The EEMG--a practical index of cortical activity and muscular relaxation.

In a continuing study of the EEG as a monitor of cortical activity during anesthesia using a time-domain wave analyzer, the contamination of the EEG by scalp muscle activity was observed. A measure of this EMG activity was extracted from the combined EEG/EMG wave obtained from two electrodes placed over the left frontalis muscle and right mastoid process. Clinical work with this variable showed that the EMG can serve both as a reliable guide to patient muscular relaxation and an indicator of unacceptable muscle contamination of the EEG. This extracted variable in conjunction with the analyzed EEG has served as a valid indicator in the assessment of the anesthetic state. The system has been successfully employed in the management of nearly 1000 surgical patients.