Bispectral Index (BIS) Monitoring


  • BIS monitor was developed by Aspect Medical Systems
  • BIS = bispectral index
  • other ‘depth of anaesthesia’ monitors exist (e.g. Entropy)


  • used to monitor the depth of sedation or anaesthesia


  • Sensor – consists of disposable, wet gel electrodes
  • Cable
  • Monitoring module – interfaces with anaesthesia machine or monitoring systems


  •  best describe as a monitor of the depth of the hypnotic component of anaesthesia
  • frontoparietal application of electrodes
  • the monitor generates a number
    — 100 = normal cortical activity (maximum alertness)
    — 0 = cortical electrical silence
  • an EMG signal and Signal Quality Index is displayed as well as a single channel EEG

The BIS algorithm

  • EEG data recorded from healthy adults who underwent repeated transitions between consciousness and unconsciousness using different anaesthetic regimens
  • analysis identified those features of EEG recordings that best correlated with clinical depth of sedation/anaesthesia – in essence, the complexity of the EEG reduces with increasing depth of anaesthesia
  • these were fitted to a model by multivariate logistic regression
  • the resulting algorithm generates a bispectral index (BIS)
  • it has been validated on healthy and now other patient populations



  • BIS < 60 results in very low levels of post-op recall
  • shown to lower anaesthetic consumption
  • slightly quicker awaking
  • safe with diathermy and defibrillation


  • opioids produce changes in depth of consciousness not discernible by BIS
  • BIS is not able to predict movement in response to surgical stimulation
  • BIS values display incredible variability
  • not helpful when using N2O or ketamine
  • cannot use in children under 5


  • low BIS associated with increasing delirium -> this can increase morbidity and mortality
  • currently no high level evidence to guide use in ICU
  • possible role for patients who are paralysed in the titration of adequate amounts of sedation and analgesia
  • may have an emerging role in out-of-hospital cardiac arrest

References and Links


Journal articles

  • Ball J. How useful is the bispectral index in the management of ICU patients? Minerva Anestesiol. 2002 Apr;68(4):248-51. PMID: 12024092.
  • Bennett C, Voss LJ, Barnard JP, Sleigh JW. Practical use of the raw electroencephalogram waveform during general anesthesia: the art and science.Anesth Analg. 2009 Aug;109(2):539-50. PMID: 19608830
  • Bigham C, Bigham S, Jones C. Does the bispectral index monitor have a role in intensive care? JICS 2012 Oct;13(4):314-319

CCC 700 6

Critical Care


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