TITLE: EEG-BISPECTRAL INDEX MONITORING OF MIDAZOLAM-KETAMINE ANESTHESIA

AUTHORS: Martin S. Mok, MD, Chih-Cheng Wu, MD, Sin- Ru Han, MD
AFFILIATION: Taipei Medical College Hospital, Taipei, Taiwan Republic of China.

INTRODUCTION: EEG-bispectral Index (BIS) is a processed EEG monitor that has been validated as a measure of the hypnotic effect of anesthetic drugs. The present study evaluated the BIS changes associated with midazolam-ketamine anesthesia.
METHODS: Twenty ASA class I-II adult patients without history of cardiovascular disease scheduled for elective surgeries were enrolled into this single blind, randomized, prospective study. No premedication was given to the patients. EEG-BIS was recorded continuously from the frontal electrode using Aspect A- 1000 monitor on arrival to the operating room. Blood pressure and heart rate were also recorded every minute throughout the study. After steady baseline readings patient received midazolam 0.05 mg/kg I.V. followed by ketamine 1.5 mg/kg I.V. at 3 min. later. At 5 min after ketamine, rocuronium 0.6 mg/kg I.V. was given to facilitate endotracheal intubation after which inhalation of sevoflurane 1.5 MAC in 60% N20 and 40% O2 was used for anesthesia maintenance. BIS values were recorded throughout the entire anesthetic course till patient was fully awake.
RESULTS: The average value of BIS during the awake basal state was 93 and it gradually declined to 83 at 3 min. after administration of midazolam and all the patients were tranquil but readily arousable. After ketamine all the patients drifted into an anesthetized "dissociate state" in 2 min at which time the BIS values increased to 90, 95 and 96 at the 3^rd, 4^th, and 5^th min. respectively. However upon the administration of inhalation anesthetics the BIS index decreased gradually to below 60 in about 6 min and remained below that value throughout surgery. After termination of anesthetics all patients became awake within 15 min. with BIS above 80. No patient had recall, delirium, or hallucination when questioned in the recovery room.
CONCLUSION: It is generally accepted that BIS index below 60 indicates adequate hypnosis, which has been shown to be true with all the intravenous and inhalation anesthetics. Our study showed that ketamine, on the contrary, produced elevated BIS values after midazolam induction when the patient appeared to be anesthetized. The use of BIS in the assessment of anesthetic depth with ketamine remains a challenging problem.
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