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Currently available in the United States:

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  • Bispectral Index (BIS™, Covidien)
  • SEDLine™ Patient State Index (PSI™, Masimo)
  • M-Entropy™ (GE Healthcare)
  • SNAP II™ (Stryker)

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Also available in other countries:

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  • Cerebral State Index (CSI™, Danmeter)
  • Narcotrend™ (Drager)
  • NeuroSENSE™ (Carefusion)

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All monitors work on the principle that EEG changes caused by GABA agonists are, in general:

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  • Predictable
  • Observable
  • Reproducible

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Anesthetics and EEG
Agents that correlate with processed EEG valuesAgents that do not correlate with processed EEG values
  • Halogenated volatile gases
  • Propofol
  • Benzodiazepines
  • Barbiturates
  • Ketamine
  • Nitrous oxide
  • Etomidate
  • Opioids
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Processed EEG monitors, through differing algorithms, generate an index value (0–100) that reflects the state of frontal EEG activity and quasi-linearizes the relationship between dose of GABA agonist and index value.

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EEG changes seen with GABA agonists include changes in:

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  • Predominant frequencies
  • Relative power distribution
  • Phase relationship and randomness
  • Pattern changes, such as burst suppression

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EEG Waveform as a Function of Anesthetic Depth
Wave nameAssociated frequencies (Hz)Usual anesthetic state when prominent
γ (gamma) or β230–80Conscious
β (beta)12–30Conscious or initial excitation with induction
Image not available. (alpha)8–12Conscious with eyes closed and relaxed or general anesthesia (with spindles)
θ (theta)4–8General anesthesia
δ (delta)0–4Deep anesthesia
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Pros and Cons of Using Processed EEG Monitors
AdvantagesDisadvantages
  • On average, less use of volatile agents or propofol
  • On average, faster emergence
  • On average, less time before eligible for PACU discharge
  • Potential to better identify unexpected outliers: those that require either more or less agent than average
  • Possible decrease in recall of unintended consciousness during surgery, particularly in the high-risk patient (but perhaps not better than utilizing end-tidal gas monitoring with alarms enabled)
  • Allow real-time display of one to four channels of raw frontal EEG
  • Cost
  • Ideal use involves watching and interpreting raw EEG wave: most anesthesiologists do not have appropriate basic knowledge
  • No absolute value assures unconsciousness or consciousness
  • Values not real time: some processing time and smoothing functions cause a delay between change in state and change in index
  • In some patients, there appears to be a more quantal relationship between agent level and EEG than a linear one. In these patients with a “plateau,” titrating agent levels down may result in a sudden state change with patient movement
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Various EEG Monitors
MonitorRecommended range for GANotes
BIS™40–60β ratio (related to activity in 30–47 Hz range) is component of upper range of index. Facial muscle EMG activity will increase this value and when EMG activity is present, BIS may be higher than expected. In this condition, BIS will decrease ...

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