Does morphine premedication influence the pain and consumption of postoperative analgesics after total knee arthroplasty?

Evidence of pre-emptive analgetic effect of opioid would offer great potential benefit to patients with postoperative pain, a better pain relief with less opioid. The aim of this double blind randomised trial was to study the effect of intramuscular morphine premedication on postoperative pain. Forty-one patients undergoing total knee arthroplasty were randomly allocated to four groups. Two groups received epidural morphine, 4 mg immediately after operation and 3 mg ten hours later, and two groups the same volume of saline. All patients had access to intravenous PCA-fentanyl: One epidural morphine and one epidural saline group (PreEpiMo and PreMo, respectively) received morphine, 0.14 mg/kg i.m. as premedication. Pain was measured with a visual analogue scale (VAS). Respiration was monitored by means of pulseoximetry, arterial blood gas analysis and rate of breathing. Morphine premedication reduced postoperative pain in the immediate postoperative period in patients with epidural placebo (PreMo), but the effect was absent in patients with PreEpiMo. Epidural morphine (EpiMo) provided stable analgesia with reduced need of PCA-fentanyl. Two patients (10%) (one in EpiMo and one in PreEpiMo) developed respiratory depression requiring naloxone treatment. The dosage of epidural morphine used in this study was a likely explanation of this depression. Nausea, vomiting, itching and urinary retention were the most frequent side effects without significant differences between the groups. In conclusion, morphine premedication had a temporary rest effect on the postoperative pain. Epidural morphine provides a better analgesia than intravenous PCA-fentanyl.