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.

Method

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

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