Bis and depth of anaesthesia monitoring

Clinical approach to depth of anaesthesia (DoA) monitoring

Define the problem: DoA has multiple components (hypnosis/unconsciousness, analgesia/nociception, immobility, autonomic responses) and no single monitor measures all.
- Processed EEG (e.g. BIS) primarily tracks cortical hypnotic state, it does not directly measure analgesia or neuromuscular blockade.
Start with basics: ensure adequate oxygenation/ventilation, haemodynamics, temperature, glucose, and exclude equipment/drug delivery problems before interpreting a DoA index.
Use DoA monitoring selectively: highest value when risk of awareness is increased or when titrating anaesthetic to avoid excessive depth.
- Examples: TIVA (especially with NMB), haemodynamic instability, elderly/frail, high-risk obstetric/trauma, prior awareness, difficult airway with prolonged induction, when using low volatile targets.
Interpret in context: correlate BIS/processed EEG with end-tidal agent (MAC), infusion rates, clinical signs, and EMG/signal quality indicators.

What BIS is (principles and outputs)

BIS (Bispectral Index) is a proprietary processed EEG parameter derived from frontal EEG features (time, frequency, and phase relationships) to estimate hypnotic depth.
Scale: 0–100 (approx). Target for general anaesthesia typically 40–60, awake usually &gt,80, isoelectric EEG ~0.
- Values are probabilistic and drug/context dependent, do not treat as a direct measure of awareness.
Common displayed parameters (device dependent): BIS index, Signal Quality Index (SQI), EMG indicator, suppression ratio (SR), sometimes spectral edge frequency (SEF).
- Suppression ratio: proportion of time EEG is suppressed/isoelectric over a recent epoch, high SR suggests very deep anaesthesia, hypothermia, severe cerebral dysfunction, or drug effect.
Electrodes: typically 3–4 electrode frontal montage (forehead/temple). Correct placement and skin prep are essential to reduce artefact.

EEG changes with anaesthetic depth (exam-relevant patterns)

Light sedation: increased beta activity, with increasing hypnotic depth, alpha and delta power increase and EEG becomes more synchronous.
Surgical anaesthesia (typical): prominent frontal alpha oscillations with slow-delta activity (agent dependent, classic with propofol and volatiles).
Very deep: burst suppression, then isoelectric EEG.
Ketamine/nitrous oxide can increase high-frequency activity and may raise BIS despite adequate hypnosis, BIS may not track dissociative anaesthesia reliably.

Indications and when BIS is most useful

TIVA with neuromuscular blockade: helps reduce risk of awareness when end-tidal agent is unavailable.
High-risk awareness scenarios: previous awareness, emergency surgery (obstetric/trauma), haemodynamic compromise limiting anaesthetic dose, difficult airway with prolonged induction, use of low volatile concentrations.
Avoiding excessive depth: elderly/frail, neuroprotection strategies, haemodynamic instability, can reduce anaesthetic dose and hypotension in some settings.
ICU sedation: adjunct to clinical sedation scales, interpret cautiously due to encephalopathy, artefact, and drug combinations.

Evidence and guideline-aligned points (exam framing)

Processed EEG monitoring can reduce awareness compared with clinical signs alone in some contexts, when compared with end-tidal agent concentration protocols, benefit may be less clear for volatile-based anaesthesia.
For TIVA (especially with paralysis), many departments and national guidance recommend considering processed EEG to reduce awareness risk.
Very low BIS with high suppression ratio has been associated in observational studies with hypotension and worse outcomes, causality is uncertain but supports avoiding unnecessarily deep anaesthesia.

Limitations and artefacts (high-yield)

BIS is vulnerable to artefact: EMG (facial muscle activity), poor electrode contact, diathermy, warming devices, pacemakers (rare), movement, shivering.
- High EMG can falsely elevate BIS (appearing lighter). Neuromuscular blockade can reduce EMG and may lower BIS without a true increase in hypnosis.
Drug-specific issues: ketamine and nitrous oxide may increase BIS, dexmedetomidine produces sleep-like EEG and may lower BIS at lighter clinical sedation, opioids reduce arousal responses but have limited direct effect on BIS.
Neurological confounders: dementia, stroke, head injury, encephalopathy, seizures, hypothermia can alter EEG and BIS interpretation.
BIS does not measure analgesia: a patient may have adequate hypnosis (BIS 40–60) but inadequate analgesia leading to sympathetic responses, conversely deep opioids may blunt signs despite light hypnosis.
Awareness can occur with BIS in target range, BIS reduces risk but does not eliminate it.

Practical use in theatre (how to apply and troubleshoot)

Before induction: apply electrodes to clean, dry skin, check impedance/SQI, document baseline awake value.
During maintenance: trend values rather than reacting to single numbers, interpret alongside haemodynamics, end-tidal agent/MAC or TIVA infusion rates, and EMG/SQI.
If BIS unexpectedly high: check SQI/electrodes, look for EMG (light anaesthesia, pain, shivering), consider ketamine/N2O, check drug delivery (IV access, pump, line occlusion), consider seizure activity.
If BIS unexpectedly low: check for hypothermia, hypotension/low cerebral perfusion, excessive anaesthetic, burst suppression (SR), encephalopathy, review drug doses and consider reducing hypnotic if clinically appropriate.
With neuromuscular blockade: be cautious—loss of EMG can lower BIS, ensure hypnotic delivery is robust (e.g. TIVA line security, anti-siphon/anti-reflux valves where appropriate, pump settings, dedicated cannula).

Other depth of anaesthesia monitors (brief comparison)

Other processed EEG indices: Entropy (State/Response), Narcotrend, Patient State Index (PSI), SedLine, all use different algorithms and are not interchangeable.
Raw EEG and spectrogram: increasingly emphasised, may outperform a single index in recognising burst suppression, seizure, and drug-specific patterns.
Nociception/analgesia monitors (not DoA): e.g. SPI, ANI, pupillometry—measure autonomic responses rather than hypnosis.

Awareness under anaesthesia (AAGA) link to DoA monitoring

Risk factors: TIVA, neuromuscular blockade, emergency surgery, obstetrics, haemodynamic instability, difficult airway/prolonged induction, previous awareness, substance misuse, high opioid tolerance.
Prevention principles: robust delivery of hypnotic, avoid long gaps during induction/transfer, use end-tidal agent monitoring for volatiles, consider processed EEG for TIVA/paralysis, maintain adequate anaesthetic concentration, check equipment and IV access.
If suspected awareness intra-op: deepen anaesthesia promptly, treat pain, consider benzodiazepine (not guaranteed to prevent explicit recall), maintain haemodynamic stability, document events, and arrange postoperative follow-up and support.

Test yourself…

Explain the principles of BIS monitoring and what physiological signal it measures.

Aim: describe input signal, processing concept, and what BIS represents clinically.

Input: frontal EEG via surface electrodes, BIS is a processed EEG index derived from multiple EEG features (time domain, frequency domain, and phase relationships/bispectral analysis).
Output: dimensionless number 0–100 estimating probability of consciousness/hypnotic state, lower numbers generally indicate deeper hypnosis.
BIS does not directly measure analgesia, immobility, or autonomic responses, it is mainly a cortical hypnosis monitor.

What BIS values correspond to awake, sedation, general anaesthesia, and very deep anaesthesia?

Awake: typically &gt,80 (often 90–100).
Sedation: roughly 60–80 (wide overlap, depends on drug and patient).
General anaesthesia target: ~40–60 (commonly used range to reduce likelihood of awareness).
Very deep: &lt,40, burst suppression may appear as BIS falls further with rising suppression ratio, near-isoelectric EEG approaches 0.

In which clinical situations would you choose to use BIS (or another processed EEG monitor), and why?

TIVA, especially with neuromuscular blockade: no end-tidal agent concentration available, processed EEG helps titrate hypnotic and reduce awareness risk.
High-risk AAGA cases: previous awareness, emergency/obstetric/trauma, haemodynamic compromise, difficult airway with prolonged induction, planned low volatile technique.
To avoid excessive depth: elderly/frail, significant cardiovascular disease, where hypotension from overdosage is a concern, also to detect burst suppression.

A common FRCA theme: compare end-tidal agent monitoring with BIS for preventing awareness during volatile anaesthesia.

End-tidal agent monitoring provides a direct measure of delivered volatile concentration (surrogate for brain partial pressure) and is strongly linked to MAC-based unconsciousness probabilities.
BIS provides an EEG-based surrogate of hypnotic effect, it can be affected by artefact and drug-specific EEG patterns.
For volatile-based anaesthesia, maintaining adequate end-tidal MAC (age-adjusted) is highly effective, BIS may add value in selected cases but is not a substitute for ensuring adequate agent delivery.

Your BIS suddenly rises from 45 to 75 during surgery. Give a structured approach to diagnosis and management.

Think: artefact → inadequate hypnosis → unusual causes, act while investigating if awareness risk.

Immediate actions: check patient and anaesthetic delivery, consider increasing hypnotic temporarily if concern about awareness, while troubleshooting.
Check signal quality: SQI/impedance, electrode position, dried gel/sweat, cable disconnection, diathermy interference.
Check EMG: shivering, light anaesthesia, pain, inadequate opioid/analgesia, note that EMG can falsely elevate BIS.
Check drug delivery: TIVA pump running, correct rate, no occlusion/kink, IV cannula patency/extravasation, dead space/3-way tap position, for volatiles check vaporiser setting, fresh gas flow, circuit leak, end-tidal agent.
Consider drug effects: ketamine/N2O may raise BIS, consider seizure activity if clinically plausible.

Your BIS is 25 with a high suppression ratio. The patient is hypotensive. How do you interpret this and what do you do?

Interpretation: very deep hypnotic state with burst suppression, may reflect excessive anaesthetic dose, hypothermia, low cerebral perfusion, or underlying cerebral pathology.
Actions: treat hypotension (vasopressor/fluids as appropriate), review anaesthetic dose and reduce hypnotic if safe, check temperature, ensure adequate oxygenation/ventilation and haemoglobin, reassess surgical stimulus/analgesia balance.
Confirm monitor validity: check SQI/electrodes, review raw EEG if available to confirm suppression rather than artefact.

How do neuromuscular blocking drugs affect BIS interpretation?

NMB reduces facial muscle activity (EMG), which can lower the BIS value independent of true hypnotic depth (less EMG contamination).
Clinical implication: a low BIS in a paralysed patient may overestimate depth, conversely, paralysis removes movement as a sign of light anaesthesia, increasing reliance on robust hypnotic delivery and monitoring.

Discuss how specific drugs can produce misleading BIS values (give examples).

Ketamine: increases high-frequency EEG activity, BIS may be higher than expected despite adequate dissociative anaesthesia.
Nitrous oxide: can increase BIS/alter EEG, BIS may underestimate hypnotic contribution of N2O-containing techniques.
Dexmedetomidine: produces non-REM sleep-like patterns, BIS may be relatively low at lighter clinical sedation compared with other agents.
Opioids: reduce arousal and haemodynamic responses, BIS may not change much, so a “good BIS” does not guarantee adequate analgesia and a “high BIS” may not mean the patient will respond if heavily opioidised.

Outline a plan to minimise awareness during TIVA with paralysis (viva-style).

Preparation: dedicated IV access for TIVA if possible, secure cannula, anti-reflux/anti-siphon where used locally, ensure pump drug concentrations and rates are correct, check battery/power and alarms.
Monitoring: processed EEG (BIS/Entropy/PSI) with attention to SQI/EMG, standard monitoring, consider arterial line if unstable, document targets.
Technique: use a validated TCI model where appropriate, avoid long interruptions (line disconnections, syringe changes) by planning, ensure adequate analgesia, avoid unnecessary paralysis or use minimal effective dose with neuromuscular monitoring.
Intra-op vigilance: respond to unexpected BIS rise by checking delivery and increasing hypnotic, maintain communication and documentation, consider volatile “backup” if appropriate and feasible.

A previous FRCA-style prompt: ‘Discuss the advantages and disadvantages of depth of anaesthesia monitoring.’

Advantages: may reduce awareness risk in selected patients (notably TIVA/paralysis), helps titrate hypnotic dose, may reduce excessive depth, hypotension, and drug consumption, provides trend information when clinical signs are unreliable.
Disadvantages: cost and disposables, artefact susceptibility, drug-specific limitations, does not measure analgesia, false reassurance possible, requires training and correct interpretation (including SQI/EMG/SR and ideally raw EEG).

How would you explain BIS monitoring to a patient who is anxious about awareness?

Explain that BIS uses forehead sensors to monitor brain electrical activity patterns and helps guide anaesthetic dosing, especially when other measures are limited (e.g. TIVA).
Clarify limitations: it reduces risk but cannot guarantee awareness will not occur, you will also use multiple other checks (drug delivery, standard monitoring, end-tidal agent if used).
Reassure with process: risk assessment, careful technique, and postoperative follow-up if any concerns.