Bispectral Index

Describe the principles behind the BIS

Bispectral Index (BIS) is a proprietary signal-processed EMG and EEG monitor used to estimate depth of anaesthesia.

The BIS outputs four values:

  • BIS
    Dimensionless index between 0 and 100 where:
    • 0 represents cortical electrical silence
    • 85-100 represents normal awake cortical activity
    • 40-60 is consistent with general anaesthesia
  • Signal Quality Index (SQI)
    Dimensionless index between 0 and 100 which gives an indication of the accuracy of the BIS value.
  • Electromyography
    Gives an indication of the influence of muscle activity on BIS values.
  • Suppression Ratio (SR)
    Percentage of previous 63 seconds where EEG is isoelectric.


Proprietary, but involves:

  • Multivariate logistic regression of EEG features that correlate with clinical levels of sedation
  • Initial validation on a cohort of healthy volunteers, not undergoing surgery
  • Use of four frontotemporal EEG monitors

Analytic techniques:

  • Compressed Spectral Array
    • The signal over a short period (e.g. 5-10 seconds) of EEG recordings are analysed together
      Each period is known as an epoch.
    • A Fourier transformation is performed
      This breaks the EEG signal down into the sine waves used to produce it.
    • A histogram of each frequency is plotted
    • As anaesthesia deepens, lower frequencies begin to dominate
    • The spectral edge frequency is the frequency greater than 95% of the frequencies in the compressed spectral array
      It is an indicator of anaesthetic depth, but not of drug concentration.
  • Coherence
    Under anaesthesia, the electrical activity in different sections of the brain falls out of phase.


  • Reduced anaesthetic awareness in high risk patient groups
    Trauma, GA caesarian section, cardiac surgery.
  • Non-invasive
  • Use appears to result in reduced anaesthetic use and more rapid emergence


  • Proprietary algorithm
  • Expensive
  • May be inaccurate with:
    • Hypothermia
    • Hypercarbia
    • Hypoxia
    • Muscle relaxants
      BIS may fall inappropriately.
    • Non-GABAergic agents (e.g. ketamine, nitrous oxide)
      May not fall appropriately.


  1. Aston D, Rivers A, Dharmadasa A. Equipment in Anaesthesia and Intensive Care: A complete guide for the FRCA. Scion Publishing Ltd. 2014.
Last updated 2019-07-18

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