Pre-emptive analgesic effect of low dose N-methyl-D-aspartate receptor antagonists: ketamine and magnesium in conjunction with spinal anesthesia

ABSTRACT

The role of NMDA receptor antagonists in mitigating acute and chronic pain is well documented. Both ketamine and magnesium are among the best known and available NMDA antagonists. In this trial, we tested the hypothesis that intravenous administration of small doses of ketamine or magnesium prior to spinal anesthesia may positively affect the characteristics of spinal block and improve the quality of postoperative pain relief. 45 patients were enrolled in this study, randomly assigned to 3 equal groups, group K received iv ketamine, group M received iv magnesium sulphate, and group S received saline placebo 5 minutes prior to induction of spinal anesthesia using hyperbaric bupivacaine. The following parameters were recorded mean arterial blood pressure, onset and duration of sensory and motor blockade, time till first analgesic request, total amount of analgesics administered to patients in the first 24 hours postoperatively, and visual analogue scale [VAS] scores for pain. Onset of both sensory and motor blocks was significantly delayed in group M compared with both K and S groups. Duration of both sensory and motor blocks was significantly longer in K and M groups compared with group S [K>M>S]. Again, time to first analgesic was significantly delayed in K and M groups compared to S group and this applies also to the total dose of opioids given which was significantly lower in K and M groups compared with S group. Finally, VAS scores were significantly lower in K and M groups in the first 3 hours postoperatively, an effect that continued with K group for the first 8 hours. Both ketamine and magnesium potentiated bupivacaine spinal anesthesia, prolonged the period of effective postoperative analgesia, reduced opioid consumption, and decreased pain scores especially in the early postoperative period