Effect of isoflurane (Forane) on intraoperative electrocorticogram

Isoflurane, an inhalation agent often used for general anesthesia during craniotomy, has been reported to suppress spike activity in the intraoperative electrocorticogram (ECoG) during epilepsy surgery. We studied the effect of isoflurane concentrations of 0.25, 0.5, 0.75, 1, and 1.25% on the number of spike bursts per 5-min epochs in 15 patients undergoing ECoG during epilepsy surgery. N2O in O2 was maintained at 50% in 10 patients, at 60% in 2, and at 70% in 3. End tidal CO2 concentration was maintained in the hypocarbic range, and analgesia was maintained with the narcotic alfentanil in the range of 0.5-2 micrograms/min. The median number of spikes for each isoflurane concentration was 29 (range 3-107) at 0.25%, 27 (range 2-73) at 0.5%; 29 (range 5-90) at 0.75%, 33 (range 2-100) at 1%, and 40 (range 32-140) in 5 patients who tolerated 1.25% without occurrence of burst suppression pattern. No significant difference (Student's paired t test) was noted in the number of spikes for each isoflurane concentration. Therefore, if isoflurane concentrations are maintained between 0.25 and 1.25% or before burst suppression pattern occurs and N2O/O2 is maintained in the 50-70% range, isoflurane has no significant effect on spike activity.