Auditory steady-state response, upper facial EMG, EEG and heart rate as predictors of movement during isoflurane-nitrous oxide anaesthesia.

We have studied the relationship between patient movement and changes in the auditory steady-state evoked potential, upper facial muscle electromyogram (FEMG), electroencephalographic-zero crossing frequency (EEG-ZCF) and heart rate during emergence from anaesthesia. Twelve healthy patients underwent surgery during stable isoflurane-nitrous oxide-oxygen anaesthesia without neuromuscular block. After skin closure, anaesthesia was discontinued abruptly while mechanical ventilation was continued until the patient moved. The magnitude of change in each physiological signal was evaluated in decibels (dB). Both the auditory steady state evoked potential and FEMG showed significant increases in amplitude during the last 5-min period before movement (6.1 and 10.7 dB, respectively). EEG-ZCF increased rapidly after anaesthesia was discontinued (2.5 dB) but there was no further increase in activity before movement. Heart rate did not change before movement. The use of the decibel transformation offers a promising method of displaying and interpreting changes in physiological variables during anaesthesia.