduled for gynecologic surgery under general anesthesia. Patients were randomly allocated to one of three groups (n = 40 for each group). The concentration of naloxone and morphine respectively was 0 mu g/mL and 1 mg/mL in group 1, 0.1 mu g/mL and 1 mg/mL in group 2 (1: 10,000), and 1 mu g/mL and 1 mg/mL in group 3 (1: 1000). Morphine consumption, verbal rating score of wound pain at rest and with exertion, and morphine-related side effects were investigated at 1, 2, 4 and 24 hours postoperatively. Results: A total of 112 patients completed the study (3 7 in group 1, 3 6 in group 2, 3 9 in group 3). The incidence of nausea during the postoperative 4-24 hours was significantly lower in group 3 than in group 1 (23.1% vs. 56.8%, p < 0. 05). Furthermore, the overal 1 incidence of severe nausea was significantly lower in group 3 than in group 1 (2.6% vs. 24.3%, P < 0.05) as was the rescue antiemetic requirements (5.1% vs. 24.3%, P < 0.05). However, there were no significant differences between groups 2 and 1. The pain scores (at rest and with exertion) and 24-hour morphine consumption were not significantly different among the three groups. Conclusion: The antiemetic efficacy of ultralow-dose naloxone combined with PCA morphine is limited by a cut-off ratio of naloxone to morphine of 1: 10,000.