Background: A naloxone infusion is effective in reducing epidural and intrathecal opioid-related side effects. The use of naloxone infusion concomitant with intravenous morphine patient-controlled analgesia (PCA) has not been evaluated, probably because of an expected direct antagonism of the systemic opioid effect. The authors compared the incidence of morphine-related side effects and the quality of analgesia from two small doses of naloxone infusion. Methods: Sixty patients classified as American Society of Anesthesiologists physical status 1, 2, or 3 who were scheduled for total abdominal hysterectomies were enrolled in the study. Patients received a standardized general anesthetic. In the postanesthetic care unit, patients received morphine as a PCA. They were randomized to receive either 0.25 mu g.kg(-1).h(-1) naloxone (low dose), 1 mu g.kg(-1).h(-1) (high dose), or saline (placebo) as a continuous infusion. Verbal rating scores for pain, nausea, vomiting, and pruritus; sedation scores; requests for antiemetic; and morphine use were recorded for 24 h. Blood pressure, respiratory rate, and oxyhemoglobin saturation were also monitored. Results: Sixty patients completed the study. Both naloxone doses were equally effective in reducing the incidence of nausea, vomiting, and pruritus compared with placebo (P < 0.05 by the chi-squared test), There was no difference in the verbal rating scores for pain between the groups. The cumulative morphine use was the lowest in the low-dose group (42.3 +/- 24.1 mg; means +/- SD) compared with the placebo (59.1 +/- 27.4 mg) and high-dose groups (64.7 +/- 33.0 mg) at 24 h (P < 0.05 by analysis of variance), There was no incidence of respiratory depression (<8 breaths/min) and no difference in sedation scores, antiemetic use, respiratory rate, and hemodynamic parameters among the groups, Conclusions: Naloxone is effective in preventing PCA opioid-related side effects. Naloxone infusion at 0.25 mu g.kg(-1).h(-1) not only attenuates these side effects but appears to reduce postoperative (beyond 4-8 h) opioid requirements, This dosing regimen can be prepared with 400 mu g naloxone in 1,000 ml crystalloid given in 24 h to a patient weighing 70 kg.
