Magnesium sulfate attenuates tourniquet pain in healthy volunteers

Preoperative administration of an N-methyl-D-aspartate (NMDA) receptor antagonist has been shown to attenuate tourniquet-induced blood pressure increase under general anesthesia, suggesting that the mechanism of this blood pressure increase includes NMDA receptor activation. The attenuation of this increase may be associated with the pain relief induced by NMDA receptor antagonism. We tested the hypothesis that magnesium sulfate, an NMDA receptor antagonist, attenuates tourniquet pain.

Twenty-four healthy volunteers were randomly assigned to four groups (n = 6 each): control (normal saline), M1 (magnesium, 1 g), M2 (magnesium, 2 g), and M4 (magnesium, 4 g). Normal saline or magnesium solution was given intravenously over a 15-min period, in a double-blind fashion, before tourniquet inflation, which was continued for 60 min or until the "pain score" (0 = no pain, 100 = highest tolerable pain) reached 100. Pain scores were recorded before and every 5 min during tourniquet inflation. If subjects reported a pain score of 100 before the end of the 60-min period, we adopted a pain score of 100 for the remaining period.

The duration of tourniquet inflation in the M4 group was significantly longer than that in the control group (54.3 ± 8.3 vs. 42.9 ± 9.9 min, P = 0.03). Pain scores in the M4 group were significantly lower than those in the control group from 10 through 50 min after the start of tourniquet inflation. The area under the curve for pain scores in the M4 group was significantly smaller than the areas in the other groups.

Magnesium sulfate, 4 g, significantly attenuated tourniquet pain in healthy awake volunteers, suggesting that NMDA receptor activation is involved in tourniquet pain.