Changes in cerebral electrical activity measured by the Cerebral Function Analysing Monitor following bolus injections of thiopentone.

Seven premedicated patients were anaesthetized with thiopentone administered incrementally every 180 s while cerebral electrical activity was recorded with the Cerebral Function Analysing Monitor (CFAM). Arterial blood samples were collected at 40, 90 and 165 s after each bolus of thiopentone for the measurement of plasma total and bound thiopentone concentrations. The correlation between free thiopentone concentration and the CFAM representation of mean electroencephalogram (EEG) amplitude was -0.78 (P less than 0.001), excluding values following the first two boluses. Beta band percentage activity was inversely related (r = 0.70-0.90; P less than 0.05- less than 0.001) and delta directly related (r = 0.76-0.86; P less than 0.05- less than 0.001) to log of free thiopentone concentration, including all values. The closest correlations (beta, r = 0.88, P less than 0.001; delta r = 0.80, P less than 0.001) were found at 90 s after each bolus.