Monitoring Depth of Anaesthesia

Why It Matters

1. Intraoperative awareness - the patient becomes conscious during surgery (incidence ~0.1-0.2% under general anaesthesia)
2. Anaesthetic overdose - excessive depth causing haemodynamic instability, prolonged emergence, organ toxicity
3. Titration - optimising drug dosing to the individual patient's needs

1. Clinical (Traditional) Signs of Anaesthetic Depth

Residual Clinical Signs Still Used

• Eye signs: lacrimation, conjugate gaze vs divergent (light plane), dilated pupils (deep or light)
• Cardiovascular: tachycardia and hypertension suggest inadequate depth; bradycardia/hypotension suggest excessive depth
• Somatic: movement, swallowing, coughing - though NMBDs mask these
• Autonomic: sweating, flushing

2. Pharmacological Surrogates

Minimum Alveolar Concentration (MAC)

• Defined as the alveolar concentration of an inhaled anaesthetic that prevents movement in 50% of patients exposed to a standard surgical stimulus
• 1 MAC is the baseline; 0.5 MAC = sedation; 1.3 MAC = 95% of patients immobile
• MAC-awake (~0.3 MAC) is the concentration at which 50% of patients open their eyes on command - relevant for emergence
• MAC-BAR (Blockade of Adrenergic Response) ~1.5 MAC - prevents autonomic response to incision

3. EEG-Based Monitoring (Processed EEG)

• Awake: high frequency (beta, 14-30 Hz), low amplitude
• Sedated: alpha (8-13 Hz) spindles
• Surgical anaesthesia: delta (0.5-4 Hz), high amplitude, slow waves
• Deep/overdose: burst suppression → isoelectric EEG

1. Activation of high-frequency waves (14-30 Hz)
2. Level of synchronised low-frequency waves
3. Occurrence of burst suppression
4. Presence of isoelectric EEG

3a. Bispectral Index (BIS) - Aspect Medical/Medtronic

• The B-Aware trial (Myles et al., 2004, Lancet) showed BIS-guided anaesthesia reduced awareness incidence
• The B-Unaware and BAG-RECALL trials subsequently showed BIS was NOT superior to end-tidal anaesthetic concentration guidance for preventing awareness in high-risk patients
• Recent meta-analyses (2024) suggest small risk reductions for delirium with BIS guidance but evidence remains mixed

• ~30-60 second lag from real-time EEG changes
• Artefact susceptibility (EMG, cautery, pacemakers)
• Ketamine and N₂O increase high-frequency power → falsely high BIS (paradoxically lower number = deeper drug effect but BIS reads high)
• Dexmedetomidine causes slow oscillations but at lighter levels → can give misleadingly low BIS
• Does not account for baseline brain status (dementia, prior brain injury)
• Proprietary algorithm - the exact weighting is not publicly disclosed
• Pressure ulcers if electrode strip used in prone position

3b. Entropy (GE Healthcare/Datex-Ohmeda)

• State Entropy (SE): frequency range 0.8-32 Hz - EEG only - reflects cortical/hypnotic depth; range 0-91; target 40-60 during surgery
• Response Entropy (RE): frequency range 0.8-47 Hz - EEG + frontalis EMG - reflects cortical + somatic arousal; range 0-100; target 40-60

3c. SEDLine (Masimo) / Patient State Index (PSI)

3d. Narcotrend (MonitorTechnik)

4. Auditory Evoked Potentials (AEP)

• Mid-latency AEP (MLAEP) at 10-100 ms post-stimulus is most relevant
• As anaesthesia deepens, the response amplitude falls and latency increases
• The AEP Index is a processed version: range 0-100 (similar to BIS scale)

5. Near-Infrared Spectroscopy (NIRS) / Cerebral Oximetry

6. Isolated Forearm Technique (IFT)

7. Minimum Monitoring Standards

• ECG - rhythm and ischaemia detection
• Pulse oximetry (SpO₂) - non-invasive oxygen saturation
• Non-invasive blood pressure (NIBP)
• End-tidal CO₂ (capnography) - confirms correct intubation, ventilation adequacy
• Inspired oxygen concentration
• End-tidal volatile anaesthetic agent concentration (when inhaled agents used)
• Temperature (for prolonged surgery)

8. Limitations and Current Controversies

Recent Evidence (2024-2026)

• EEG-guided anaesthesia and POCD (Yin et al., BMC Anaesthesiol, 2025) - EEG monitoring reduces postoperative cognitive dysfunction in elderly patients [PMID: 40859118]
• BIS-guided sevoflurane in paediatrics (Derylo et al., Paediatr Anaesth, 2026) - BIS guidance reduces sevoflurane consumption in children [PMID: 41273159]
• EEG monitoring and delirium across age groups (Chen et al., BMC Anaesthesiol, 2026) - EEG-guided depth monitoring reduces delirium incidence [PMID: 41606493]

Summary

Guidelines on Awareness Prevention: AAGBI (2019) and ASA (2006)

Guideline Status at a Glance

Important note: Neither the AAGBI 2019 NAP5 Handbook nor the ASA 2006 Advisory have been superseded or formally updated as of mid-2026. The AAGBI 2021 monitoring standards document updates some monitoring requirements but does not replace the awareness-specific guidance.

1. AAGBI/RCoA: The NAP5 Handbook (2019)

Incidence (from NAP5)

• Overall AAGA: ~1 in 19,000 general anaesthetics
• With neuromuscular blocking drugs (NMBDs): ~1 in 8,000
• Without NMBDs: ~1 in 136,000
• Under TIVA: significantly higher than with volatile anaesthesia

Three Unique Emphases of the NAP5 Handbook

1. NMBDs are the single most important risk factor for AAGA - management of NMBDs is the primary prevention strategy
2. Consent guidance - provides template wording for practitioners (first guideline to do so for AAGA)
3. A precise, evidence-based approach to emergence and NMBD reversal - previously not clearly articulated

Key Recommendations (Selected)

Pre-operative / Consent

• All patients must be informed of the risk of AAGA before surgery
• Written information should include AAGA as a recognised risk (RCoA patient leaflets available at rcoa.ac.uk)
• At consent, emphasise: (a) AAGA is generally uncommon or rare; (b) the patient will have opportunity to discuss concerns with the anaesthetist on the day

Intraoperative - General

• Avoid inadequate anaesthetic doses - the most common cause of AAGA in NAP5 was human error in drug delivery
• Use the AC-WHO checklist (ABCDE checklist) to ensure continuous delivery of anaesthetic agents before and during surgery:
  • A - Anaesthetic machine/delivery device checked
  • B - IV access checked and working
  • C - Vaporiser/infusion checked and filled/running
  • D - Drug doses reviewed and correct
  • E - End-tidal agent concentration/BIS monitored

NMBDs - The Central Prevention Strategy

• Avoid using NMBDs unless genuinely necessary for the surgical procedure
• If NMBDs are used:
  • Use a nerve stimulator (quantitative neuromuscular monitoring) to confirm adequate depth of block and complete reversal
  • Do NOT extubate until full neuromuscular function is confirmed (TOF ratio ≥0.9 by quantitative monitoring)
  • Ensure the anaesthetic depth is adequate before giving an NMBD
• Sugammadex is preferred over neostigmine for reversal of rocuronium/vecuronium when rapid or complete reversal is required

TIVA-Specific

• TIVA carries a higher awareness risk than volatile anaesthesia
• When using propofol TIVA:
  • Use target-controlled infusion (TCI) with an appropriate pharmacokinetic model (Schnider or Marsh for propofol)
  • Check the IV cannula, syringe pump, and line integrity before and during anaesthesia
  • Consider use of processed EEG monitoring (BIS or Entropy) as an adjunct - particularly important because there is no equivalent of end-tidal agent concentration as a real-time check
  • Ensure the pump is running and the drug is actually entering the patient

Monitoring

• End-tidal anaesthetic agent concentration (ETAC): maintain at ≥0.7 MAC for hypnosis in most adults (adjusted for age - MAC decreases with age)
• Brain function monitoring (BIS/Entropy): should be considered particularly in:
  • TIVA
  • Cardiac surgery
  • Caesarean section under GA
  • High-risk patients (known awareness history, tolerance to opioids/alcohol/sedatives)
  • Emergency surgery
  • Trauma surgery
• The guideline does not mandate BIS for all patients, recognising the evidence that BIS is not superior to ETAC-guided anaesthesia for awareness prevention in all cases

High-Risk Groups Identified by NAP5

• Patients receiving NMBDs
• TIVA without EEG monitoring
• Obstetric GA (especially caesarean section - limited agents often used)
• Cardiac surgery (deliberately lighter anaesthesia to maintain haemodynamics)
• Emergency/trauma surgery (haemodynamic compromise limits doses)
• Obese patients (pharmacokinetic variability)
• History of prior awareness
• Chronic opioid or benzodiazepine users (tolerance)
• Female sex and younger age (pharmacodynamic variability)

2. ASA Practice Advisory (2006)

Note: This advisory predates NAP5 (2014), the B-Unaware trial (2008), and the BAG-RECALL trial (2011), which significantly changed understanding of BIS utility.

Core Recommendations

Pre-operative

• Identify patients at increased risk of awareness using risk factors
• Pre-medication with benzodiazepines should be considered in high-risk patients (e.g., midazolam preoperatively) - this is supported by recent evidence (Wang et al., Br J Anaesth 2023, which found benzodiazepines reduced awareness risk RR 0.26, 95% CI 0.12-0.58) [PMID: 36621439]
• Inform patients who are at high risk about the possibility of awareness

Intraoperative Monitoring

• Check for purposeful or reflex movement
• Monitor for autonomic signs: tachycardia, hypertension, lacrimation, sweating, pupil dilation

• ECG
• Blood pressure
• Heart rate
• End-tidal anaesthetic agent analyser
• Capnography

• Cardiac surgery
• Caesarean section under GA
• TIVA
• High-risk patients

Intraoperative Management if Awareness Suspected

• Immediately deepen the anaesthetic - increase volatile concentration or infusion rate
• Administer a benzodiazepine (midazolam, lorazepam) if awareness is suspected during the procedure - however, the advisory acknowledges these are unreliable for retrograde amnesia
• Consider administering a scopolamine or other amnestic agent

Post-operative Management

1. PACU interview (Structured postoperative questions - Brice questionnaire):
   • What is the last thing you remember before going to sleep?
   • What is the first thing you remember on waking?
   • Do you remember anything in between?
   • Did you dream?
   • What was the worst thing about your operation?
2. A second interview at 1-2 days post-operatively (many patients do not report awareness immediately due to fear of not being believed or cognitive fog)
3. If awareness is confirmed:
   • Do not dismiss or minimise the patient's experience
   • Express empathy and regret (not necessarily a legal admission)
   • Provide a full explanation of what happened
   • Offer referral for psychological counselling or psychiatric support (PTSD can develop in up to 70% of patients with AAGA)
   • Document in the medical record
   • Report through institutional incident reporting systems
   • Conduct a root-cause analysis

3. Key Differences Between AAGBI 2019 and ASA 2006

4. What Has Changed Since These Guidelines

B-Unaware Trial (2008) and BAG-RECALL Trial (2011)

ENGAGES Trial (2019) and ENGAGES-Canada Trial (2024)

Benzodiazepine Meta-analysis (Wang et al., Br J Anaesth 2023) [PMID: 36621439]

Indian Delphi Consensus on pEEG (2026) [PMID: 42145302]

5. Practical Prevention Checklist (Synthesised from Both Guidelines)

Before induction:

• Assess and document risk factors for AAGA
• Obtain consent that includes mention of AAGA risk
• Pre-medicate with benzodiazepine in high-risk patients
• Check anaesthetic machine, vaporiser filled, IV access working
• Apply depth of anaesthesia monitor (BIS/Entropy) for TIVA, cardiac, OB-GA, high-risk cases

Induction:

• Confirm adequate anaesthetic depth before giving NMBD
• Use appropriate induction dose (avoid underdosing)
• Start end-tidal agent monitoring or check TIVA pump is running

Maintenance:

• Maintain ETAC ≥0.7 age-adjusted MAC or BIS 40-60 for TIVA
• Check IV line/pump integrity regularly in TIVA
• Respond promptly to signs of light anaesthesia (tachycardia, hypertension, movement, lacrimation)
• Use quantitative neuromuscular monitoring if NMBDs are in use

Emergence:

• Confirm TOF ratio ≥0.9 before extubation (quantitative monitor)
• Use sugammadex for complete reversal when required

Post-operatively:

• Conduct Brice interview in PACU and again at 1-2 days
• If AAGA reported: validate, explain, document, refer for psychological support