The sedative and electroencephalographic effects of regional anaesthesia

Abstract

We studied sedation scores, the processed electroencephalogram and mean arterial pressure pre-operatively in patients before and after spinal (n = 15) and lumbar epidural (n = 15) anaesthesia. We compared them with changes occurring over time in a control group (n = 15), using the responsiveness component of the Observer Assessment of Alertness and Sedation rating scale. The electroencephalographic variables studied were bispectral index, 95% spectral edge frequency, median frequency and the power distribution between frequency bands α, β, θ and δ. No differences were detected within groups in sedation scores before and after intervention, or between groups at either stage. No between-group differences were detected in electroencephalographic variables. In the spinal group, spinal anaesthesia caused an increased in bispectral index, 95% spectral edge frequency, median frequency and β frequencies, and a decrease in δ frequencies and mean arterial pressure. The increase in β frequencies is similar to that seen in patients with low plasma concentrations of midazolam and may represent subclinical sedation.