Postoperative pain management with intravenous patient-controlled morphine: comparison of the effect of adding magnesium or ketamine.

BACKGROUND AND OBJECTIVE

This double-blind randomized study tested whether the addition of magnesium or ketamine to morphine for intravenous patient-controlled analgesia resulted in improved analgesic efficacy and lower pain scores compared with morphine patient-controlled analgesia alone after major abdominal surgery.

METHOD

Ninety patients (3 x 30) were randomly allocated to receive either morphine 0.4 mg mL(-1) (Group M) by patient-controlled analgesia, morphine 0.4mg mL(-1) + MgSO4 30mg mL(-1) (Group MM) or morphine 0.4 mg mL(-1) + ketamine 1 mg mL(-1) (Group MK). Postoperative analgesia was started when the verbal rating scale was > or = 2. Patients were first given a standardized loading dose (0.05 mg kg(-1)) of the study solution. They were then allowed to use bolus doses of this solution (0.0125 mg kg(-1) every 20 min without time limit). Discomfort, sedation, pain scores, cumulative morphine consumption and adverse effects were recorded up to 24 h after the start of the patient-controlled analgesia.

RESULTS

The level of discomfort, level of sedation and verbal rating scores decreased significantly with time in all groups (P < 0.05). Both verbal rating and discomfort scores were significantly lower in Groups MM and MK at 15, 30 and 60 min compared with Group M (P < 0.001). Cumulative morphine consumption after 12 and 24 h was significantly higher in Group M alone (median 26 and 49 mg, respectively) compared with Group MM (24.2 and 45.7 mg) and Group MK (24.4 and 46.5 mg).

CONCLUSIONS

In the immediate postoperative period, the addition of magnesium or ketamine to morphine for intravenous patient-controlled analgesia led to a significantly lower consumption of morphine. However, these differences are unlikely to be of any clinical relevance.