The influence of frontal muscle electromyography on electroencephalography‐based depth of anaesthesia monitoring

Abstract

PhD dissertation abstract Background: Electroencephalography (EEG)-based depth of anaesthesia monitoring has become increasingly popular during the last 15 years. It has been reported to decrease the incidence of intraoperative awareness, to reduce the amount of anaesthetic drugs given to the patient, and to enhance recovery after anaesthesia. The EEG analysed by the depth of anaesthesia monitors is collected from the forehead of the patient, rendering EEG-based depth of anaesthesia monitoring vulnerable to confounding factors, such as electromyography (EMG) of the frontal muscles. Also, EEG arousal can produce changes in the numerical values of the depth of anaesthesia monitors without a change in the actual depth of anaesthesia, making the interpretation of depth of anaesthesia monitors very challenging for an anaesthesiologist. Objective: The aim of this doctoral thesis was to study the effect of EMG and EEG arousal on the numerical values of bispectral index scale (BIS) and Entropy, and to investigate if EMG arousal can be detected visually on the anaesthesia monitor. Methods and results: In Study I, the patients were anaesthetised with propofol-nitrous oxide (N2O) or propofol-N2O-remifentanil. The one-channel EEG collected by the Entropy strip was analysed offline. Intubation of the trachea produced EMG arousal in 13/16 and 15/15 patients with and without remifentanil, respectively. EMG arousal caused a significant rise in Entropy values even during propofol-induced burst suppression. The EMG activity started at frequencies below 20 Hz, contaminating the interpretation of the Entropy values. In Study II, the one-channel EEG collected by the Entropy strip was analysed to discover the incidence of skin incision-associated EMG and EEG arousal under sevoflurane-N2O-rocuronium or sevoflurane-N2O anaesthesia. Skin incision caused EMG arousal in 0/19 and 13/19 patients with and without rocuronium, respectively. Skin incision produced EEG arousal in 17/19 and 15/19 patients with and without rocuronium. EEG arousal was classified as beta arousal in 30 patients, causing an increase in Entropy values. EEG delta arousal (two patients) caused a decrease in Entropy values. The power spectra of EEG and EMG overlapped significantly. In Study III, during light propofol-remifentanil anaesthesia, the neuromuscular blockade (NMB) was antagonised with neostigmine or sugammadex, and the numerical values of BIS and Entropy were studied.