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BIS and Processed EEG

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Monitoring II · 9 min read

BIS isn't magic; it's a processed EEG number that helps in TIVA + high-risk patients and is unreliable with ketamine + N₂O.

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Interpret BIS values and recognize 'too deep' vs 'awareness risk.'
List the drugs that produce paradoxical BIS readings.
Use BIS appropriately during TIVA and in the frail/elderly.
Differentiate burst-suppression patterns.

0-100 scale interpretation

KeyBIS 100 = awake.

80 = sedated, responsive.

60-80 = light anesthesia, possible recall.

40-60 = general anesthesia target.

20-40 = deep anesthesia.

0-20 = burst suppression or isoelectric.

Target 40-60 for most GA.

Below 40 prolongs emergence + may correlate with postop delirium (especially elderly).

Above 60 risks awareness.

Trend matters more than absolute number.

Processed EEG (BIS) limitations - mechanism/sequence: derived from raw EEG via frequency analysis, suppression ratio, and burst suppression algorithm into 0-100 scale. Targets 40-60 for general anesthesia; sequence of EEG processing produces a unitless number that lags real-time state.

Processed EEG inputs

KeyProprietary algorithm (BIS) integrates spectral analysis (frequency content), bispectral analysis (phase coupling), burst suppression ratio.

SedLine (Masimo) uses different proprietary algorithm with similar output.

Display also shows raw EEG + electromyography signal + signal quality index.

EMG values >30 suggest patient activity contaminating signal — interpret BIS cautiously when EMG high.

Processed EEG (BIS) comparison/flowchart: alternatives include entropy, SedLine, raw EEG; algorithm proprietary. Flowchart guides interpretation when BIS doesn't match clinical state - check signal quality, EMG, agent type before titrating anesthesia.

Drug-specific unreliability

KeyKetamine activates EEG BIS rises despite dissociative anesthesia.

Patient is profoundly anesthetized but BIS reads light.

N₂O has minimal EEG effect BIS doesn't reflect N₂O contribution.

Dexmedetomidine produces sleep-like EEG — BIS may be informative but pattern differs from volatile.

Etomidate causes myoclonic activity early spurious BIS swings.

Use clinical judgment alongside the number.

Depth of anesthesia monitoring: SedLine PSI and BIS index process EEG to display 0-100 scale; general anesthesia target 40-60; complements clinical signs to reduce awareness and over-sedation.

EMG and cautery artifact

KeyEMG from forehead muscle activity contaminates BIS — common cause of spuriously high BIS in paralyzed patients.

NMB administration drops BIS apparently by suppressing EMG, not by deepening anesthesia.

Cautery causes wild BIS swings — ignore during active cautery.

Hairy forehead, sweat, electrode displacement all degrade signal.

Verify electrode contact, dry skin, signal quality bars green.

Processed EEG (BIS) clinical pearls/pitfalls: don't rely on BIS alone for awareness prevention - use minimum MAC and benzodiazepines in high-risk cases; check signal quality, EMG artifact, electrode placement; correlate with clinical signs and end-tidal anesthetic concentration.

BAG-RECALL trial

KeyB-Unaware (Avidan 2008): BIS-guided vs end-tidal-guided anesthesia in high-risk patients showed equivalent awareness rates.

BAG-RECALL (2011): confirmed in larger trial.

Conclusion: end-tidal volatile concentration ≥0.7 MAC equally protective.

End-tidal monitoring is universally available, cheaper, simpler.

BIS adds incremental value in TIVA (no end-tidal to monitor) + when MAC must be <0.7 by necessity (e.g., severe cardiac).

Awareness during anesthesia: risk factors include light anesthesia, paralysis, TIVA misuse, cardiac/OB/trauma cases; prevention with adequate dosing, depth monitoring, MAC awareness.

Burst suppression and elderly delirium

KeySustained BURST SUPPRESSION (BIS <20) in elderly correlates with postoperative delirium incidence + duration.

Recent recommendation: avoid >5 min cumulative burst suppression in age >65.

Use the LIGHTEST EFFECTIVE anesthetic depth.

ENGAGES trial (2019) didn't show mortality benefit from avoiding burst suppression but observational data link it to cognitive outcomes.

Modern protocol

titrate to BIS 40-60 in elderly
document any deeper periods
reduce anesthetic dose if BIS <40 sustained

Evoked potentials and raw EEG patterns: SSEP, MEP, BAEP can be affected by volatile anesthetics; minimize agent doses, use TIVA when monitoring evoked potentials.

Density spectral array (DSA) — the new modality

KeyModern processed EEG monitors (SedLine, Masimo, NeuroSENSE) display DENSITY SPECTRAL ARRAY (DSA) — a color spectrogram showing power across EEG frequency bands over time.

Different anesthetic states produce characteristic DSA patterns: AWAKE (high-frequency content, multi-band), SEVOFLURANE/PROPOFOL surgical depth (alpha-spindles 8-12 Hz dominant + slow oscillations), TOO-DEEP (burst suppression — alternating bursts + flat lines visible), KETAMINE (high-gamma activity).

DSA is more pattern-based + intuitive than the single BIS number; rapidly being adopted in cardiac + neurosurgery + elderly care for richer information.

SedLine (PSI) waveform shows multichannel frontal EEG with PSI 25-50 target for general anesthesia; provides raw EEG, density spectral array, and processed index for depth monitoring.

Practical use cases + limitations

USE BIS/processed EEG when

TIVA cases (no end-tidal volatile to monitor depth)
high awareness risk (cardiac, OB GA emergency, trauma)
elderly avoiding burst suppression
NMB-paralyzed patients where motor signs absent

DON'T rely on BIS as only depth measure with: ketamine-based anesthesia (BIS unreliably high), high-dose dexmedetomidine, N2O-heavy techniques, EMG-contaminated traces, neurologic disease affecting EEG.

Combine with end-tidal MAC + clinical signs for best practice.

Document BIS values periodically + rationale for any prolonged deep state.

BIS (bispectral index) scale 0-100 derives from processed frontal EEG: 40-60 target for general anesthesia, 100 = awake, 0 = isoelectric. Reduces awareness risk and helps titrate anesthetic depth in TIVA and inhaled techniques.

⚠ Common pitfalls

Targeting BIS 40-60 in the very elderly — they often need less anesthetic; titrate to clinical effect.
Trusting BIS during ketamine or dexmedetomidine — readings are unreliable.
Ignoring high EMG signal — false elevation suggests inadequate paralysis or surgical stimulation.
Stopping the volatile during burst suppression in a fragile elderly — that's the goal in some cases.

💎 Clinical pearls

BIS <40 + burst suppression in elderly is associated with worse delirium outcomes (BAG-RECALL secondary analysis).
BIS use in TIVA without paralysis is one of the strongest awareness-prevention strategies.
Pediatric BIS thresholds are not well-validated — use clinical signs + age-adjusted MAC.
If BIS reads high despite high MAC, check for paradoxical drugs (ketamine), EMG, or actual awareness.

Recap

BIS <40 + burst suppression in elderly is associated with worse delirium outcomes (BAG-RECALL secondary analysis).
BIS use in TIVA without paralysis is one of the strongest awareness-prevention strategies.
Pediatric BIS thresholds are not well-validated — use clinical signs + age-adjusted MAC.
If BIS reads high despite high MAC, check for paradoxical drugs (ketamine), EMG, or actual awareness.

Mark each section done to complete the module.