The influences of nitrous oxide and ketamine on electroencephalogram during propofol-anesthesia

BACKGROUND: The influence of nitrous oxide and ketamine on electroencephalogram (EEG) during the induction of general anesthesia with propofol was quantitatively analyzed. METHODS: Anesthesia was induced with propofol using TCI (target controlled infusion) system. Twenty-five adult patients (ASA I-II) were randomly divided into three groups: 1. anesthesia induced with propofol alone [Group-P (n=7)], 2. anesthesia induced with inhalation of nitrous oxide in addition to propofol [Group-PN (n= 10)] and 3. anesthesia induced with intravenous injection of ketamine in addition to propofol [Group-PK (n= 8)]. We studied the influence of nitrous oxide and ketamine on EEG by analyzing the relations between effect-site concentration of propofol and EEG-indices. RESULTS: Additional injection of ketamine (Group-PK) increased BIS index during the maintenance of anesthesia from 43.4 +/- 8.2 to 71.8 +/- 6.5 in comparison with the contrasting group (Group-P). Nitrous oxide also increased the high frequency component in EEG. CONCLUSIONS: When anesthesia is induced with nitrous oxide and/or ketamine together with propofol, and BIS is taken as an index of depth of anesthesia, the intracerebral concentration of propofol becomes excess.