BIS

Question 1

Last sense to go during anesthesia is..

Therefore, which EP type is a good candidate to measure patient consciousness?

Answer 1

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

Question 2

Basic description of BIS monitor

Answer 2

Forehead sensors gather EEG and EMG data which is analyzed and an algorithm spits out a dimensionless number that correlates with pt consciousness

Question 3

Uses for BIS monitors

Answer 3

• Patient awareness monitor
• Used to titrate anesthetics in order to prevent anesthetic overdose and facilitate better emergence and post-op experience

Question 4

EEG wave types + associated states (linked to each wave’s dominant presence)

Answer 4

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)

Question 5

First order analysis of EEG - what are we looking at and trying to identify ?

Answer 5

• 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)

Question 6

Suppression ratio (SR) definition..

Burst suppression indicates..

Answer 6

(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

Question 7

BIS monitor values and anesthetic states

Answer 7

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

Question 8

Second order analysis of EEG: what are we looking at and trying to identify?

Answer 8

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)

Question 9

Spectral edge frequency (SEF)

Answer 9

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)

Question 10

Third order analysis of EEG: what are we looking at and what is its signifiance?

Answer 10

AKA Bispectral analysis

• Measures correlation between phase differences
• Physiological significance unclear! Might be helpful for noise reduction

Question 11

BIS algorithm incorporates:

Answer 11

SR (suppression ratio), power spectrum analysis (CSA/DSA), bispectral analysis of frontal EEG and EMG

Note: EMG is a relatively highly weighted component

Question 12

BIS limitations

Answer 12

• 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)