Depth of anaesthesia & awareness

Overview & definition

The depth of anaesthesia is the degree of the central nervous system depression — the assurance that the patient is the unconscious, the amnesic, and the unresponsive to the noxious stimulus. The accidental awareness under the general anaesthesia (the AAGA) is the failure of this assurance — the patient becomes consciously aware during the surgery, the most-feared complication by the patients and the psychologically-traumatic. The NAP5 (the Fifth National Audit Project of the Royal College of Anaesthetists and the Association of Anaesthetists) is the definitive study of the AAGA in the United Kingdom, the largest of its kind, and the basis of the modern prevention. The processed EEG (the BIS, the entropy) is the technological aid whose role is the nuanced.[1][4]

The components of the anaesthetic depth

The anaesthetic depth has the several components (the anaesthetic triad and the broader):[1]

• The hypnosis (the unconsciousness, the amnesia). The cortical depression — the loss of the awareness and the recall. The target of the depth monitoring.
• The immobility. The absence of the movement in response to the surgical stimulus — the spinal-reflex suppression, the target of the MAC and the relaxant.
• The analgesia. The suppression of the autonomic and the nociceptive response.
• The muscle relaxation. The neuromuscular blockade — the not the anaesthesia, the separate component that can mask the inadequate hypnosis (the paralysed-but-aware). [1]

The dissociation of the components (the deep relaxation, the light hypnosis) is the source of the awareness risk. The depth monitoring targets the hypnosis.[1]

The awareness — the definition and the types

The AAGA is the conscious awareness during the intended general anaesthesia. The types (the NAP5):[4]

• The explicit recall — the conscious recollection of the events during the anaesthesia (the auditory, the tactile, the pain, the paralysis). The most-disturbing, the psychologically-traumatic. The patients recall the conversations, the surgical manipulation, the pain, the inability to move.
• The implicit recall — the unconscious influence of the intraoperative events on the later behaviour (the no conscious memory), the difficult to the detect and the less-the-clinically-significant.
• The dreaming — the common, the usually-the-not-traumatic, the not-the-true-awareness.
• The painless vs the painful awareness — the painful awareness (the pain with the recall) is the most-traumatic.
• The movement during the anaesthesia — the Pandit work on the movements and the psychologically-traumatic awareness.[5]

The NAP5 — the incidence and the risk factors

The NAP5 (the 2014, the UK-wide) is the definitive study of the AAGA — the largest prospective collection of the cases, the analysis of the causation, and the recommendations. The overall incidence: about 1 in 19 000 the general anaesthetics (the lower than the earlier estimates), but the much higher in the high-risk groups.[4][6]

The NAP5 risk factors for the AAGA:[4][6]

• The neuromuscular blockade. The paralysed patient — the highest risk; the muscle relaxant masks the movement, the only sign of the light anaesthesia. The "the paralysed-but-aware" is the classic NAP5 scenario. The avoidance of the relaxant-without-the-hypnotic is the key.
• The total intravenous anaesthesia (the TIVA). The higher risk than the volatile (the no end-tidal monitor, the infusion error, the disconnection, the underdosing). The NAP5 flagged the TIVA.
• The cardiac, the thoracic, the caesarean, the trauma, the emergency. The light anaesthesia for the haemodynamic compromise, the rapid-sequence, the high-stimulus.
• The difficult airway and the prolonged intubation. The light anaesthesia during the airway manipulation.
• The obesity, the substance use, the age (the young).
• The equipment failure. The vaporiser empty, the TIVA pump disconnected, the no-alarm. [1]

The risk factors — the detailed

The detailed NAP5 causation (the themes):[4][5]

• The medication-related (the commonest). The underdosing, the neuromuscular blockade without the adequate hypnotic, the TIVA error, the rapid-sequence with the light hypnotic.
• The patient-related. The difficult-to-anaesthetise (the obesity, the chronic opioid, the substance use, the young age), the cardiac/haemodynamic compromise necessitating the light anaesthesia.
• The situational. The cardiac, the caesarean, the trauma, the emergency, the difficult airway — the light anaesthesia for the haemodynamics.
• The system and the equipment. The vaporiser failure, the TIVA pump error, the no-alarms, the inadequate monitoring. [1]

The processed EEG — the BIS and the entropy

The processed EEG (the pEEG) analyses the raw frontal EEG and reduces it to the dimensionless index of the hypnotic depth:[1][2]

• The Bispectral Index (the BIS). The 0 to 100 index (the 100 the awake, the 40 to 60 the recommended range, the below 40 the deep, the 0 the isoelectric). The incorporates the frequency, the bispectrum, the burst suppression. The first and the most-studied.
• The entropy (the state and the response). The 0 to 100 index, the derived from the EEG and the facial EMG. The 40 to 60 the recommended.
• The patient-state index, the Narcotrend, the cerebral-state index — the alternatives. [1]

The recommendation: the pEEG in the high-risk (the TIVA, the paralysed, the cardiac, the caesarean, the total intravenous, the high-risk-of-awareness), and the goal of the routine (the Berger-Estilita case for the standard of care).[2]

The evidence on the processed EEG

The evidence on the pEEG and the awareness prevention is the nuanced:[1][4]

• The B-Unaware (2008) and the BAG-RECALL (2011) trials. The randomised trials of the BIS-guided versus the end-tidal-anaesthetic-concentration (the ETAC) -guided anaesthesia in the high-risk. The results: the BIS did NOT clearly reduce the awareness compared with the ETAC protocol; the ETAC protocol was the effective. The trials challenged the routine BIS.
• The meta-analyses. The modest reduction of the awareness with the pEEG in the high-risk, but the no-clear-benefit over the careful ETAC monitoring for the volatile.
• The NAP5. The pEEG did NOT prevent the majority of the AAGA cases (the pEEG was the not-in-use, or the pEEG was the in-use-but-the-awareness-occurred, or the awareness was the "the pEEG looked adequate"). The pEEG is the not-the-guarantee.
• The current position. The pEEG is the adjunct, the especially-valuable in the TIVA (the no-ETAC) and the high-risk; the not-the-substitute for the adequate dosing and the vigilance. The goal of the routine monitoring is the evolving.[2][3]

The limitations of the processed EEG

The pEEG is the imperfect:[1][3]

• The artefact. The neuromuscular blockade (the artefactual suppression), the electrocautery, the warming, the movement, the pacemaker.
• The confounding agents. The ketamine and the nitrous oxide (the paradoxical rise), the dexmedetomidine (the different EEG), the elderly EEG (the slowed baseline).
• The not-the-awareness-guarantee. The pEEG may the "look adequate" during the awareness; the not-the-perfect-correlation.
• The deep-anaesthesia harm. The pEEG-driven deep anaesthesia (the BIS under 40) associated with the harm (the mortality, the delirium, the stroke in the some-studies — the "the too-deep").
• The cost and the training. [1]

The end-tidal volatile and the MAC

For the volatile anaesthesia, the end-tidal anaesthetic concentration (the ETAC) and the age-adjusted MAC are the practical depth monitors — the more-reliable than the pEEG for the volatile. The maintenance of the age-adjusted MAC above about 0.7 (the 0.7 to 1.3 the typical) reduces the awareness risk; the ETAC monitoring with the alarm is the standard. The ETAC is the not-available for the TIVA, hence the pEEG role.[1][4]

The high-risk situations

The situations with the elevated awareness risk, where the special attention and the monitoring are warranted:[4][6]

• The paralysed patient. The highest risk; the avoidance of the relaxant-without-the-hypnotic; the pEEG; the vigilance.
• The TIVA. The pEEG (the no-ETAC); the careful pump setup; the line check; the disconnection alarm.
• The cardiac anaesthesia. The light anaesthesia on the bypass; the high-stimulus; the pEEG.
• The caesarean section. The light anaesthesia for the neonate; the high awareness rate historically (the NAP5 obstetric — the Odor incidence).[6]
• The trauma and the emergency. The haemodynamic compromise; the rapid sequence; the light anaesthesia.
• The difficult airway. The light anaesthesia during the airway manipulation.
• The obese, the chronic-opioid, the substance use, the young. The difficult-to-anaesthetise.

The explicit and the implicit awareness — the follow-up

The AAGA report — the patient reports the memory of the events during the anaesthesia — requires the structured follow-up (the NAP5 recommendation):[4]

• The validation and the listening. The taking-the-report-seriously, the not-the-dismissal, the apology, the empathy, the explanation. The validation is the therapeutic.
• The documentation — the detailed account (the timing, the content, the pain, the paralysis, the impact), the timeline of the anaesthetic.
• The causation analysis — the root-cause (the underdosing, the equipment, the TIVA, the high-risk), the learning.
• The psychological support — the referral to the counselling, the psychological therapy (the CBT for the PTSD). The AAGA can cause the post-traumatic stress disorder (the PTSD), the anxiety, the dental fear, the avoidance of the medical care.
• The explanation — the honest, the detailed, the what-happened, the why, the prevention.
• The follow-up — the weeks, the months; the monitoring for the PTSD. [1]

The psychological consequences

The AAGA can cause the significant psychological harm — the PTSD (the re-experiencing, the avoidance, the hyperarousal, the nightmares), the anxiety, the depression, the dental fear, the avoidance of the medical care, the relationship strain. The early recognition and the psychological support (the CBT, the EMDR) improve the outcome. The minimising of the harm (the Graham work) is the NAP5 priority.[4][5]

The prevention — the checklist and the recommendations

The NAP5 and the consensus recommendations for the AAGA prevention:[4][2]

• The adequate dosing. The induction dose sufficient, the maintenance adequate (the age-adjusted MAC above 0.7, the TIVA adequate).
• The avoidance of the relaxant without the hypnotic. The never-give-the-relaxant-without-the-hypnotic; the check before the relaxant; the "the paralysed-but-aware" prevention.
• The equipment check. The vaporiser (the level, the agent), the TIVA pump (the setup, the line, the disconnection alarm), the breathing system (the leak, the disconnect).
• The monitoring. The ETAC and the alarm for the volatile; the pEEG for the TIVA and the high-risk.
• The alarm for the vaporiser failure. The low-anaesthetic-concentration alarm.
• The awareness prompt at the emergence. The asking of the patient, the documentation.
• The structured follow-up of any report.[4]

The paediatric awareness

The paediatric AAGA (the Sury NAP5) has the distinct features: the incidence the similar or the higher (the difficult-to-assess in the young), the different causes (the induction, the breath-holding, the inhalational), the different report (the children the not-always-the-volunteer, the parent-the-key), the behavioural changes (the sleep disturbance, the regression, the play-acting-the-anaesthetic). The age-appropriate assessment (the Brice questionnaire modified), the parent-the-informant, and the psychological support. The prevention: the adequate dosing, the avoidance of the trauma at the induction.[7]

[1] [1]

Red flags

[1] [1] [1] [1] [1]

References

1. [1]Yu H, et al. Depth of anesthesia monitoring: Current evidence, clinical impact, and future directions J Int Med Res, 2026.PMID 42333677
2. [2]Berger-Estilita J, et al. Making brain monitoring routine: why processed EEG monitoring should be standard practice J Clin Monit Comput, 2026.PMID 42247119
3. [3]Rieck E, et al. The potential of processed EEG monitoring systems in predicting postoperative cognitive deficits J Clin Anesth, 2026.PMID 42160881
4. [4]Graham M, et al. Minimizing the Harm of Accidental Awareness Under General Anesthesia: New Perspectives From Patients Misdiagnosed as Being in a Vegetative State Anesth Analg, 2018.PMID 28922237
5. [5]Pandit JJ, et al. Movements During Intended General Anesthesia and Psychologically Traumatic Accidental Awareness: Explanatory Role of the Efference Copy Network Anesth Analg, 2026.PMID 40911489
6. [6]Odor PM, et al. Incidence of accidental awareness during general anaesthesia in obstetrics: a multicentre, prospective cohort study Anaesthesia, 2021.PMID 33434945
7. [7]Sury MR, et al. Accidental awareness during anesthesia in children Paediatr Anaesth, 2016.PMID 27059416