Intrathecal (IT) magnesium has antinociceptive effects on animals and has been reported to prolong spinal opioid analgesia in humans. This study examined the effect of IT magnesium on spinal anesthesia and postoperative epidural analgesia.

Sixty patients undergoing total knee replacement were enrolled in this study. Before the IT injection of 0.5% isobaric tetracaine (10 mg), group C and group M received 0.9% saline or 50% magnesium sulfate 0.1 ml, respectively. The epidural solution for postoperative analgesia contained 0.2% ropivacaine (100 ml) only in group M, and 0.2% ropivacaine plus morphine (50 µg/ml) in group C. The verbal rating scale (VRS) scores for pain, sensory block level, intensity of motor block and side effects were recorded at 5, 60, and 120 minutes after the IT injection and at 1, 12 and 36 hours after surgery in the post-anesthesia care unit (PACU).

The VRS score at 120 minutes after the IT injection were lower in group M than in group C (P < 0.05). There were no differences in the VRS scores and the use of supplemental analgesics at the postoperative period. The incidence of PONV, pruritus and urinary retention was significantly lower in group M than in group C at 12 and 36 hours after surgery.

IT magnesium can be used as a local anesthetic adjuvant to strengthen the analgesic effect of spinal local anesthesia and to intensify the analgesic effect of epidural local anesthesia for postoperative pain control to the extent of 5 mg epidural morphine.