Effect of halothane on spinal somatosensory evoked potentials in sheep

The effects of increasing concentrations of halothane on the morphology of spinal somatosensory evoked potentials (SEP) were studied in seven anaesthetized sheep. Anaesthesia was induced with thiopentone and maintained with nitrous oxide in oxygen. Ventilation was controlled throughout. Arterial pressure, temperature and end-tidal carbon dioxide tension were monitored. A lumbar laminectomy was performed, the sciatic nerve exposed and a baseline spinal SEP was recorded from the surface of the lumbar spinal cord. Halothane was introduced in incremental steps of 1% up to 3% and further SEP were recorded. These recordings revealed that the spinal SEP peak latencies and general waveform configuration were stable under halothane anaesthesia. Baseline SEP amplitude was similar to that obtained with 1% halothane; however, at concentrations of 2% halothane or greater, there was a significant (P less than 0.005) attenuation of all components of the spinal SEP. These findings have important anaesthetic implications for patients undergoing spinal cord monitoring using SEP, although interspecies differences may occur.