Auditory stimuli as a contributor to consciousness while under general anesthesia

Abstract

BackgroundThe incidence of intraoperative awareness under general anesthesia approaches 1% in high-risk patients. Anesthesiologists commonly utilize processed electroencephalograms (EEG) in order to monitor “depth” of anesthesia, the most common of which is the Bispectral Index (BIS). The B-aware and B-Unaware trials, which were designed to test the efficacy of the BIS monitor, noted an auditory component in 16 of 17 confirmed cases of intraoperative awareness. Implicit auditory memory formation has been documented under general anesthesia. Small studies have documented a significant effect of noise on BIS scores during monitored anesthesia care. MethodsTwenty-two patients undergoing general anesthesia received earplugs after the induction of anesthesia. Every ten minutes the earplugs were reinserted or removed. Noise levels were recorded every 0.125s and both average and maximal BIS scores were recorded every minute. Non-parametric analysis of both populations (with and without earplugs) was performed. A mixed effects model with one degree of freedom (with and without earplugs) was generated to take into account the effect of anesthetic agents on BIS scores. Results3009min of data were recorded. The median and range (25–75%) BIS scores were 39 (29–46) and 39 (28–44) with and without earplugs in place, respectively. Earplugs were associated with lower BIS scores (p=0.0183). The mixed effects model confirmed this relationship (p<0.001). Subgroup analysis of BIS scores in which the potential for awareness existed (maximum BIS>60 in any one minute epoch) showed a 32% reduction in the incidence of maximal BIS scores exceeding 60 (p=0.0012). There was no relationship between ambient noise level and average maximal BIS score (R2=0.003). ConclusionsOur study suggests that earplugs may reduce the incidence of BIS scores >60 in patients undergoing total intravenous anesthesia and that auditory stimuli may affect EEG interpretation. Because of the low cost and safety of noise reduction, as well as the catastrophic implications of intraoperative awareness, further studies to explore the effects of auditory stimuli on awareness and anesthesia are warranted.