Anaesthetic Awareness

Accidental intraoperative awareness is the ability to recall any events which occurred during the period at which it was intended that the patient was unconscious. Awareness:

Occurs in ~1:20,000 cases
- Incidence with NMB is ~1:8200 cases
- Incidence without NMB is ~1:135,000
- Dynamic phase of anaesthesia is the most common time
  i.e. Induction and emergence.
May lead to memory formation
Memories may be:
- Explicit
  Memory of exact events, which may be:
  - Recalled spontaneously
  - Provoked by post-operative events or questioning
- Implicit
  Not consciously recalled events, but may affect later behaviour.
May cause significant psychological harm
- Insomnia
- Depression
- Anxiety
- PTSD

Risks

Key factors affecting risk of awareness include:

Depth of anaesthesia
- Inspired agent concentration
  - Rare when MAC >0.8
  - Virtually non-existent when MAC >1
    Once steady state has been reached.
- Poor anaesthetic technique
  - Omission or late commencement of volatile
  - Underdosing of induction agent
  - Underdosing during hypotensive episodes
    Titration of anaesthetic agent to BP ↑ risk of awareness.
  - Failure to recognise signs of light anaesthesia
Patient factors
↑ anaesthetic requirement in:
- Young age
- Hyperthyroidism
- Obesity
- Anxiety
- Drug exposures
  - Smokers
  - Heavy alcohol
  - Recreational drug users
  - Repeated anaesthetics
Neuromuscular blockade
Greatly ↑ both the incidence and severity of awareness. Particularly:
- Use of non-depolarising agents
- Failure to monitor effect of non-depolarising agents
High risk surgery
Usually due to intentional dose reduction to minimise haemodynamic effects.
- Cardiac surgery
- Emergency surgery
- Significant haemorrhage
- Caesarian section
Equipment malfunction

Clinical Manifestations

May be masked by β-blockers and anti-muscarinics, among other things

Relate to sympathetic activation:

↑ HR
↑ BP
Sweating
Crying
Movement/grimacing
Tachypnoea
Pupillary dilatation and reactivity

Management

Includes:

Preventative
Intraoperative management
Post-operative management

Preventative

Premedication
Use of benzodiazepines reduces incidence.
Use of at least 0.8 MAC of volatile

Intraoperative Management

Use of processed EEG monitoring
- May ↓ awareness in high risk patients (NNT ~140)
- Effect confounded by muscle relaxants
Rapidly deepen anaesthesia
If hypotension is present, support BP whilst deepening anaesthesia
Consider administration of IV benzodiazepine
Will not provide retrograde amnesia, but anterograde amnesia reduces evidence of further recall.

Postoperative Management

Obtain and document a detailed account of the experience:
* Perioperative timing * Distinguish between dreaming and awareness * Note details of recalled events
* The Brice Questionnaire is a traditional approach to assessing events: * Performed twice * First at 24-48 hours * Second at 7-8 days * Consists of five questions: * What was the last thing you remembered happening before you went to sleep? * What is the first thing you remember on waking? * Did you dream, or have any other experience whilst you were asleep? * What was the worst thing about the operation?” * What was the next worst thing?

Provide:

Support and sympathy
Denial of veracity of events worsens psychological outcome.
Early referral to psychiatric and counselling services

References

Hardman JG, Aitkenhead AR. Awareness during anaesthesia. Contin Educ Anaesth Crit Care Pain. 2005 Dec 1;5(6):183–6.
Pandit JJ, Andrade J, Bogod DG, Hitchman JM, Jonker WR, Lucas N, et al. 5th National Audit Project (NAP5) on accidental awareness during general anaesthesia: summary of main findings and risk factors. Br J Anaesth. 2014 Oct 1;113(4):549–59.
Brice DD, Hetherington RR, Utting JE. A Simple Study of Awareness and Dreaming During Anaesthesia. British Journal of Anaesthesia. 1970 Jun;42(6):535–42.