Deriving a Real Time Depth of Anaesthesia Index from the Middle Latency Auditory Evoked Potential (MLAEP)

This paper describes the development of a monitor for depth of anaesthesia based on auditory evoked potential (AEP). An auditory stimulus was applied and the mid-latency response (15 - 110 ms) was measured. A PIC microcontroller was used to handle the low level control and the data was sent to a PC via a serial link. Data processing was carried out using programmes written in MatLab. Mixed language programming under MS Windows using Dynamic Link Library (DLL) and ActiveX was employed. The user interface plotted the signal, stored the data and obtained a level of arousal score (LAS) from 256 stimuli in less than 1.5 seconds. Clinical trials of the system were carried out on 20 patients undergoing general anaesthesia. The anaesthetist was blind to these figures so that it did not influence clinical judgement. None of the patients recalled any awareness after the operation. LAS was also measured in 10 control subjects. Results showed significant difference (p < 0.05) between the awake and asleep LAS values.