• EEG for depth of anaesthesia
    • Awareness (explicit)
      • 0.07-0.2% in non-cardiac, non-obstetric surgery
      • 0.4% for LSCS under GA
      • 1% for cardiac surgery
    • The EEG
      • Surface recording of summated electrical potentials arising from pyramidal neurons in the cerebral cortex
      • Raw EEG is too difficult to interpret for measuring depth of anaesthesia
      • EEG can be altered in different ways depending on the agent used
    • Time domain analysis
      • Examines voltage changes over time
      • Cerebral function monitor (CFM) records the voltages from bilateral parietal electrodes
        • Voltage plotted against a compressed time axis
        • Limited use clinically due to complex dose-response interpretation
    • Frequency domain analysis
      • Raw EEG is processed by Fourier analysis
      • Analysed with respect to frequency distribution,
        power, and phase relationship
      • Power spectral analysis
        • Looks at relationship between power and frequency over a period of time (epoch)
        • Graphically, power vs frequency forms a spectral array
        • These spectral arrays can be superimposed to build up a display of changes in the location of power within frequencies over time
        • Compressed spectral array (CSA) is obtained by superimposing linear plots of successive epochs on each other generating a 3D display
        • Peak power frequency, median frequency and spectral edge frequency attempt to give a figure relating to depth of anaesthesia
      • BIS
        • This considers phase relationship between component waves of different frequencies that make up the EEG.
        • These undergo multi variate logistic regression to generate the bispectral index (BIS)
        • Measured by a dimensionless scale between 0-100. The probability of post-op recall is low if BIS is <60 intra-op

    • Link:ceaccp.oxfordjournals.org/content/5/6/183.full.pdf
    • CEACCP
      Link:ceaccp.oxfordjournals.org/content/3/4/106.full.pdf