Awareness During General Anaesthesia

Definitions

Explicit Awareness

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?

Implicit Awareness

Factors That Increase the Likelihood of Awareness

Patient-Related

Surgery-Related

Anaesthetic Technique-Related

• Total intravenous anaesthesia (TIVA) — no end-tidal agent concentration available to confirm delivery; reliance entirely on infusion pump integrity
• Vaporiser malfunction or disconnection — unrecognised interruption of volatile delivery
• Low-dose techniques — in compromised patients where haemodynamic tolerance is limited
• Neuromuscular blockade — abolishes movement as a clinical sign of light anaesthesia; false reassurance
• Medication errors — administering neuromuscular blocker before induction agent (syringe swap); approximately 20% of ASA closed claims for awareness involved awake paralysis
• Nitrous oxide-only "anaesthesia" — insufficient hypnotic component

Techniques Used to Assess Depth of Anaesthesia

1. Clinical Signs

2. End-Tidal Anaesthetic Agent Concentration

3. Processed Electroencephalography (EEG) Monitors

• Provides real-time feedback on CNS drug effect, especially valuable in TIVA where there is no end-tidal gas to measure
• Useful in haemodynamically compromised patients (emergency surgery, caesarean section under GA, trauma) where clinical tolerance limits titration
• May help avoid anaesthetic overdose and reduce postoperative cognitive complications
• Can guide sedation when neuromuscular blockade precludes clinical assessment in the ICU

• Algorithms trained on young, healthy volunteers — performance degrades in heterogeneous populations; a BIS of 50 in one patient does not equal BIS 50 in another; different individuals become aware at BIS values ranging 40–90
• Ketamine causes cortical excitation, falsely elevating the index (suggesting lighter anaesthesia is being given than is actually the case)
• Opioids have minimal EEG effect; the index primarily reflects the hypnotic component
• EMG artefact — electromyographic activity from facial/frontal muscles falsely elevates BIS
• Neuromuscular blockade paradox — loss of EMG after suxamethonium can drop BIS into the target range in an awake, paralysed patient; this is a critical false-negative scenario
• Randomised trials (including the B-Unaware and BAG-RECALL trials) have failed to demonstrate superiority of BIS over end-tidal agent concentration monitoring in preventing awareness when volatile agents are used
• Specificity/sensitivity in IFT studies typically only 40–85%; predictive value ~0.7
• The alpha-delta EEG pattern itself does not preclude IFT-detectable consciousness

4. Raw EEG / Spectrogram (Electroencephalography)

5. Isolated Forearm Technique (IFT)

6. Mid-Latency Auditory Evoked Potentials (MLAEPs)

7. Cerebral Oximetry (NIRS)

Summary: Value in MD (Medical Doctor / Clinical) Anaesthesia Practice

Stages of Anaesthesia — Guidelines for MD Anaesthesia

1. The ASA Continuum of Depth of Sedation — the clinically current, guideline-defined framework
2. Guedel's Four Stages — the classical, historically foundational staging system

Part 1: The ASA Continuum of Depth of Sedation/Anaesthesia

• Sedation is a continuum, not a series of discrete steps — the line between levels is subtle and often crossed unexpectedly
• Deep sedation requires an independent provider dedicated solely to patient monitoring (separate from the proceduralist)
• For deep sedation: ACLS-trained personnel must be physically present; for moderate sedation: ACLS must be immediately available (1–5 minutes)
• Monitoring requirements increase with depth: continuous pulse oximetry, capnography (mandatory for deep sedation/GA), haemodynamic monitoring, and response to commands
• Recognised high-risk groups requiring anaesthesiologist consultation: morbid obesity, OSA, severe cardiopulmonary disease, extremes of age, pregnancy, substance use disorders
• Appropriate reversal agents (naloxone, flumazenil) and resuscitation equipment must be immediately accessible at all levels

Part 2: Guedel's Four Classical Stages of Anaesthesia

Stage I — Analgesia

• Begins with onset of the anaesthetic; consciousness retained
• Initially: analgesia without amnesia — patient feels pain but less intensely; oriented and responsive
• Later in Stage I: analgesia with amnesia — pain sensation blunted, short-term memory impaired
• Respiration: normal and regular
• Reflexes: fully intact
• Clinical correlate: equivalent to light sedation/anxiolysis in modern terms

Stage II — Excitement (Delirium)

• Loss of consciousness; patient appears delirious, may vocalize, struggle, hold breath, vomit
• Complete amnesia for this stage
• Physiological features:
  • Respiration: irregular, rapid, can include breath-holding
  • Heart rate and blood pressure: elevated (sympathetic activation)
  • All reflexes are exaggerated — laryngospasm, bronchospasm, vomiting can occur
  • Pupils: dilated
• Most dangerous stage — risk of airway compromise, laryngospasm, aspiration
• Goal: pass through Stage II as rapidly as possible by quickly increasing agent concentration
• Modern induction with IV agents (propofol) virtually eliminates this stage by bypassing it

Stage III — Surgical Anaesthesia

Stage IV — Medullary Depression

• Life-threatening stage — severe CNS depression reaching the medulla
• Vasomotor centre in the medulla: depressed → cardiovascular collapse, profound hypotension
• Respiratory centre in the brainstem: depressed → apnoea
• Without immediate circulatory and respiratory support, death ensues rapidly
• In clinical practice this is an overdose state — reached by inadvertent overdose, equipment failure, or failure to recognise deepening anaesthesia

Part 3: The Modern 3-Stage Procedural Framework

1. Induction

• Time from drug administration to establishment of unconsciousness
• Adults: IV propofol (30–40 seconds to unconsciousness); IV NMB (rocuronium/succinylcholine) for intubation
• Paediatrics without IV access: inhalational induction with sevoflurane (non-pungent)
• Rate of induction depends on speed of effective anaesthetic concentration reaching the brain (influenced by cardiac output, blood-gas partition coefficient, alveolar ventilation)

2. Maintenance

• Sustained period of surgical anaesthesia, carefully titrated to the stimulus
• Volatile agents (sevoflurane, isoflurane, desflurane) — end-tidal concentration monitoring
• TIVA with propofol ± remifentanil infusion — requires processed EEG monitoring
• Balance between hypnosis, analgesia, and muscle relaxation (the anaesthetic triad)
• Opioids provide analgesia (volatile agents alter consciousness but do not reliably abolish pain perception)

3. Emergence (Recovery)

• Begins with discontinuation of the maintenance agent; ends with return of consciousness and all protective reflexes
• Mechanism: primarily redistribution from CNS (not metabolism) for most agents
• Neuromuscular blockade reversed pharmacologically (neostigmine/sugammadex) unless spontaneously metabolised
• Confirm: spontaneous ventilation, intact airway reflexes, adequate haemodynamics, return of response to commands
• Emergence is essentially the reverse of induction — washout of agent from the brain

Summary: Guedel's Stages at a Glance

INCREASING DEPTH OF ANAESTHESIA →

Stage I          Stage II          Stage III (4 Planes)          Stage IV
Analgesia    →  Excitement    →   Surgical Anaesthesia      →   Medullary Depression
                (DANGER ZONE)   (TARGET OPERATING ZONE)        (OVERDOSE/DEATH)

• Stage I: describes light sedation / anxiolysis on the ASA continuum
• Stage II: bypassed by IV induction; guides management of difficult/slow inhalational inductions
• Stage III: the clinical target; the four planes guide titration and identify when depth is appropriate vs excessive
• Stage IV: mandates immediate recognition and resuscitation