Improving the acquisition of the middle latency response with a view to measuring conscious awareness during anaesthesia

It has been suggested that the Middle Latency Response or MLR (an auditory evoked potential) may be used as an indicator of conscious awareness during anaesthesia. However, the SNR of the response is poor and this is one limit on the clinical utility of the technique. The MLR is a few tenths of a microvolt in amplitude and is embedded in the spontaneous Electroencephalogram (EEG) waveform which has an amplitude typically of 10 to 30 V. Thus the signal-to-noise-ratio (SNR) is less that 1:10 (?20 dB). This poor SNR must be improved in order to obtain repeatable responses. The traditional method for improving the SNR of the MLR is to use a synchronised ensemble average of many successive responses, where the onset of the stimulus triggers the synchronisation process. Theoretically, for n averages the SNR increases by a factor ?n. Many studies of the MLR use click stimuli presented at 5 Hz. Typically 1-2000 averages are needed in order to obtain a clear waveform and even then SNR may be poor. The high number of averages required results in a long acquisition time. In a joint research project between audiology, anaesthetics and biomedical engineering, we have aimed to improve the acquisition of the MLR. We have been investigating the use of new stimulation techniques in order to improve the acquisition of the MLR. Specifically we have used the maximum length sequence (MLS) technique, which allows the use of fast stimulation rates to record the MLR, and the use of chirp stimuli designed to compensate for frequency dispersion on the basilar membrane instead of clicks. The optimal stimulation paradigm we have developed to obtain the MLR uses chirp stimuli at an MLS rate of 167 stimuli/s. This stimulation technique improves acquisition time of the MLR by more than an order of magnitude compared to using conventional stimulation at 5 clicks/s. The technique improves the SNR of the MLR, which should in turn makes the response more repeatable. We are currently conducting a clinical trial investigating changes in the MLR acquired with this technique during anaesthesia. The study takes place at Southampton General Hospital on patients awaiting heart surgery. The MLR is recorded from patients as they are anaesthetised prior to surgery. They are allowed to emerge from the anaesthetic, then re-anaesthetised and corresponding changes in the MLR are tracked. Recruitment of subjects for the anaesthetics study has been slow and there have been a number of technical difficulties with recording the MLR in the anaesthetics room (such as mains interference). However, there is evidence that the MLR shows a categorical change between awake and asleep states. This may reflect the activity of attention dependant auditory centres in the brain. It does not show a graded change that might indicate depth of anaesthesia, rather it appears to categorically indicate conscious awareness. If the acquisition difficulties can be overcome, the MLR has potential as a clinical indicator of awareness during surgery (although hearing loss would prevent use of the technique). However more subjects are needed to confirm these findings.