Abstract

Key worh: anesthesia, general; coma; electromyography; facial muscles; facial nerve; intracranial pressure; mimetic muscles; monitoring. During general anaesthesia and in lowered vigilance states such as after major trauma and during heavy sedation or analgesic medication, patients’ ability to communicate with their surroundings is limited. Subjective intuitional interpretation may be the only means to ascertain a patient’s emotional state, mood, and pain perception. Electromyographic detection and quantification of minimal and covert facial mimic muscle activity in anaesthesiology and critical care was an interesting concept worth further evaluation. In this study, the behaviour of quantitative surfacedetected electromyographic activity (qEMG) was investigated during common anaesthetic events, postoperatively, and in volunteers as well as in experimental animals. A review of the methodology includes the necessary details for reproduction of the studies, including computerized processing of numerical data available in the commercial equipment. Results from the monitoring of 218 patients, seven volunteers and 31 rats are discussed. Conclusions are based on 32 testable null-hypotheses, the earlier documented literature and the author’s own experience. The qEMG signal was derived from two electrodes placed on the frontal area and on the mastoid process behind the ipsilateral ear. After amplification, the signal was filtered to obtain a portion containing electrical activity between 60-300 Hz, which was considered to represent electromyographic activity. The signals were thereafter full-wave rectified and averaged with a l-s time constant. The output of the processing unit consisted of a graphics display and a numeric computer output. A variety of clinical conditions and drug effects were studied in order to evaluate the method’s applicability in research and in routine anaesthetic practice. The facial muscles turned out to be less sensitive to the effects of neuromuscular blocking drugs than the hand muscles, the normal monitoring site of neuromuscular transmission. Although muscle relaxants had a suppressing effect on spontaneous EMG activity, they did not abolish the ability of facial muscles to react to noxious stimuli. Also abdominal muscles retained this capability, though presumably through a different motor mechanism. Depolarizing and non-depolarizing neuromuscular blocking drugs had disparate effects on spontaneous EMG activity; succinylcholine seemed to facilitate the qEMG amplitude during recovery of the block. Quantitative surface electromyography revealed impending arousals during lightening anaesthesia, although this was not a consistent phenomenon. Inadequate anaesthesia was always reflected by an increase in facial qEMG, albeit this often was also evident to the naked eye. The arousal at the end of anaesthesia was always associated with an abrupt increase in facial qEMG activity, which often was preceded by a more gradual, predictive rise. Auditory stimulation was also effective in increasing qEMG. Relief of postoperative pain was quantitatively reflected as a fall in facial qEMG amplitude. In volunteers, an increase of painful stimulation was associated with a rise in qEMG amplitude which, in turn, correlated with subjective pain ratings. Return of facial muscle activity after a hypoxic brain insult was always associated with long-term survival in experimental animals. In conclusion, quantitative facial surface electromyography enables discrimination of adequate vs. inadequate anaesthesia. It is also a useful measure of drug effect, vigilance levels and central nervous system integrity.