Last sense to go during anesthesia is..
Therefore, which EP type is a good candidate to measure patient consciousness?
sense of hearing - it is also the first to come back as anesthesia wears off
auditory evoked potential = good way to measure pt consciousness as those signals are the last to be suppressed by anesthesia
Basic description of BIS monitor
Forehead sensors gather EEG and EMG data which is analyzed and an algorithm spits out a dimensionless number that correlates with pt consciousness
Uses for BIS monitors
- Patient awareness monitor
- Used to titrate anesthetics in order to prevent anesthetic overdose and facilitate better emergence and post-op experience
EEG wave types + associated states (linked to each wave’s dominant presence)
Beta: awake, conscious (15-30 Hz)
Alpha: eyes closed, relaxed; propofol and inhaled agents (8-13 Hz)
Theta: light sleep (Stage 1 and 2) (4-8 Hz)
Delta: deep sleep (Stage 3 and 4) (0.5-4 Hz)
First order analysis of EEG - what are we looking at and trying to identify ?
- Looking at amplitude mean and variance of waves
- Identifying periods of EEG suppression (0.5+ sec intervals during which EEG voltage < |5.0 mV|
(slide 18)
Suppression ratio (SR) definition..
Burst suppression indicates..
(SR)
% of time over the previous 63 seconds that EEG waveform exhibited suppression
- Should be 0 during normal levels of anesthesia
- Burst suppression: EEG pattern which marks the point at which you have maximally decreased O2 in the brain without causing cellular damage
BIS monitor values and anesthetic states
0-20: cerebral suppression/stage 4 anesthesia [in this range, the BIS value is entirely dependent on burst suppression in a LINEAR relationship]
>>SR 50 = BIS 20 >>SR 100 = BIS 0
40-60: optimal anesthetic state
60+: patient sedated but at risk for awareness
Second order analysis of EEG: what are we looking at and trying to identify?
EEG waveforms are combined (Fourier analysis) and displayed as a CSA (compressed spectral array) or DSA (density spectral array)
Can (somewhat unreliably) predict anesthetic depth through SEF monitoring
(slide 21)
Spectral edge frequency (SEF)
Frequency below which a certain percentage of the power of a signal is located
SEF95: 95% of the pt’s brain frequency is below this value
Studied as an independent predictor of anesthetic depth with a cutoff around 14-15 Hz, but a poor positive predictive value for awareness (goes down initially but as pt gets deeper, it can go up)
(slide 23; SQI = signal quality insert)
Third order analysis of EEG: what are we looking at and what is its signifiance?
AKA Bispectral analysis
- Measures correlation between phase differences
- Physiological significance unclear! Might be helpful for noise reduction
BIS algorithm incorporates:
SR (suppression ratio), power spectrum analysis (CSA/DSA), bispectral analysis of frontal EEG and EMG
Note: EMG is a relatively highly weighted component
BIS limitations
- Many sources of artifacts exist (electrocautery, Bairhugger, pacemakers, etc..) ; SQI does not always ID these noises
- Signal processing lag time = 14-155 sec which limits our ability to prevent awareness
- Fails to account for certain drugs with known anesthetic effects (Ketamine, N2O, Precedex, etc..)
- Paradoxical changes (BIS increases with deepening anesthetic, and decreases just prior to awakening)
- Pathologies change BIS readings (hypoglycemia, decrease in CBF, brain problems, hypothermia)
- Beta-adrenergic agents have all been shown to increase BIS
- No good in moving patients (too much background noise)
