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From the Department of Anesthesiology, The University of Tokyo, Tokyo, Japan.
Address correspondence and reprint requests to Tomoki Nishiyama, MD, PhD, 3-2-6-603 Kawaguchi, Kawaguchi-shi, Saitama 332-0015, Japan. Address e-mail to nishit-tky{at}umin.ac.jp.
BACKGROUND: The click sounds of auditory evoked potentials (AEP) might have some effect on electroencephalogram indices and Bispectral Index (BIS) but many studies, unconcerned about this effect, have measured both indices simultaneously. In this study, I examined the effects, of AEP click sounds on the BIS, and also on the response entropy (RE) and state entropy (SE) of the entropy monitor.
METHODS: Forty patients aged 40–70 yr and scheduled for surgery of lower extremities under spinal anesthesia were anesthetized with 0.5% bupivacaine or tetracaine. Patients were sedated with midazolam 1 mg followed by propofol infusion started at 1 mg · kg–1 · h–1. Propofol infusion was controlled to keep BIS or SE at 80, 60, or 40 for several minutes, and then click sounds (65 dB) of the AEP were given for 60 s. The changes in BIS, RE, and SE were observed continuously for 60 s after the click sounds had stopped.
RESULTS: BIS, SE, and RE significantly increased during the click sounds. The longest duration of increase was at BIS or SE 60.
CONCLUSION: AEP monitor click sounds transiently increased the simultaneously measured BIS, RE, and SE during different levels of sedation by propofol infusion during spinal anesthesia. Therefore, the effects of the click sounds should be considered when these monitors are used simultaneously in the same patient.
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