BIS Flashcards
Levels of unconsciousness
Perception of explicit memory
Perception and no explicit memory
No perception and implicit memory
No perception and no memory
Components of anesthesia
Unconsciousness/hypnosis
Analgesia
Muscle relaxant
Explicit memory
Refers to intentional or conscious recollection of prior experiences
Implicit memory
Information not associated with any conscious recollection
Recall may occur during dreaming, hypnosis, or other psychological methods
Percent of legal claims against anesthesia providers
Two
Worst thing they have ever experienced
Awareness under anesthesia
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
Individual patient response variance to anesthetic
Unique tolerance (some predictable, others not) Fluctuations in hemodynamics stability
Circumstantial variance in anesthetic requirement
Surgical stimulus
User error
Delivery device failure
Measuring depth - clinical signs
Heart rate Blood pressure Sweating Lacrimation Pupil diameter
Measuring depth - isolated forearm technique
Tourniquet applied to one arm prior to MR
Spontaneous movement or movement to command indicates light anesthesia
Measuring depth - skin impedance
Quantitative measure of sweat production
Factors that affect sweating (atropine, autonomic neuropathy) reduce accuracy
Measuring depth - surface electromyelogram
Only useful in patients that are not receiving full MR
Most widely applied technology for measuring anesthetic depth
EEG
Last sense to be suppressed by anesthesia
Auditory
Used alone or in combination with EEG
Auditory evoked potentials
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
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
