EEG Flashcards
Advantages of EEG Monitoring
- Monitoring of depth when unable to access head
- Traditional sites/methods of monitoring eg eye position, m tone unreliable in face of NMBA
- Reflection of hypnotic properties only
Disadvantages of EEG
- Subject to large drug variations
- Conflicting information: arousal pattern or deeper level of ax depending on magnitude of stimulus, level of anesthesia
- No reflection of analgesic properties
- Not more accurate than physical monitoring of depth
General EEG Patterns
o Low‐amplitude, high‐frequency wave pattern during awake state
high‐amplitude
Low‐frequency pattern during ax to burst suppression (intermittent periods of electrical silence)
o Persistent electrical silence with deep levels of anesthesia
EEG Limitations
- Requires vol of recording, specialized training/expertise for interpretation
o EEG voltage (power) changes as function of time (time domain)
o Generation of indices: total EEG power (TOTPOW), median power frequency, burst suppression
o Interpretational algorithms (Fourier transformation): signal activity as function of frequency (frequency domain)
SEF95
spectral edge frequency 95: frequency below which 95% of total EEG power resides
MF
= Median frequency, median EEG power frequency
delta
0.5-3.5Hz
theta
3.5-7.0Hz
alpha
7-13Hz
beta
13-30hz
EEG Frequencies?
Dinner Tonight is Alfredo with Broccoli
delta 0.5-3.5
Theta 3.5-7
Alpha 7-13
Beta 13-30
Bispectral Index
EEG that quantifies degree of ax-induced cortical depression, represents weighted value derived from four subparameters
Burst suppression ratio (time domain)
QUAZI value (time domain)
Beta2 power ratio in 30–47 Hz range vs 11–20 Hz range (frequency domain)
Bispectral biocoherence ratio of peaks in 0.5–47 Hz range vs 40–47 Hz range (frequency domain)
Advantages of BIS
No calibration required, bar graph – signal quality, amt of muscle artifact
Used to help assure that patients are well anesthetized, pain-free, unaware
Disadvantages of BIS
Excessive m movement can interfere with BIS computation
BIS index highly variable in dogs, did not track ETiso well
MAC(BIS)
MAC at which nociceptive stimulus causes increase in BIS to 60 (cats)
BIS: Effect of different anesthetics
Strong depression: propofol, midaz, TP
Intermediate effect: inhalants
Little effect: opioids
Increase BIS: ketamine, N2O
BIS Measurements
- Quantitative, 0-100
BIS >90
compatible with awake, alert
BIS 80-90
anxiolysis
BIS 60-80
with moderate obtundation
* 60-70: compatible with completion of sx in dogs
BIS <60
Loss of recall
BIS <50
Unresponsive to verbal stimuli
BIS <40
loss of muscular movement in response to a noxious stimulus
BIS <20
burst suppression (deep anesthesia)
BIS 0
isoelectric activity
Cerebral state index (CSI) using a cerebral state monitor (CSM)
o 3 clip electrodes: forehead (+), occipital (-), parietal (reference)
o Index calculated via algorithm of two energy bands (beta, and alpha), B:A ratio, burst suppression (BS)
BS: quotient that considers time to zero EEG activity
EEG, CSI, BS, EMG, signal quality index (SQI)
CSI >90
Awake
CSI 80-90
sedation
CSI <80
Unconscious
Anything lower not correlated well with physical signs of depth
Narcotrend
Auditory evoked EEG responses used primarily to assess neurologic function, CNS disease
Also used to assess depth and awareness/recall
Analyzes raw EEG data, categorizes level of sedation:
Narcotrend: A0
Awake
Narcotrend: A1/A2
Subviligant
Narcotrend: B0/B1/B2
sedation
Narcotrend: C0/C1/C2
Ax
Narcotrend: D0/D1/D2
Moderate Ax
Narcotrend: E
Deep anesthesia, burst suppression
Narcotrend: F
coma/electrical silence
Electroencephalography
○ Recording of spontaneous electrical brain activity from scalp electrodes
■ Surface signals represent summation of activity from millions of neurons in the cerebral cortex
■ Result from excitatory and inhibitory Postsynaptic Potentials (PSPs) in large pyramidal neurons located in the lower layers of the cerebral cortex
○ Amplitude and frequency are modulated by AFFERENT inputs from sensory-specific thalamic nuclei
Gamma bands
Problem solving, concentration
EnGrossment
Beta bands
Busy, active mind
Alpha bands
Reflective, restful
alPha _ pensive
Theta bands
drowsiness
T for tired
Delta bands
sleep, dreaming
D for dreaming
EEG Activity
activity associated with consciousness is generated from pacemaker neurons within ascending reticular activating system (ARAS)
mediated, modulated through thalamic connections
■ Pacemaker neurons oscillate 8-12 Hz
■ 𝛂 rhythm dominates resting EEG
Awake State
consciousness maintained by circulating activity among ARAS, intralaminar nuclei, cerebral cortex
■ Additional sensory stimulation will cause cortical arousal → desynchronization of 𝛂 oscillators with appearance of a faster rhythm in the 𝛃 frequency range (12-25 Hz)
■ Desynchronization: shift in EEG pattern from LOW-voltage, FAST-wave to a HIGH-voltage, SLOW-wave pattern
As anesthetic depth increases…
predominant EEG pattern characterized by DECREASE in 𝛃, INCREASE in 𝛂/𝜭 (anteriorization with loss of consciousness LOC)
Progression of Anesthetic Depth
, 𝛅 and 𝜭 waves appear in Centroposterior regions with subsequent anterior spread
Very Deep Anesthesia
indicated by flat areas with interspersed 𝛂 and 𝛃 activity (Burst Suppression Pattern) followed by a complete loss of electrical discharged (isoelectricity or electrical silence)
EEG synchronization or desynchronization during anesthesia associated with clinical signs
mydriasis, hypertension, and/or tachycardia
● Poor correlation between EEG and hemodynamic responses to noxious stimulation reported
Power-Spectrum Analysis
computer-processed EEG analysis has been employed for 30+ years
■ Minute-to-minute assessment of anesthetic depth
■ Produce Quantitative EEG (QEEG)
QEEG
● Median frequency (MF)
● Spectral edge frequency (SEF)
● Percentage distribution of total power into the frequency bands
● Power band ratios: 𝜭/𝛅 ; 𝛂/𝛅; 𝛃/𝛅
CSA: Compressed Spectral Array
allow readability of EEG
Hills and valleys in the CSA represent frequency bands with higher and lower power, respectively
CSA: Increasing ax depth
○ increase in total power, increase in relative power of 𝛅 and 𝜭; decrease in 𝛂 and 𝛃, decrease in MF/SEF
CSA: decreasing ax depth
Shift in power from low (𝛅 and 𝜭) to high (𝛂 and 𝛃),
decrease MF, SEF
BSR
Burst Suppression Ratio
Burst Suppression Ratio
percentage of isoelectric periods occurring over a certain period of time
■ Burst suppression indicates non-specific reduction in cerebral metabolic activity
● Ex. Trauma, drugs, hypothermia
BSR Measurements
1 indicates no activity; 0 indicates active brain
● Ratio increases as the brain becomes increasingly inactive
EEG isoelectricity: ETiso vs EThalo
○ Studies have shown that EEG isoelectricity might be achieved with lower end-tidal Isoflurane concentrations compared to halothane
■ Dogs, horses, rats
■ Anesthetic depth as determined by clinical signs and MAC multiples → Isoflurane produced lower SEF, MF and/or 𝛃/𝛅 ratio values but higher amplitude measures than Halothane
Conclusion: brain reacts more sensitively than spinal cord to Isoflurane
Primary Goal of Intraop EEG
■ Maintenance of a HIGH-voltage, SLOW-wave EEG
■ Avoid sudden changes to pattern in response to noxious stimuli
Types of Evoked Potentials
Mid-latency Auditory Evoked Potentials (MLAEPs)
Somatosensory-evoked potentials (SSEPs)
SSEPs
electrical signals generated by the nervous system following mechanical or electrical stimulation in the periphery and subsequent sequential activation of neuronal structures along the somatosensory pathway
■ Appear as negative waves (Ni- initial; Ns- second) superimposed on was surface positivity (Pi- initial; Ps- second)
SSEPs for Most Anesthetics
● increase in anesthetic depth indicated by an increase in latency and decrease in the amplitude of Pi and Ni waves
● Decreased depth/Arousal: Decreased latency and increase in amplitude
■ Effects of anesthetics on SSEPs supports that information transfer through the thalamus is interrupted
Which two anesthetic agents do not follow the normal rule for SSEPs?
Propofol, etomidate
Usefulness of Evoked Potentials
○ Neither MLAEPs or EEG-derived variables can be used to predict movement response to noxious stimuli → Limited utility for Intraoperative monitoring
BIS
Index derived from EEG signal processing
Dimensionless number intended to indicate the patient’s level of consciousness
■ Range 0-100
● 100: awake
● 0: isoelectric EEG
● 55: recommended as the upper limit that might assure adequate depth of surgical anesthesia
BIS Uses
○ Dose-dependent DECREASE in BIS values with increasing ET concentrations/doses (iso, sevo and propofol)
○ No benefit found with BIS monitoring compared to end-tidal anesthetic agent concentration (ETAC) with respect to avoiding awareness
■ Changing volatile anesthetic concentration based solely on BIS NOT RECOMMENDED
■ Reported to be significantly MORE accurate than targeted or measured propofol concentrations during sedation/hypnosis
Nacrotrend Index
○ Algorithm based on pattern recognition of the raw EEG
■ Classifies EEG traces into stages from A (awake) to F (burst suppression/isoelectricity)
■ Dimensionless index (NI): 100 (awake) - 0 (isoelectricity)
○ NI and BIS considered equally effective in monitoring anesthetic depth in human patients
Otto et al 2012 (NI)
Compared NI with clinical signs of anesthesia during experimental cardiac surgery in isoflurane-anesthetized sheep - significant correlation between NI and increasing anesthetic depth
Entropy
○ EEG parameter based on frequency spectrum analysis (MF, SEF)
■ Reflect linear signal properties
■ Analyzes the irregularity, complexity, and unpredictability of EEG signals
Types of Entropy
- State Entropy (SE)
- Response Entropy
■ Dimensionless numbers
■ Overlap of EEG and EMG frequencies (30-50 Hz)
■ At deep levels of anesthesia (EMG power = 0), SE and RE are equal
State Entropy
computed over EEG-dominate spectrum (0.8-32 Hz)
■ Reflects the Cortical State of the patient
Response Entropy
Includes EEG and EMG
■ Indirect measure of adequacy of analgesia
■ EMG activity increases as result of nociceptive stimulation and during decreasing levels of anesthesia
Entropy Uses
○ Recommended range: 40-60
■ SE > 60 → Increase depth of anesthesia
■ RE > 60 → Additional analgesics should be considered
○ Poor performance during ketamine and NO administration
Index of Consciousness (IoC)
Derived from a combination of symbolic dynamics, 𝛃-ratio and EEG suppression rate
IoC Ranges
■ 0 = isoelectric
■ 99 = awake
■ 80 = associated with sedation
■ 40-60 = recommended for general anesthesia
Cerebral State Index/Monitor (CSM)
○ EEG based monitor used to measure the depth of hypnosis during GA
Are there any currently used EEG-based monitors that can unfailingly detect wakefulness?
NO
Electromyography
Measurement of the electrical activity within the muscle
■ Needle inserted in muscle
■ Analysis of waveforms and firing rates of single or multiple motor units can give diagnostic information
