AWARENESS DURING GENERAL ANAESTHESIA

MD Anaesthesia Examination Answer - 50 Marks

1. DEFINITION AND CLASSIFICATION

• Explicit memory (declarative): episodic recall of events - this is AWR
• Implicit memory: subconscious processing without recall - may still cause psychological harm

2. INCIDENCE

• General surgical population: 1-2 per 1,000 cases (range 1:500 to 1:20,000)
• Cardiac surgery: 1.5%
• Obstetric (Caesarean section) GA: 0.4%
• Major trauma surgery (early studies): up to 43% recall
• ~2% of all ASA Closed Claims Project cases relate to awareness
• NHS Litigation Authority (1995-2007): 19 of 93 relevant claims were for "awake paralysis"
• Consciousness without recall is substantially more common than AWR

"When formally assessed, this complication occurs in approximately one to two cases per 1000 and is associated with a high incidence of posttraumatic stress disorder." - Miller's 10e, Ch. 8

3. AETIOLOGY AND RISK FACTORS

A. Drug Delivery Failure

• Vaporizer malfunction or empty vaporizer
• Breathing circuit disconnection or leak
• IV line tissued (extravasation) - TIVA patient receives no drug
• Drug labeling/administration errors (e.g., NMB given before induction agent)

B. Deliberate Dose Limitation

• Haemodynamically compromised patients (trauma, cardiac tamponade, severe aortic stenosis)
• Emergency surgery where speed precedes adequate induction
• Caesarean section - concern for neonatal CNS depression historically led to underdosing

C. Increased Anaesthetic Requirements

• Chronic substance use/abuse (alcohol, opioids, benzodiazepines, amphetamines)
• Younger patients (higher MAC requirements)
• Genetic variability in pharmacodynamic response

D. Masking of Clinical Signs

• Neuromuscular blocking drugs - abolish movement, the most reliable clinical sign
• High opioid doses blunt autonomic response without providing hypnosis
• Beta-blockers, antihypertensives masking tachycardia and hypertension

Patient Risk Factors (Miller's, Box 36.1):

• Female sex
• Early-to-middle age
• Previous history of awareness
• Out-of-hours cases (junior operator)
• TIVA without EEG monitoring
• Rapid sequence induction
• Neuromuscular blockade

4. TIMING OF AWARENESS

• Two-thirds of awareness episodes occur during the induction or emergence phase
• Maintenance phase events are less common but more distressing
• Most dangerous period: just after induction if NMB is given before adequate hypnosis, and during rewarming from cardiopulmonary bypass (CPB)

5. CLINICAL FEATURES AND SEQUELAE

Intraoperative signs (indirect - no pathognomonic sign):

• Tachycardia, hypertension
• Sweating, lacrimation
• Mydriasis
• Breath-holding, bucking
• Patient movement (may be masked by NMB)

Patient experience may include:

• Hearing sounds/voices (most common)
• Feeling of pressure, pain, or surgical manipulation
• Paralysis and inability to communicate (most terrifying)
• Emotional/emotional distress

Psychological Sequelae (Morgan & Mikhail 7e, Ch. 54):

• Mild anxiety
• Sleep disturbances
• Nightmares
• Social difficulties
• Post-Traumatic Stress Disorder (PTSD) - high incidence with AWR
• Claims for "awake paralysis" - the most distressing subset

6. MONITORING - DEPTH OF ANAESTHESIA

A. Clinical Monitoring (Traditional)

B. End-Tidal Anaesthetic Concentration (ETAC)

• Maintaining ≥ 0.7 MAC (age-corrected) is an established, evidence-based strategy
• Modern anaesthesia machines provide real-time ETAC measurement
• B-Unaware and BAG-RECALL trials showed no superiority of BIS over ETAC monitoring in preventing AWR in high-risk patients

C. EEG-Based Indices

• Scale: 0 (isoelectric EEG) to 100 (fully awake)
• Target range for GA: 40-60
• Values < 40: deep anaesthesia/burst suppression
• Values > 60: increasing risk of awareness

• Meta-analysis of 52 studies (41,331 participants): BIS monitoring more than halved AWR compared to clinical monitoring alone (OR = 0.36, 95% CI 0.21-0.60)
• B-Aware Trial: BIS significantly reduced AWR vs. standard care
• B-Unaware / BAG-RECALL: No difference vs. ETAC-guided anaesthesia
• Mashour et al. (MACS trial, 18,836 patients): No significant overall difference BIS vs. ETAC alarms; BUT post-hoc analysis with equipment failure excluded showed lower awareness in BIS arm
• TIVA-specific trial: BIS guidance reduced awareness from 0.65% to 0.14% - clinically important for propofol-based TIVA

• Less reliable in elderly (acceptable anaesthesia at relatively high index values)
• EMG artifact from facial muscles causes falsely elevated readings
• Electrocautery, ECG, pacemaker interference
• During cardiac surgery: pump head rotation, hypothermia cause falsely high BIS values
• Data smoothing introduces a 15-30 second lag behind clinical events
• Poor performance for non-GABAergic drugs (ketamine, nitrous oxide)

• Entropy (State Entropy / Response Entropy)
• Narcotrend
• Patient State Index (PSI)
• A-line ARX Index (AAI - auditory evoked potentials)

• Spectral edge frequency (SEF)
• Power spectral density / spectrogram - shows characteristic anesthetic patterns
• Burst suppression ratio (BSR)

"It is crucial to see the EEG monitor as one component in a multidimensional evaluation of the state of the patient." - Miller's 10e, Ch. 36

7. PREVENTION

Pre-operative:

1. Risk stratification - identify high-risk patients (cardiac surgery, obstetrics, trauma, TIVA, NMB use, substance misuse)
2. Informed consent - discuss possibility of awareness for high-risk procedures
3. Pre-medication with benzodiazepines (midazolam) or scopolamine - enhances amnesia
4. Equipment check - vaporizer levels, TIVA pump function, IV line patency

Intraoperative:

1. Adequate induction agent before NMB - never give NMB before confirming hypnosis
2. Maintain ETAC ≥ 0.7 MAC for inhalational anaesthesia
3. BIS/EEG monitoring - especially for TIVA, high-risk patients, cardiac surgery
4. Use benzodiazepines (midazolam) if volatile agents must be restricted
5. Peripheral nerve stimulator to ensure NMB is at required level - avoid unnecessary deep blockade
6. Monitor for clinical signs (movement, autonomic response)
7. During CPB: begin propofol or dexmedetomidine infusion at CPB start, continue through rewarming; restart volatile agent once ventilation restarted
8. Documentation of ETAC values and amnesic drug doses

Specific to TIVA:

• Dual IV access / check cannula patency before NMB
• Target-controlled infusion (TCI) with TIVA
• BIS monitoring is most valuable here (no ETAC to guide)
• AAGA-TIVA risk is higher than with inhalational agents

8. MANAGEMENT OF SUSPECTED INTRAOPERATIVE AWARENESS

Immediate:

• Increase volatile agent or propofol dose
• Administer benzodiazepine (midazolam 1-2 mg IV) for amnesia
• Administer opioid for analgesia
• Reassure the patient verbally ("You are in the operating room, everything is fine, you will not remember this")

9. POSTOPERATIVE MANAGEMENT OF CONFIRMED AWR

1. Detailed history - obtain full account of what was experienced (timing, sensory details, emotional impact)
2. Empathetic approach - most patients reporting awareness are dissatisfied with how their concerns are handled (North American Anaesthesia Awareness Registry)
3. Answer patient questions honestly
4. Psychological referral - if PTSD features present (nightmares, flashbacks, anxiety)
5. Formal incident reporting and root cause analysis
6. Medico-legal documentation - awareness claims form ~2% of ASA Closed Claims

10. SPECIAL SITUATIONS

Caesarean Section under GA:

• Historically underdosed to avoid neonatal CNS depression
• Now recognized as unacceptable - adequate thiopental/propofol dose, add volatile agent after intubation
• Incidence now 0.4% with modern practice

Cardiac Surgery:

• Highest-risk setting (1.5% incidence)
• CPB-related: dilution of drugs, awareness during rewarming
• NMB required - movement cannot be relied upon
• BIS falsely elevated by pacemaker, pump motor, hypothermia

Rapid Sequence Induction (RSI):

• High-dose suxamethonium given at induction - complete paralysis before airway secured
• Failed intubation + awake paralysis is the most distressing scenario
• Ensure induction agent given before NMB

Paediatric Awareness:

• Incidence similar to adults (approximately 0.2-1%)
• Children less likely to volunteer experience spontaneously
• Structured interview (Modified Brice Interview) should be used

11. INTERVIEWING FOR AWARENESS - THE BRICE QUESTIONNAIRE

1. What was the last thing you remember before going to sleep?
2. What was the first thing you remember on waking?
3. Do you remember anything between these two points?
4. Did you dream during your operation?
5. What was the worst thing about your operation?

SUMMARY TABLE

• Miller's Anesthesia, 10th Edition - Chapters 8 (Consciousness & Memory), 36 (Brain Monitoring), 50 (Cardiac Anaesthesia)
• Morgan & Mikhail's Clinical Anesthesiology, 7th Edition - Chapter 54 (Complications)