BISPECTRAL INDEX

Abstract

S58 INTRODUCTION: Awareness during general anesthesia, particularly with use of neuromuscular blocking drugs, is a potential risk. Bispectral index (BIS), a processed EEG, has been shown to be useful in monitoring the level of consciousness [1]. This study measured arousal and level of consciousness reflected by BIS during endotracheal intubation, a noxious stimulus, during general anesthesia (GA) at our institution. METHODS: With IRB approved consent, 100 patients aged 18 to 70 yrs of either sex, ASA class I or II, were studied. Following placement of intravenous canula in the non-dominant arm and routine monitors and BIS (Model A 1000 EEG, Aspect Medical Systems, Inc.), a tourniquet (pressure > 150 above SBP) was applied on the dominant arm. Patients practiced squeezing the investigator's hand on command, first once and then following a positive response, twice. Anesthesia was then induced by the method of choice of the anesthesia team. Patients were commanded to squeeze the hand every 30 seconds from the time of induction until 5 min. following endotracheal intubation. BIS, heart rate and blood pressure were continuously recorded. The isolation of the forearm was assured with a peripheral nerve stimulator. Throughout the study, the anesthesia team was unaware of the patients' responses to command or the BIS. All patients were interviewed twenty four hours later about awareness and recall. Data were analyzed using Student's t-test and linear regression. P< 0.05 was considered significant. RESULTS: Sixteen patients responded by squeezing the hand twice on command (arousal group). Two patients responded by squeezing the hand once and were included in the non-arousal group. (Table 1)Table 1The increase in BIS from the start of laryngoscopy to the end of endotracheal intubation in the arousal group was approximately 10 units vs. 3.5 in the non-arousal group (P<0.001). There was no difference in heart rate or mean arterial pressure. None of the patients reported any awareness or recall. DISCUSSION: The study demonstrates that 16% of the patients were aroused by noxious stimulus during routine induction of anesthesia. Patients with higher BIS values (higher levels of consciousness) had an increased incidence of arousal and their BIS response to noxious stimulus was significantly greater than patients with lower initial BIS values. BIS may be valuable in determining the level of consciousness (depth of hypnosis) prior to noxious stimuli encountered during GA.