Use of the median EEG frequency and pharmacokinetics in determining depth of anaesthesia

Abstract

Summary The median frequency in the EEG is the most useful derived variable from the EEG with which to measure the depth of anaesthesia. In awake subjects, a median frequency of 10 Hz was observed. During anaesthesia with a variety of different anaesthetic agents, there was a reduction in median frequency. A median frequency of less than 5 Hz was consistent with the abolition of a number of signs of awareness. These included reaction to verbal command, impaired orientation in time and space, and eyelash reflex. Pharmacokinetic and pharmacodynamic analysis permitted a close correlation to be obtained between median frequency, signs of awareness and concentration of anaesthetic in blood and brain. The possibility of feedback control of anaesthetic administration has been explored.