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Over the past 70 years there have been attempts to use the EEG as a monitor
of anesthetic depth. With increasing depth of anesthesia the EEG does show
a common progression from low amplitude and high frequency through high
amplitude and low frequency to isoelectricity. However the raw EEG is too
complex for the untrained and interpretation is too non-specific and
inconsistent. Similarly attempts at using derivatives of the EEG such as
power analysis, median frequency and spectral edge frequency have not
proved to be clinically useful. More recently a number of potentially
better EEG derivatives have been developed including:
- Bispectral index (BIS), Aspect
Medical
- Patient State Index (PSI-
Sedline), Hospira
- State and Response Entropy,
GE Healthcare
- SNAP Index, Everest
- Cerebral State Monitor,
Dannemeter
Another monitoring technique is based on Auditory Evoked Potentials.
While the EEG is derived from the electrical activity of brain cells
scattered throughout the cortex and sub-cortical areas and therefore is not
anatomically specific, the auditory evoked potentials are a response to a
specific auditory stimulus and the anatomical pathways involved are better
defined.
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