Spinal anaesthesia indirectly depresses cortical activity associated with electrical stimulation of the reticular formation

Background. Neuraxial blockade reduces the requirements forsedation and general anaesthesia. We investigated whether lidocainespinal anaesthesia affected cortical activity as determinedby EEG desynchronization that occurs following electrical stimulationof the midbrain reticular formation (MRF). Methods. Six goats were anaesthetized with isoflurane, and cervicallaminectomy performed to permit spinal application of lidocaine.The EEG was recorded before, during and after focal electricalstimulation (0.1, 0.2, 0.3 and 0.4 mA) in the MRF while keepingthe isoflurane concentration constant. Results. During lidocaine spinal anaesthesia, the spectral edgefrequency (SEF) after MRF electrical stimulation (13.6 (SD 1.0)Hz, averaged across all stimulus currents) was less than theSEF during control and recovery periods (18.6 (3.6) Hz and 17.2(2.2) Hz, respectively; P<0.05). Bispectral index valueswere similarly affected: 69 (10) at control compared with 55(6) during the spinal block (P<0.05). Conclusions. These results suggest that lidocaine spinal anaesthesiablocks ascending somatosensory transmission to mildly depressthe excitability of reticulo–thalamo–cortical arousalmechanisms. Br J Anaesth 2003; 91: 233–8