Role of magnesium sulfate in postoperative analgesia

N-methyl-D-aspartate antagonists may play a role in the prevention of pain. An assessment was made of the effect of the physiologic N-methyl-D-aspartate antagonist magnesium on analgesic requirements, pain, comfort, and quality of sleep in the postoperative period.

In a randomized, double-blind study, 42 patients undergoing elective abdominal hysterectomy with general anesthesia received 20% magnesium sulfate or saline (control) 15 ml intravenously before start of surgery and 2.5 ml/h for the next 20 h. Postoperative morphine requirement was assessed for 48 h using patient-controlled analgesia. Maximum expiratory flow (peak flow), pain at rest and during peak flow, and discomfort were evaluated up to the 48th postoperative hour, and 1 week and 1 month after surgery. Insomnia was evaluated after the first and second postoperative nights.

Compared to control subjects, magnesium-treated patients consumed less morphine during the first 48h (P<0.03), which was most pronounced during the first 6 h (P<0.004), and experienced less discomfort during the first and second postoperative days (P<0.05-0.005). The magnesium-treated group revealed no change in postoperative sleeping patterns when compared to preoperative patterns. Control patients showed an increase in insomnia during the first and second postoperative nights (P<0.002 and P<0.005, respectively) compared to preoperative values.

This is the first clinical study showing that the perioperative application of magnesium sulfate is associated with smaller analgesic requirement, less discomfort, and a better quality of sleep in the postoperative period but not with adverse effects. Magnesium could be of interest as an adjuvant to postoperative analgesia.