Magnesium presents analgesic effects by blocking the non-competitive N-methyl-D-aspartate receptor in the spinal cord. The purpose of this study was to evaluate the effects of epidural magnesium on cumulative dose of ropivacaine in patients with patient-controlled epidural analgesia (PCEA) after a thoracotomy.

In a randomized, prospective, double-blind trial, we enrolled 40 patients undergoing PCEA after a thoracotomy. Control group (n = 20) received an initial dose of 0.375% ropivacaine 0.1 ml/kg, and following demand doses of 0.2% ropivacaine 2 ml with 15 min lockout interval on the patient demand. Magnesium group (n = 20) additionally received MgSO₄ 100 mg in the initial dose and 4 mg in the demand doses. Cumulative dose of ropivacaine, the resting and coughing visual analog scale (VAS) score were measured at 3, 6, 12, 24, and 48 hours after surgery.

Magnesium group showed lower cumulative dose of ropivacaine than control group at 12, 24, and 48 hours after the surgery (P < 0.05), but not at 3 and 6 hours (P < 0.05). The VAS score was not different between 2 groups (P < 0.05).

Epidural magnesium may be useful as an adjuvant to ropivacaine after a thoracotomy. However, because magnesium showed no beneficial effect at the early period after the surgery, more investigation about proper initial dose is required.