Preemptive analgesia may prevent nociceptive inputs generated during surgery from sensitizing central neurons and, therefore, may reduce post-operative pain. But, preemptive analgesia has been shown to decrease postinjury pain in animals, studies in human have provided controversial results. We studied whether intravenous morphine infusion before induction could affect post-operative pain and analgesic demands, when compared with intravenous morphine infusion after the closure of the peritoneum.

Female patients scheduled for total abdominal hysterectomy were randomly assigned to one of two groups of prospectively studied in a double-blind manner. Group I (n=23) and II (n=20) received intravenous morphine (0.1 mg/kg) before induction of anesthesia and after the closure of the peritoneum, respectively. Either group had continuous infusion of morphine (1.5 mg/hr) immediately after i.v. bolus morphine. Postoperative pain relief was provided with i.v. morphine from a PCA system (Medex Walkmed). Postoperative visual analogue pain scores (VAS), analgesics requirements and side effects were examined and compared between groups for 2 postoperative days.

VAS were significantly less in group I (3.3±0.4) than in group II (5.3±0.5) 2hrs after surgery (p<0.01). Patient-cantrolled morphine cumulative consumption in group I was significantly less than in group II for 24hours (21.9 mg vs 35.3 mg) and 48hours (37.4 mg vs 55.0 mg) after operation (p<0.01).

Preemptive analgesia with intravenous morphine reduces postoperative pain and analgesic requirements. Lower postoperative analgesic requirements in preemptive analgesic group indicate that intravenous morphine prevents development of injury-induced peripheral or central sensitization.