Awareness Under Anesthesia

Definition

• Intraoperative consciousness without recall — patient is conscious but forms no explicit memory (occurs far more frequently)
• Sedation-related awareness — expected during regional or monitored anaesthesia care
• Dreams during anaesthesia — which are not true intraoperative awareness

Incidence

Causes and Risk Factors

Patient Factors

• Chronic alcohol or substance use (increased anaesthetic requirement)
• Long-term opioid or benzodiazepine use
• Obesity
• Difficult intubation (distraction during induction)
• Previous history of awareness

Anaesthetic/Surgical Factors

• Cardiac surgery — deliberately light anaesthesia to preserve cardiac output
• Obstetric (caesarean) surgery — anaesthetic restricted before delivery to protect the neonate
• Major trauma — hypotension limits drug dosing
• Neuromuscular blocking drugs (NMBDs) — muscle paralysis masks movement, the most reliable clinical sign of light anaesthesia; NMBDs have been directly implicated in contributing to awareness
• Use of nitrous oxide–opioid TIVA without volatile agents
• Vaporiser malfunction, empty vaporiser, circuit leaks, oxygen flushing diluting volatile agents
• Drug-labelling errors (e.g., NMBD administered before induction)

Clinical Features and Patient Experience

• Hearing conversations or sounds
• Feeling pressure, pain, or surgical stimuli
• Sensing inability to move or communicate
• Emotional distress, panic, sense of helplessness

• Post-traumatic stress disorder (PTSD) — the most serious sequela
• Sleep disturbances, nightmares
• Anxiety, depression, social difficulties
• Legal action — ~2% of ASA Closed Claims involve awareness; 20% of these were "awake paralysis" cases

Monitoring and Detection

1. Clinical Signs (Unreliable)

• Tachycardia, hypertension, sweating, tearing, pupillary dilation — these are autonomic responses but are not reliable indicators of consciousness. Awareness can occur in the absence of sympathetic signs, and autonomic instability occurs without conscious recall. (Miller's Anesthesia, 10e)

2. End-Tidal Agent Monitoring

• Maintaining an age-adjusted MAC ≥ 0.7 of a volatile agent is a practical target for preventing awareness
• Current evidence suggests end-tidal agent monitoring is at least as effective as processed EEG for preventing AWR

3. Processed EEG Monitoring

• Bispectral Index (BIS): ranges 0–100; target range 40–60 for general anaesthesia
• Entropy monitors (GE Healthcare), SedLine/spectrogram (Masimo)
• EEG monitoring reduces AWR by more than 50% compared to clinical monitoring alone, but its advantage over end-tidal agent monitoring is less clear
• Limitations of BIS:
  • EMG/muscle artefact causes falsely elevated values (NMBDs can drop BIS in awake patients — "false negative")
  • Less reliable in the elderly, ketamine anaesthesia, or nitrous oxide–opioid techniques
  • Individual variability: awareness has occurred at BIS values between 40–90
  • Different BIS values carry different meaning in different individuals
• ASA position: Processed EEG monitors are not a standard of care — use is left to clinician judgment. Large randomised trials (B-Aware, BAG-RECALL) have not demonstrated superiority of BIS over end-tidal agent monitoring in general surgical populations.

4. Isolated Forearm Technique (IFT)

• A tourniquet is applied before NMBD administration to isolate one forearm from neuromuscular blockade
• Allows detection of purposeful motor responses to commands during paralysis
• Considered the gold standard for detecting connected consciousness in research settings
• Rarely used clinically

5. Modified Brice Interview (Post-operative)

1. What is the last thing you remember before going to sleep?
2. What is the first thing you remember 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?

Prevention

1. Adequate premedication — benzodiazepines (midazolam) provide anxiolysis and anterograde amnesia
2. Appropriate induction doses — avoid underdosing in sick or haemodynamically compromised patients
3. Volatile agent monitoring — maintain end-tidal volatile concentration ≥ 0.7 MAC
4. Vaporiser and circuit checks — pre-anaesthetic equipment checklist to detect leaks, empty vaporisers
5. Caution with NMBDs — use only when necessary; monitor depth of NMB; ensure adequate hypnosis before paralysis
6. BIS/processed EEG — consider in high-risk patients (cardiac surgery, trauma, TIVA)
7. TIVA protocols — target-controlled infusion (TCI) of propofol to maintain appropriate plasma levels
8. Avoid errors in drug administration — clear labelling, confirmed drug identity before injection

Management When Awareness is Suspected or Confirmed

1. Acknowledge and communicate — speak to the patient post-operatively; do not deny or dismiss reports
2. Psychological support — early referral for counselling; screen for PTSD
3. Incident documentation — report via hospital governance systems
4. Review intraoperative records — identify the likely cause (equipment failure, underdosing, drug error)
5. Follow-up — long-term psychological support if PTSD develops
6. Legal considerations — documentation is critical for medicolegal defence

Summary

Neuroprotection and Anesthesia

1. Introduction

• High rate of oxygen and glucose consumption
• Inability to store substrate
• Inability to rapidly dispose of toxic metabolites

2. Mechanisms of Ischemic Brain Injury

a) Excitotoxicity

• Ischemia → failure of Na⁺/K⁺-ATPase → membrane depolarisation → excessive glutamate release
• Glutamate activates NMDA and AMPA receptors → massive Ca²⁺ influx
• Ca²⁺ overload triggers proteases, phospholipases, nitric oxide synthase, and mitochondrial damage

b) Energy Failure

• Loss of ATP → ion pump failure → cytotoxic oedema
• Anaerobic glycolysis → lactic acidosis → worsened ischaemic injury

c) Apoptosis (Delayed Neuronal Death)

• Mitochondrial injury → cytochrome c release → caspase-9 → caspase-3 activation
• Bcl-2 (antiapoptotic) / Bax (proapoptotic) balance determines outcome
• Delayed neuronal death continues for days to months after the ischaemic insult — expansion of infarction is a dynamic process

d) Inflammation

• Excitotoxic injury (hours 0–6) → inflammatory mediator release (days 1–3) → apoptotic death (days to months)
• Cerebral inflammation has been demonstrated 6–8 months after the primary ischaemia in animal models

e) Reperfusion Injury

• Restoration of flow generates free radicals (reactive oxygen species, ROS)
• Microvascular injury, BBB disruption, and oedema worsen outcome despite reperfusion

3. Physiological Basis: CBF, CMR, and Coupling

Cerebral Metabolic Rate (CMR) and Cerebral Blood Flow (CBF)

• Normal CBF: ~50 mL/100g/min; CMRO₂: ~3.5 mL/100g/min
• Flow–metabolism coupling: CBF is tightly coupled to CMRO₂ under normal conditions
• Anesthetics that reduce CMR will secondarily reduce CBF — this is the basis of metabolic suppression neuroprotection

Autoregulation

• CBF is maintained constant over a MAP of ~50–150 mmHg
• Volatile anesthetics impair autoregulation in a dose-dependent manner
• Loss of autoregulation makes the brain pressure-passive — hypotension directly reduces CBF

CO₂ Reactivity

• Hypercapnia → cerebral vasodilation → ↑CBF
• Hypocapnia → vasoconstriction → ↓CBF
• CO₂ reactivity is preserved at ≤1 MAC for isoflurane, desflurane, and sevoflurane

4. Anesthetic Effects on Cerebral Physiology

5. Pharmacological Neuroprotection by Anesthetic Agents

a) Barbiturates

• Most studied for neuroprotection
• Reduce CMRO₂ by up to 60% (EEG isoelectric = maximum metabolic suppression)
• Mechanism: GABA-A agonism → neuronal inhibition; also free-radical scavenging
• Protective in focal ischemia animal models
• Clinical evidence is disappointing: barbiturates are ineffective after complete global ischemia (cardiac arrest)
• EEG burst suppression is NOT required for maximal neuroprotection from pentobarbital in focal ischaemia models
• Clinically used during temporary clipping in cerebrovascular surgery

b) Propofol

• Produces CMR suppression comparable to barbiturates (EEG burst suppression achievable)
• Antioxidant properties (phenolic structure — scavenges lipid peroxyl radicals)
• Animal data: cerebral infarction significantly reduced vs. awake controls; similar protection to pentobarbital in direct comparisons
• Protection is not sustained with moderate-severe ischemia — only mild ischemia benefits durably
• Used clinically during carotid endarterectomy and aneurysm surgery anecdotally

c) Volatile Anesthetics (Isoflurane, Sevoflurane, Desflurane)

• Reduce CMRO₂; mechanisms include:
  • Reduction of ischaemia-induced glutamate release
  • Activation of ATP-dependent K⁺ channels (KATP)
  • Augmentation of antiapoptotic mediators
  • Reduction of excitotoxic stress
  • Augmentation of CBF in ischaemic regions
• Isoflurane: historically considered the volatile of choice for neuroprotection (greatest protection vs. other volatiles); reduces CMRO₂ and provides relative cerebral vasodilation maintaining tissue PO₂
• Desflurane: shown to improve neurologic outcome in rat incomplete ischaemia models; in piglets on low-flow CPB, better outcome than fentanyl/droperidol; increases brain tissue PO₂ during administration and maintains PO₂ better than thiopental during temporary arterial occlusion in human cerebrovascular surgery
• Sevoflurane: comparable profile; TREK-1 K⁺ channel activation contributes to neuroprotective effects
• Anesthetic preconditioning: Brief sublethal exposure to volatile agents induces tolerance to subsequent ischaemia — similar to ischaemic preconditioning; involves KATP channels, PKC, and mitochondrial pathways
• Practical limitation: differences in neuroprotection between volatile agents are not considered clinically important

d) Ketamine

• Increases CMR and CBF — traditionally avoided in neurosurgical patients
• NMDA receptor antagonism → reduces excitotoxic Ca²⁺ influx → potential neuroprotective mechanism
• S(+)-ketamine: may influence antiapoptotic protein expression after cerebral ischaemia/reperfusion
• Concern about ICP elevation now considered largely unfounded in mechanically ventilated patients
• Clinical neuroprotective use remains rare — awaiting randomised controlled trial data

e) Etomidate

• Produces burst suppression and CMR suppression equivalent to barbiturates; GABA-A agonist
• Not neuroprotective in clinical practice: In focal ischaemia models, injury volume was larger than halothane controls
• In patients with temporary vessel occlusion, etomidate produced greater tissue hypoxia and acidosis than desflurane
• IHAST trial: etomidate supplementation for neuroprotection showed no efficacy
• Mechanism of harm: imidazole ring may directly bind NO + inhibit NO synthase → reduced vasodilation + haemolysis
• No scientific evidence supports etomidate for cerebral protection

f) Xenon

• Inert gas with anaesthetic properties; NMDA and KATP channel modulation
• Neuroprotective in experimental models
• TREK-1 channel activation contributes to neuroprotective effects (shared with sevoflurane)
• Not widely available clinically

6. Hypothermia as Neuroprotection

Mechanism

• Hypothermia reduces CMRO₂ far more than any anaesthetic can — reduces even the basal homeostatic oxygen requirement of neurons
• Animal data: protection from even a 1°C reduction in core temperature, suggesting mechanisms beyond metabolic suppression alone
• Additional mechanisms: reduced glutamate release, reduced free-radical generation, reduced inflammation, reduced apoptosis

Clinical Evidence

Hyperthermia

• Must be actively avoided in the setting of cerebral ischaemia
• Focal cerebral infarct size triples for each 1°C rise in core temperature in animal models

7. Non-Anaesthetic Neuroprotective Pharmacology

8. Anaesthetic Management Strategies for Neuroprotection

Intraoperative Principles

1. Maintain CPP — avoid hypotension (particularly during carotid endarterectomy, temporary vessel occlusion, cardiac surgery)
2. Maintain normocapnia — hypocapnia causes vasoconstriction and may worsen ischaemia; hypercapnia increases ICP
3. Avoid hyperthermia — actively monitor and treat; avoid any temperature rise in at-risk patients
4. Normoglycaemia — hyperglycaemia (>180 mg/dL) worsens cerebral acidosis under ischaemic conditions; hypoglycaemia equally detrimental
5. Avoid anaemia — adequate haemoglobin for oxygen-carrying capacity
6. EEG monitoring — guide barbiturate/propofol dosing to burst suppression when temporary vessel occlusion is planned
7. Cerebral perfusion monitoring — transcranial Doppler, cerebral oximetry (NIRS, rSO₂), jugular venous O₂ saturation, brain tissue PO₂ (Licox)

Specific Clinical Scenarios

• Carotid endarterectomy: isoflurane, desflurane, or propofol; EEG/SSEP monitoring; maintain MAP; consider barbiturate burst suppression if EEG ischaemia
• Cerebral aneurysm clipping: temporary clipping tolerance augmented by burst suppression with barbiturates or propofol; IHAST showed hypothermia adds no benefit and increases infections
• Cardiac surgery (CPB): pH-stat vs. alpha-stat management; pulsatile flow; embolic load reduction; cerebral oximetry
• Neonatal/paediatric: hypothermia for HIE within 6h of delivery

9. Postoperative Cognitive Dysfunction (POCD)

• Volatile anaesthetics linked in animal models to neuroinflammation, neurotransmitter imbalance, and decreased BDNF
• Clinical data challenging: well-conducted RCTs have not demonstrated differences in cognitive outcomes between regional and general anaesthesia in high-risk older adults
• Reducing volatile agent concentration with EEG guidance reduces postoperative delirium in the first 5 days and may lessen cognitive impairment at 1 year
• N₂O does not appear to alter short-term delirium or cognitive decline progression when added to volatile agent

10. Summary