Awareness under general anesthesia

1. Unintended Awareness and
Depth of Anaesthesia Monitoring
Dr. Ahmed Eldesoky

2. Media Coverage
• ‘AWAKE’ will do to anaesthesia what
‘JAWS’ did to swimming in the ocean.
• Movie producer Joana Vicente,
commenting on her movie “Awake”
• A “psychological thriller” that tells the story
of a man who is awake but paralyzed
during surgery.

3. • General anaesthesia is a state of drug-induced, reversible
loss of consciousness.
• Implicit in this description (and consistent with patient
expectations) is that from the time of induction of
anaesthesia to emergence, patients will not be conscious
of their surgery or their surroundings.

4. Awareness
• The situation that occurs when a patient under general
anesthesia becomes aware of some or all events during
surgery or a procedure, and has direct recall of those
events.
• Most episodes of awareness are avoidable.
• Awareness during anesthesia is a very disturbing event if
encountered.
• Memories of the event are remembered spontaneously
or provoked by post-op events.
• Recall of such events specially if awareness of paralysis
and painful stimuli is the issue then ones life maybe
changed permanently.

5. INCIDINCE
A WORLD WIDE PROBLEM

6. Types of Memory

7. • Explicit memory may be recalled spontaneously, or may
be provoked by postoperative events or questioning.
• Implicit memory may not be consciously recalled, but
may affect behavior or performance at a later time
• General anaesthesia suppress cortical activity, and are
thought to disrupt the connectivity of cortical areas and
subcortical–cortical connections in a dose-dependent
fashion.
• Some degree of information processing occurs in lighter
planes of anaesthesia even though patients appear to be
adequately anaesthetized.

8. Intraoperative Awareness
WHY AWARENESS OCCURS ?

9. ASA Closed Claims
Causes of Awareness in ASA Closed Claims (n=71)

10. Risk Factors

11. Role of muscle relaxants
• Under general anesthesia, the patient's muscles may be
paralyzed in order to facilitate tracheal intubation,
surgical exposure or mechanical ventilation.
• It is incorrect to think that physiological signs such as
tachycardia, hypertension, mydriasis, sweating, and
lacrimation will continue to occur normally in response to
pain in the anesthetized state.
• If neuromuscular blocking drugs are used this causes
skeletal muscle paralysis and interferes with the
functioning of the autonomic nervous system. The
patient cannot signal their distress and they may not
exhibit the signs of awareness that would be expected to
be detectable by clinical vigilance.

12. Depth of Anaesthesia Monitoring
• Isolated Forearm Technique
• EEG Derived Monitors:
 Bispectral Index
 The Narcotrend
 M-Entropy
 aepEX

13. Isolated Forearm Technique

14. Isolated Forearm Technique
• There is acceptance amongst a majority of experts in the
field of awareness that the isolated forearm technique,
which measures responsiveness to command as a
surrogate for consciousness, is the ‘gold standard’
technique against which other monitors should be
validated.
• However, only 50% of patients who respond to
command with an isolated forearm can later recall doing
so.

15. Bispectral Index

16. Bispectral Index
• BIS combines power spectral analysis with analysis of
phase relationships (bispectral analysis) between the
component frequencies of the EEG signal.
• Measures patients response to sedative/hypnotics
administration.
• Non-invasive.
• Converts the generated EEG Data into a number.
• Ideal numbers under GA are between 40-60.
• According to some studies, BIS uses led to a more
precise dosing of medication and less time till recovery
leading to high turnover of patients.

17. Bispectral Index
• Other studies, found claims of awareness in spite a
value of 40 intra-op. This studies concluded that BIS
maybe effective in measuring hypnotic state yet
awareness still can occur even with a low BIS value.
• A recent study concluded that values between 50-60
were insufficient to prevent awareness during intubation
with propofol or alfentanil use.

18. Narcotrend

19. Narcotrend
• A new EEG monitor designed to measure the depth of
anaesthesia.
• Narcotrend uses power spectral analysis and automated
pattern recognition algorithms to classify the EEG into
stages from A (awake) to F (general anaesthesia with
increasing burst suppression) and generate an index of
depth of anaesthesia.
• In comparison with bispectral index monitoring during
propofol-remifentanil-anaesthesia. The Narcotrend
stages D or E are assumed equivalent to BIS values
between 64 and 40 indicating general anaesthesia.

20. M-Entropy

21. M-Entropy
• It analyses the amount of disorder in the EEG signal
(‘state’ entropy). During anaesthesia, the EEG signal
becomes more regular, resulting in decreased entropy.
• M-Entropy also measures the irregularity of the frontalis
electromyogram (FEMG), which diminishes as
anaesthesia deepens, providing an indication of
analgesic adequacy (‘response’ entropy).
• During anaesthesia, state entropy and response entropy
normally have the same value, but if response entropy
diverges by more than 10 points from the state entropy
value the ‘analgesic’ component of the anaesthetic may
be inadequate.

22. aepEX

23. aepEX
• This device generates loud clicks via earphones at 7 Hz
and records the EEG response.
• The evoked responses are generated by synapses
during the passage of the signal from the cochlea,
through the brainstem to the cortex, and are extracted
from the EEG signal by digital averaging.
• All anaesthetics decrease the amplitude and increase
the latency of the early cortical (mid-latency) responses
following the auditory stimulus in a dose-dependent
manner.

24. • All of these technologies may be affected by electrical
interference (e.g. surgical diathermy, pacemakers,
muscle artifact) and depressed cortical metabolism as a
result of ischaemia or hypothermia.
• All the monitors have a variable lag-time between a
state change and a displayed change in index values.
• Although this is unimportant in steady-state conditions,
it may be relevant where there is a sudden surge in
noxious stimulation and where analgesia may not be
adequate.
• The use of any depth of anaesthesia monitor should
always augment individual practitioner judgement, and
complement standard clinical methods of assessment.

25. Strategies for preventing and dealing
with awareness
• Most awareness is avoidable, and the incidence can be
reduced by 50% through audit and education.
• Therefore, merely drawing attention to the issue and
improving anaesthetists’ understanding is an important
preventive measure in itself.

26. Vigilance
• Preoperative assessment will identify relevant risk factors.
• All apparatus should be checked before the start of every
list and if necessary between cases.
• Drug errors may be minimized by meticulous preparation
according to best practice.
• A depth of anaesthesia monitor should be considered in the
context of TIVA, when an additional risk factor for
awareness is present, or both.

27. Vigilance
• Supplementary doses of induction agent should be given in
the event of an unexpected difficult intubation ( particularly
for rapid sequence intubation).
• Extra caution is required during certain types of surgery,
during transfer, and at the end of a case in order to avoid
premature lightening of anaesthesia.
• Persistent tachycardia or hypertension require
investigation.

28. Anaesthetic technique
• A protocol-guided approach using end-tidal agent alarms
is effective in reducing the incidence of awareness.
• Nitrous oxide and benzodiazepines are only weak
hypnotics, and have unreliable effects on memory
formation.
• In particular, short-acting benzodiazepines (such as
midazolam) are likely to become ineffective during the
maintenance phase without subsequent redosing.
• Benzodiazepines do not retrogradely block memory for
events, and their use in this setting does not prevent
awareness or recall.

29. Anaesthetic technique
• An age-corrected MAC value of 0.7 should be
maintained at all times (preferably using an alarm), as
this correlates well with suppression of consciousness.
• Target-controlled TIVA systems are based on mean
population pharmacokinetics. The measured plasma
concentration (Cp) of the drug shows great inter-
individual variability.
• So that many TIVA practitioners induce anaesthesia
slowly with a target-controlled infusion, and pay attention
to the estimated Cp where verbal response is lost as a
guide to subsequent anaesthetic requirements in
individual patients.

30. When awareness has
occurred or is suspected

31. When awareness has
occurred or is suspected
• Complaints of unintended awareness should always be
taken seriously, and should be followed up by a senior
anaesthetist.
• A full explanation and sympathy should be offered, and
counselling provided.
• Apology is not an admission of guilt, and may be all that
patients are looking for.
• If negative psychological symptoms are present and
persist for more than 4 weeks, referral to psychological
services is warranted in order to minimize long-term
harm which may be devastating for patients and their
relatives.

32. Conclusion
• The incidence of unintended awareness may be higher than
commonly perceived at around 1:660 anaesthetics.
• Neuromuscular blocking drugs remain the most commonly
implicated risk factor for unintended awareness.
• An adequate administration rate of anaesthetic drugs (MAC
0.7 for volatile agents) should be maintained and the
haemodynamic consequences of anaesthesia should be
dealt with separately.

33. Conclusion
• Benzodiazepines do not prevent awareness or recall.
• Depth of anaesthesia monitors may be helpful,
particularly for TIVA, but may not be better than close
attention to end-tidal volatile agent concentration.
• All reported cases of intraoperative awareness should be
taken seriously.
• Early counselling can reduce long-term psychological
harm.