A preliminary evaluation of a new derived EEG index monitor in anesthetized patients

An electroencephalogram (EEG) monitoring device, recently developed by Nicolet Biomedical, analyzes both high and low EEG frequencies. A processed derivative is obtained and displayed graphically and numerically on a monitor and may be used during anesthesia to indicate anesthetic "depth." However, radio-frequency interference from electrical equipment typically used in the operating room has the potential to interfere with the analysis of the high frequency components of the EEG.

The objective of this study was to determine the ability of the Nicolet EEG monitoring device to function satisfactorily and effectively in the operating arena when used for anesthetized patients.

A total of 40 patients undergoing surgery with general anesthesia were monitored using a prototype, processed EEG monitoring device. The device was used beginning prior to induction and continuing until emergence from anesthesia.

Analysis was performed on 38 of the 40 patients. Brief interruption of the derived EEG Index occurred during use of the monopolar electrosurgical unit (ESU) as well as during stimulation of the facial nerve (used to monitor pharmacologic muscle relaxation). Derived EEG Index interruption was also noted during some movements of the patient's head or the EEG electrode wires. Changes in the derived EEG Index value accompanied changes in inhaled concentration of anesthetics and, in one patient, change in the derived EEG Index preceded patient movement that was not heralded by the typical cardiovascular indications of "light" anesthesia.

The derived EEG Index monitoring system evaluated in this study functioned satisfactorily in the operating room setting in patients undergoing general anesthesia. Interference from ESU, facial nerve stimulation, and movement of the electrode wires caused brief interruptions of the derived EEG Index display and did not affect its utility in monitoring brain activity during anesthesia.