A comparison of the absolute amplitude of motor evoked potentials among groups of patients with various concentrations of nitrous oxide.
OBJECTIVE
It has been shown in previous studies that nitrous oxide (N(2)O) suppresses the amplitude of motor evoked potentials (MEPs) in individual subjects. In the present study, we compared the absolute amplitude and latency of MEPs among groups of patients with various concentrations of N(2)O.


METHODS
The subjects were 60 patients who were scheduled to undergo craniotomy with MEP monitoring. Anesthesia was induced and maintained with propofol and fentanyl. The patients were randomly assigned to one of three groups based on the concentration of N(2)O: 0% N(2)O (N0 group), 50% N(2)O (N50 group), and 66% N(2)O (N66 group). MEPs were elicited by transcranial electrical stimulation. The effect-site concentrations (ESCs) of anesthetics were calculated retrospectively. The effects of anesthetics on MEP were analyzed by analysis of covariance (ANCOVA) followed by Tukey's method.


RESULTS
MEPs were elicited in all cases. The absolute amplitude of the MEP was significantly higher in the N0 group than in the N50 and N66 groups [4.16 +/- 0.42 vs 1.00 +/- 0.26 mV and 1.00 +/- 0.27 mV, respectively (mean +/- SD); P < 0.05]. In contrast, there was no significant difference in the latency of the MEP among the three groups of subjects (N0: 16.64 +/- 0.72, N50: 16.78 +/- 0.66, and N66: 16.82 +/- 0.63 ms).


CONCLUSIONS
The results suggest that N(2)O can suppress the absolute amplitude of the MEP in patients under propofol and fentanyl anesthesia. Although monitoring of MEP as a trend is feasible even if N(2)O is used, the use of N(2)O may be better avoided.
Kunisawa
Takayuki
T
Department of Anesthesia, Shirakawa Hospital, 10-1 Rokutanyama, 961-0092 Shirakawa, Japan.
Nagata
Osamu
O
Nomura
Minoru
M
Iwasaki
Hiroshi
H
Ozaki
Makoto
M
D000293:Adolescent; D000328:Adult; D000368:Aged; D004305:Dose-Response Relationship, Drug; D019054:Evoked Potentials, Motor / Q000187:drug effects*; D006801:Humans; D008875:Middle Aged; D009609:Nitrous Oxide / Q000494:pharmacology*; D011930:Reaction Time / Q000187:drug effects
D009609:Nitrous Oxide