Awareness Under Anaesthesia

Definition

Classification / Spectrum

Incidence

• General surgical population: approximately 1-2 per 1000 patients (0.1-0.2%)
• Cardiac surgery: ~1.5%
• Caesarean section: ~0.4%
• Major trauma surgery: historically up to 43% (recall rates in early studies)
• Children: slightly higher incidence reported than adults

Causes and Risk Factors

Patient Factors

• Female sex (meta-analysis confirms higher susceptibility - PMID 37453840)
• Long-term substance abuse (opioids, benzodiazepines, alcohol) - increases anaesthetic requirements
• Chronic pain/opioid tolerance
• Obesity
• Difficult airway (reduced ability to deepen anaesthesia)
• Cardiovascular instability limiting anaesthetic dosing
• Prior awareness (sensitised CNS)

Surgical/Procedural Factors

• Emergency surgery, major trauma
• Cardiac surgery (cardiopulmonary bypass alters drug pharmacokinetics)
• Obstetric surgery (concern about fetal drug depression limits depth)
• Prolonged neuromuscular blockade (masks movement, the most reliable clinical sign of light anaesthesia)

Anaesthetic/Equipment Factors

• Vaporiser malfunction or empty vaporiser - a common preventable cause
• Drug administration errors (e.g., paralytic given before induction)
• TIVA (Total Intravenous Anaesthesia): no volatile agent, sole reliance on infusion - prone to interruption/underdosing
• Nitrous oxide as sole hypnotic agent - insufficient on its own
• Inadequate preoperative assessment of drug tolerance

Consequences

• Acute anxiety, fear, and helplessness during the event
• Post-traumatic stress disorder (PTSD) - sleep disturbances, nightmares, flashbacks
• Social difficulties and relationship problems
• Long-term psychological morbidity requiring formal counselling
• Litigation - approximately 2% of ASA Closed Claims relate to awareness; claims include both "awake paralysis" (paralytic given before induction) and recall under anaesthesia
• NHS Litigation Authority data (1995-2007): 19 of 93 relevant claims were for "awake paralysis"

Diagnosis / Detection

Postoperative Assessment - Modified Brice Questionnaire

1. What is the last thing you remembered before going to sleep?
2. What is the first thing you remembered when you woke up?
3. Can you remember anything between these two periods?
4. Did you dream during your operation?
5. What was the worst thing about your operation?

Intraoperative Monitoring

• Movement, hypertension, tachycardia, sweating, lacrimation

• A tourniquet inflated on one arm prior to NMB injection allows voluntary hand movement in response to commands
• Gold standard for detecting intraoperative consciousness in real time
• Not routinely used clinically but important as a research tool

• BIS values of 40-90 have been associated with awareness in different individuals - a BIS of 50 in one patient does not equal a BIS of 50 in another
• Administration of neuromuscular blockers (which abolish EMG artefact) can DROP the BIS significantly in awake subjects, producing false-negative values
• Correlation with IFT responses (connected consciousness) is only around 0.7 with sensitivities/specificities between 40-85%
• IFT responses have been seen even during classic alpha-delta EEG pattern typical of GABA-ergic anaesthesia

• B-Aware trial (Myles et al., Lancet 2004): BIS-guided anaesthesia significantly reduced awareness vs. standard care in high-risk patients
• BAG-RECALL trial (Avidan et al., NEJM 2011): In high-risk patients, BIS-guided and ETAC (end-tidal anaesthetic concentration)-guided protocols had comparable low awareness rates - ETAC guidance was not inferior to BIS

Prevention

General Principles

1. Preoperative: Identify high-risk patients (substance abuse, medications, prior awareness, surgical type). Inform patient and discuss possibility as part of informed consent for those at high risk
2. Administer adequate doses of induction agent; do not rely on nitrous oxide as sole hypnotic
3. Inhalational agents: Minimum Alveolar Concentration (MAC) >0.5 provides amnesia; maintain ETAC >0.7 MAC
4. TIVA: Use TCI (Target Controlled Infusion) with appropriate propofol targets; consider adding a benzodiazepine
5. Benzodiazepines or scopolamine as adjuncts when depth must be limited (e.g., haemodynamically unstable patients, caesarean section)
6. Check equipment: Vaporiser levels, IV access patency before induction
7. Document: End-tidal concentrations of volatile agents and dosages of amnesic drugs throughout
8. EEG monitoring (BIS or equivalent): Consider in high-risk cases, TIVA, and procedures requiring light anaesthesia - while RCT evidence is mixed, they provide additional information
9. Movement response during surgery should trigger immediate deepening of anaesthesia

Management if Awareness Occurs

1. Intraoperative: If awareness is suspected or reported intraoperatively, immediately deepen anaesthesia; administer a benzodiazepine (provides anterograde amnesia) ± opioid
2. Postoperative:
   • Listen empathetically and without dismissiveness
   • Obtain a detailed account of the patient's experience
   • Answer questions honestly; acknowledge what happened
   • Explain what occurred from the anaesthetic perspective
   • Document thoroughly
   • Refer for psychological counselling / psychiatric review if symptoms of PTSD develop
   • Incident report and root cause analysis (equipment check, drug audit)
3. Medico-legal: Candid documentation is essential; most claims arise partly from poor communication after the event

Summary Table

• Miller's Anesthesia, 10e (2023), Chapter 36 - EEG and Depth of Anaesthesia
• Morgan & Mikhail's Clinical Anesthesiology, 7e - Complications Chapter
• Barash, Cullen & Stoelting's Clinical Anesthesia, 9e
• Braithwaite et al. (2023) - Female sex and anaesthetic awareness: systematic review & meta-analysis, Br J Anaesth