BIS Flashcards by Helen Wang

Levels of unconsciousness

Perception of explicit memory
Perception and no explicit memory
No perception and implicit memory
No perception and no memory

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Components of anesthesia

Unconsciousness/hypnosis
Analgesia
Muscle relaxant

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Explicit memory

Refers to intentional or conscious recollection of prior experiences

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Implicit memory

Information not associated with any conscious recollection

Recall may occur during dreaming, hypnosis, or other psychological methods

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Percent of legal claims against anesthesia providers

Two

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Worst thing they have ever experienced

Awareness under anesthesia

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How big of a problem is awareness?

More common than necessary
One of every thousand patients
Three of every thousand cardiac patients
Up to forty eight with severe trauma

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Individual patient response variance to anesthetic

Unique tolerance (some predictable, others not)
Fluctuations in hemodynamics stability

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Circumstantial variance in anesthetic requirement

Surgical stimulus
User error
Delivery device failure

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Measuring depth - clinical signs

Heart rate
Blood pressure
Sweating
Lacrimation
Pupil diameter

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Measuring depth - isolated forearm technique

Tourniquet applied to one arm prior to MR

Spontaneous movement or movement to command indicates light anesthesia

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Measuring depth - skin impedance

Quantitative measure of sweat production

Factors that affect sweating (atropine, autonomic neuropathy) reduce accuracy

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Measuring depth - surface electromyelogram

Only useful in patients that are not receiving full MR

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Most widely applied technology for measuring anesthetic depth

EEG

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Last sense to be suppressed by anesthesia

Auditory

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Used alone or in combination with EEG

Auditory evoked potentials

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BIS index range

0 flatline
0-20 burst suppression
20-40 deep hypnotic state
40-60 GA
60-80 moderate sedation
80-100 responds to normal voice
100 awake

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Moderate sedation

Responds to loud commands or prodding, shaking

General anesthesia

Low probability of explicit recall

Unresponsive to verbal stimulus

BIS is used to

Prevent patient awareness
Titrate anesthetic to prevent OD
Facilitate faster wake ups, shorter PACU stays
Cost savings

How does BIS work

Analyzes analog EEG signal and incorporates facial EMG

Plugged into proprietary algorithm

EEG is

Recording of electrical activity along scalp

Arises from ion currents across cell membrane of neurons with similar spatial orientation

Contribute most to EEG signal

Pyramidal neurons of the cortex

Beta
Alpha
Theta
Delta

Awake
Eyes closed
Light sleep
Deep sleep

First order EEG analysis

Looks at amplitude mean and variance | Identifies periods of EEG suppression

EEG depression is defined as

At least a .5 second interval during which the EEG voltage does not exceed plus or minus 5 mV

Suppression ratio

Ratio of time over the previous 63 seconds that the EEG waveform exhibited suppression

Second order EEG analysis

Fourier analysis of the EEG waveform parses out the individual frequency components, creating a power spectrum Displayed as a compressed spectral array (CSA) or density spectral array (DSA)

Spectral edge frequency (SEF)

Frequency below which certain percentage of the power signal is located Has been studied as an independent predictor of anesthetic depth Poor positive predictive values for awareness

The cutoff range for SEF

14-15 Hz

SEF can be higher than 15 Hz during

Deep anesthesia or burst suppression

EEG third order (bispectral) analysis

Measures correlation of phase between different frequency components Physiological significance unclear May be helpful for noise reduction

Why are normal monitors important when using EEG and BIS?

Monitors are not intuitive Information given is only as good as the info they get Programmed to identify and disregard common artifact, but are not perfect

What do we know about the BIS algorithm?

Incorporates suppression ratio, power spectrum analysis, bispectral analysis of frontal EEG as well as facial muscle EMG EMG is high weighted component

The relationship between BIS and SR

Linear SR 50 = BIS 20 SR 100 = BIS 0

BIS waveforms - pre MR and post MR

Pre - high frequency background fuzziness Post - cleaner, underlying spindles and delta

Sources of noise

``` Electrocautery Forced air warmer Cardiac pacemaker Hair shaver Endless others ```

Limitations to BIS

Signal processing lag time - can range from 14-155 seconds Failure to account for certain drugs with known anesthetic effects - ketamine, N2O, xenon, dexmedetomidine?

T or F. Sevoflurane and isoflurane exhibit more BIS reduction than halothane

True

Paradoxical changes

Increase in BIS with deepening anesthetic Decrease in BIS just prior to awakening

Pathophysiologic conditions leading to reduction in BIS

Hypoglycemia Decrease in CBF Focal or global brain pathology Hypothermia

Beta adrenergic agents epinephrine, ephedrine, and isoproterenol have all been shown to

Increase BIS Not seen with phenylephrine (pure alpha adrenergic agonist)

B-Aware trial

Lower awareness rate in BIS group vs routine Episodes of awareness occurred when 55-59 and 79-82 Decreases awareness in 82% in high risk adults having GA