To the Editor:-The bispectral index (BIS) developed by Aspect Medical Systems is increasingly used as a clinical tool for monitoring the hypnotic state during general anesthesia. [1-3]The BIS ranges from 100 (awake) to 0 (deep hypnosis). [4]During baseline recording in an experimental pain study, we obtained BIS values using an A-1000 EEG monitor (BIS version 3.11, Aspect Medical Systems, Natick, MA) in fully conscious volunteers before administration of nitrous oxide or xenon. The EEG signal was recorded from leads F3 - Cz, F4 - Cz, P3 - Cz, and P4 - Cz (International 10/20 System). One volunteer had a BIS of 40 as his awake baseline. Electrode impedance was less than 500 Omega, and the signal quality index (SQI) was in the “good” range. On administration of nitrous oxide, the BIS further decreased, although the volunteer was still responding promptly. Experimental pain tests resulted in an increase of the BIS value. The raw EEG signal differed from other volunteers in this study only in having a slightly smaller amplitude. Similar baseline BIS values were observed in the same volunteer during the second study session 3 days later. Xenon administration resulted in an increase in the BIS, although clinically xenon caused concentration- dependent sedation. Four weeks later, our Department of Neurology obtained a 16-lead diagnostic EEG of this volunteer. A diagnosis of a genetically determined low voltage EEG was made. This is defined by amplitudes not greater than 20 mV over all head regions and occurs with an incidence of between 5-10% of the population. [5]It is not associated with any brain dysfunction.

The BIS EEG index was developed from patients with normal EEGs. It is therefore expected that the BIS may be misled by patients with abnormal EEG patterns. Most anesthesiologists will not be able to determine whether an EEG pattern is “normal” or “abnormal” from inspection of the waveform itself. Thus, it is essential to confirm a normal BIS value when the patient is awake to prevent misleading interpretations of subsequent BIS values during anesthesia.

Thomas W. Schnider,

Martin Luginbuhl,

Steen Petersen-Felix,

Institute for Anesthesiology and Intensive Care

Johannes Mathis,

Department of Neurology; University Hospital; Inselspital; Bern, Switzerland;

(Accepted for publication August 3, 1998.)

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