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 alpha, beta, theta and delta. 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 increase in bispectral index, 95% spectral edge frequency, median frequency and beta frequencies, and a decrease in delta frequencies and mean arterial pressure. The increase in beta frequencies is similar to that seen in patients with low plasma concentrations of midazolam and may represent subclinical sedation.
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