Reduced anesthetic requirement after electrical stimulation of periaqueductal gray matter

To determine whether electrical stimulation of the periaqueductal gray region decreases anesthetic requirement, the authors studied the effect of such stimulation on the MAC of halothane and 60% nitrous oxide in 33 patients. These patients, who were undergoing implantation of a radio-frequency-coupled receiver and connection of that receiver to electrodes previously implanted in the periaqueductal gray area, were assigned randomly to receive (n = 16) or not receive (n = 17) electrical stimulation 1 h before surgery. The mean value (+/- SEM) for the minimum alveolar concentration of halothane combined with 60% nitrous oxide was significantly less (P less than 0.001) for patients who were stimulated preoperatively (0.15 +/- 0.05%) than for those who were not (0.51 +/- 0.02%). The authors conclude that stimulation of the periaqueductal gray region decreases anesthetic requirements and believe that at least three mechanisms are possible: a nonspecific narcotic-like effect, a specific effect on a pain pathway, or an effect on specific neural pathways that affect anesthetic requirements secondary to changes in regional concentrations of neurotransmitters.