Anaesthetic Awareness

Anaesthetic awareness is unintended consciousness with explicit recall of events during general anaesthesia. Recall may include sounds, touch, pain, paralysis or distress.

Epidemiology. Estimates range from about 1 in 19,000 (spontaneous reports in a national audit) to about 1 in 1,000 (structured interview studies). Rates are higher when neuromuscular block is used, and in caesarean section, cardiac surgery, trauma and emergency surgery.

Causes. Drug delivery failure (syringe or vaporiser error, disconnection, empty vaporiser, cannula tissued), delay between paralysis and hypnotic, intentionally light anaesthesia for haemodynamic instability, total intravenous anaesthesia without monitoring, and increased requirement.

Risk factors. Previous awareness, young age, female sex, obesity, chronic alcohol, opioid or benzodiazepine use, difficult airway, and limited end tidal agent monitoring.

Consequences. Psychological harm in up to 50% or more of those with distressing recall, including anxiety, nightmares, sleep disturbance and post-traumatic stress disorder.

Pathophysiology.

Hypnotic concentration at the effect site falls under the threshold for loss of consciousness. Neuromuscular block removes movement, so awareness goes unrecognised.

During anaesthesia: tachycardia, hypertension, sweating, lacrimation, pupil dilation or movement, although these are unreliable. After: recall of voices, pressure, pain, paralysis, choking or helplessness; later flashbacks, nightmares and avoidance of medical care.

Red flags.

• Recall of paralysis with pain.
• Panic, intrusive memories, or nightmares in the first week.
• Suicidal thoughts.

Interview all patients with suspected awareness at 0 to 24 hours, 24 to 72 hours and about 1 week using a structured questionnaire (modified Brice interview). Classify the event with the Michigan awareness classification (class 0 to 4, with distress as a modifier). Screen for post-traumatic stress disorder at 4 weeks.

Differential diagnosis.

• Dreaming under anaesthesia.
• Recall under sedation or regional block.
• Ketamine hallucinations.
• Postoperative delirium.
• Pre-existing psychiatric disorder.

Intraoperative suspicion. Check that the hypnotic is reaching the patient (cannula, infusion pump, vaporiser, end tidal agent), then deepen anaesthesia.

• Propofol: 0.5 to 1 mg/kg IV bolus, repeated once if needed; maximum 2 mg/kg cumulative for rescue; monitor blood pressure and airway; reduce in older adults and hypovolaemia.
• Midazolam: 1 to 2 mg IV; maximum 5 mg; monitor sedation and ventilation; halve in older adults and hepatic impairment.

Keep end tidal volatile agent at 0.7 age adjusted minimum alveolar concentration or more. Processed EEG helps when total intravenous anaesthesia is used with neuromuscular block (index 40 to 60), but it does not replace clinical checks.

After the event. Meet the patient early, listen, accept the account without argument, apologise, and explain the likely cause. Record the event, report it to the department, and offer psychological support. Early psychological therapy with trauma focused cognitive behavioural therapy reduces later symptoms.

• Sertraline (specialist initiated, for post-traumatic stress disorder): 25 to 50 mg orally once daily, titrated to 50 to 200 mg daily; continue 6 to 12 months; maximum 200 mg daily; monitor mood, suicidal thoughts and sodium; reduce dose in hepatic impairment.

Prevention. Use a checklist for drug and delivery, give a hypnotic before or with the relaxant, avoid delay after paralysis, monitor end tidal agent concentration, and use higher induction doses in high risk patients.

Monitoring. Follow up at 1 week and 4 weeks.

Referral and escalation. Clinical psychology or psychiatry for persistent symptoms.