Skip to main content

Current State of Intra-operative Cerebral Monitors

Presentation on the current state of intra-operative cerebral monitors, click read more to see the slide show.


•Electroencephalography (EEG) and Anaesthesia
•Current depth of anaesthesia monitors
•Brain Anaesthesia Response Monitor (BAR Monitor)


EEG & Anaesthesia

•Anesthetic agents target a host of synaptic sites, at differing levels of the CNS.
•Synaptic (micro) modifications produce observable (macro) changes in human EEG.

EEG-based Cerebral Monitors

•All current electrophysiological measures heuristically derived
•All current depth-of-anaesthesia monitors use QEEG, MLAEP, or combination
§QEEG - median frequency, spectral edge frequency (e.g. SEF95), bispectrum, BSR, relative and absolute band powers
§Middle latency auditory evoked potential (MLAEP)
•EEG waveforms are analysed to produce a dimensionless number between 0 and 100. 

EEG Processing Methods

• BIS,
• Narcotrend,
• Entropy

Limitation of Current Depth of Anaesthesia Monitors

•Statistical process used to calculate indices which are of no physiological significance
•Subcortical mechanisms not considered in most algorithms.
•Algorithms are based on effects of commonly used anaesthetic drugs on the EEG and are blind to a range of common adjuvant anaesthetic agents - nitrous oxide, ketamine, opioids
•Shifts in levels of consciousness are not instantly reflected (processing time)

BAR Monitor - Development of physiological approaches

•Improved monitoring approaches will result if we better understand the links between the targets of drug action and their clinical effects.
•Neural Field models  seek to link micro and macro scales through meso-scale modeling.
•NFT models populations of interacting excitatory and inhibitory neurons.
•Typically scaling to a cortical macrocolumn

BAR Monitor - EEG Analysis from Theory



BAR Monitor - Cortical State and Cortical Input

Cortical State (CS): scalar representation of state of cortical filter estimated using fixed order ARMA modeling

Cortical Input (CI): RMS amplitude of estimated noise driving the filter


BAR Monitor - Typical Intravenous Induction


Monitoring limitations addressed by the BAR Monitor:

•A physiologically inspired method of EEG analysis allows more accurate monitoring during anaesthesia
•BAR indices provide measures of cortical as well as sub-cortical mechanisms
•Wider range of anaesthetic agents can be monitored
•Shifts in levels of consciousness reflected with only a 2-second delay
Current State of Intra-operative Cerebral Monitors.pdf3.61 MB