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 ofdose and timing of administration of morphine on postoperative pain andanalgesic requirements in 60 patients undergoing hysterectomy, with orwithout salpingo-oophorectomy. Patients were allocated randomly to one ofthree groups: during standardized general anaesthesia, group post receivedmorphine 0.15 mg kg-1 i.v. at peritoneal closure after hysterectomy; grouppre-low received morphine 0.15 mg kg-1 on induction of anaesthesia; andgroup pre-high received morphine 0.3 mg kg-1 on induction of anaesthesia.Median postoperative morphine consumption (first 24 h) from a PCA systemwas 68 mg (group post), 56 mg (group pre- low) and 43 mg (group pre-high),and total perioperative morphine consumption (induction of anaesthesia toend 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) weresimilar in the three groups. A large dose of morphine 0.3 mg kg-1 i.v. oninduction of anaesthesia significantly reduced postoperative PCA morphinerequirements compared with the smaller dose (0.15 mg kg-1) administered atinduction or peritoneal closure, in patients undergoing hysterectomy, withor without salpingo-oophorectomy.