Monitoring the function of the central nervous system of a patient during a surgery where an injury to the nerves can occur
is important in securing the safety of the patient. The somatosensory tract of patients undergoing corrective surgery for
scoliosis were monitored using tibial nerve somatosensory evoked potentials (tSEP) by stimulating the tibial nerve from the
ankle and recording the responses from electroencephalography (EEG) electrodes placed on the scalp of the patient.
The recorded EEG was segmented off-line into different epochs on the basis of spontaneous neuronal activity overlaying the
tSEPs. The epochs were: bursts, suppressions, spindles and technical artefacts. Ensemble averaging was carried out over the
epochs to discover how quickly the overall tSEP signal-tonoise ratio (SNR) increases as a function of N.
The tSEP SNR increased most rapidly during the suppression epochs which contain only little spontaneous EEG activity in comparison
with the tSEPs recorded during the burst and spindle epochs. Fast and reliable results can be achieved in the operating room
by using deep propofol anaesthesia where the spontaneous EEG activity is mostly low, selecting only high SNR epochs and using
ensemble averaging on those to form the overall tSEP response.
Keywords Somatosensory evoked potentials - Signal-to-noise ratio - Ensemble averaging - Anaesthesia