Monitoring evoked potentials during surgery to assess the level of anaesthesia.

The hypnotic and analgesic components of anaesthesia can be assessed using middle latency auditory evoked potentials (MLAEPs) and somatosensory evoked potentials (SEPs). To monitor these potentials reliably during clinical anaesthesia, we have developed an evoked potential (EP) system based around a portable personal computer, a DSP board and an isolated pre-amplifier unit. Unlike many currently available systems, this amplifier is largely immune to diathermy interference due to excellent isolation via a digital fibre optical link, small size and RF screening and filtering. The pre-amplifier unit has integral auditory and somatosensory stimulators, and automatic calibration and impedance checking. Stimulus intensity and profile are under software control and SEP stimulus level is constantly monitored. The unit is powered by two AA cells and battery status continuously monitored by the PC software. Up to eight channels of EEG may be recorded and displayed in a smoothly scrolling window and as moving average MLAEPs and SEPs.