Effects of halothane and enflurane on far and near field somatosensory evoked potentials.

Somatosensory potentials evoked in response to median nerve stimulation were studied in 10 patients during surgery under general anaesthesia with halothane (five patients) or enflurane (five patients). Bipolar scalp responses (near field) and scalp to non-cephalic reference (far field) potentials were recorded before the induction of anaesthesia, and after 15 min stabilization at each of 0.5, 1.0, 1.5 and 2.0% end-tidal concentrations of anaesthetic agent. Both agents produced similar effects. The latency of the bipolar responses was increased and the amplitude decreased. The amplitude of the far field, subcortically generated, potentials measured from the scalp to non-cephalic electrodes did not decrease as much as the near field potential with increasing concentrations of volatile anaesthetic, although the latency of the potential recorded at the frontal electrode increased. At higher anaesthetic concentrations the virtual elimination of the near field potential caused the frontal and rolandic potentials to appear to be identical. Since far field somatosensory evoked potentials are preserved during deep anaesthesia, they should be considered for use when measurement of evoked responses is required for monitoring purposes.