A comparison of ECG cancellation techniques applied to the surface recording of somatosensory evoked potentials

The use of somatosensory evoked potentials (SEPs) for determining spinal cord integrity in a clinical setting is well known. The poor signal-to-noise (SNR) ratio of such measurements has led to ensemble averaging being employed to extract the signal from the background noise. Over the thoracic region, the poor SNR is largely the result of interference generated by the cardiac musculature. Therefore, any reduction in the level of this cardiac interference will greatly improve the performance of any SEP monitoring system. Three methods were investigated as techniques to reduce the cardiac interference in SEP measurements. These were clipping, gating and adaptive noise cancellation (ANC). It was found that, although clipping and gating performed as well as, if not better than, ANC, these techniques were both very sensitive to the setting of a threshold level. The linear ANC scheme employed circumvented the problem associated with a threshold level, and with an order of 50, SNR improvement figures of approximately sixfold were achievable over the entire thoracic region.