Influence of dose and timing of administration of morphine on postoperative pain and analgesic requirements

In a randomized, double-blind study, we have investigated the effect of dose and timing of administration of morphine on postoperative pain and analgesic requirements in 60 patients undergoing hysterectomy, with or without salpingo-oophorectomy. Patients were allocated randomly to one of three groups: during standardized general anaesthesia, group post received morphine 0.15 mg kg-1 i.v. at peritoneal closure after hysterectomy; group pre-low received morphine 0.15 mg kg-1 on induction of anaesthesia; and group pre-high received morphine 0.3 mg kg-1 on induction of anaesthesia. Median postoperative morphine consumption (first 24 h) from a PCA system was 68 mg (group post), 56 mg (group pre-low) and 43 mg (group pre-high), and total perioperative morphine consumption (induction of anaesthesia to end of 24 h after surgery) was 77 mg (group post), 65 mg (group pre-low) and 63 mg (group pre-high). Pain scores (at rest and on movement) were similar in the three groups. A large dose of morphine 0.3 mg kg-1 i.v. on induction of anaesthesia significantly reduced postoperative PCA morphine requirements compared with the smaller dose (0.15 mg kg-1) administered at induction or peritoneal closure, in patients undergoing hysterectomy, with or without salpingo-oophorectomy.