Effects of different concentrations of sevoflurane and desflurane on subcortical somatosensory evoked responses in anaesthetized, non-stimulated patients

Twenty-four patients were recruited and given either sevoflurane or desflur ane as their sole anaesthetic. Each patient was given sequentially increasi ng or decreasing doses at 0.5 MAC intervals, and the median nerve somatosen sory evoked response recorded after an equilibration at each concentration. The N-20-P-25 and P-25-N-35 amplitudes decreased with increasing agent con centration. However, for both agents the P-15-N-20 amplitude response was q uadratic in shape. The peak inflection points were at 3.2% for sevoflurane and 4.9% for desflurane. There were no differences between the ascending an d descending groups. This increase in activity in the midbrain at 'surgical ' end-tidal anaesthetic concentrations suggests more complex neuroelectrica l responses to anaesthesia than simple global suppression.