What is the incidence of unintended awareness?
1:660 anaesthetics
What is the most commonly implicated risk factor for unintended awareness?
neuromuscular blocking drugs account for 85% of cases of awareness
What is an “adequate” administration rate of volatile agents to prevent awareness?
MAC >0.7
What is general anaesthesia?
A state of drug induced, reversible loss of consciousness whereby the patient is not conscious of their surgery or surroundings
What is awareness?
After completion of anaesthesia, an individual has an explicit recall of intraoperative events with or without pain.
What is the name of the structured interview that tries to detect awareness?
Brice Asked immediately after anaesthesia and again within a few days: - what was the last thing you remembered happening before you went to sleep? - what is the first thing you remember happening on waking? - did you dream or have any other experiences whilst asleep? - what was the worst thing about your operation? - what was the next worse?
What is subconscious awareness?
“Implicit memory” It occurs when the patient develops memory for events during anaesthesia that they do not later recall
How does awareness occur?
When too little anaesthetic is delivered to the brain. The anaesthetist has failed to administer enough anaesthetic to maintain unconsciousness.
This may occur due to:
- a result of inadequate checking at the start of lists or between cases (eg the ACGO left open, circuit disconnected or faulty equipment)
- drug errors (cefuroxime given instead of thiopentone)
- undetected failure of IV cannulae or infusion devices
What are risk factors for awareness in a patient with normal physiology?
- unrecognised equipment failure
- reduced practitioner vigilance (eg empty vaporizer)
- underdosing for LSCS due to fears for baby
- unexpected difficult intubation + non-supplementation of anaesthesia
- using sole anaesthetic N2O/O2 or midazolam
- following transfer of patient
- TIVA (failure of drug delivery or poor understanding of pharmacology)
What are the risk factors for awareness due to abnormal physiology?
Masked physiology:
- complete heart block
- hypothyroidism
- autonomic neuropathy
- beta blockers
Drug resistance:
- prior Hx of awareness (individual genetic variability)
- excessive alcohol intake
- regular amphetamine/cocaine/opiate use
- chronic pain syndromes
Poor cardiovascular reserve:
- EF <30%
- severe aortic stenosis
- ASA >=4
What life threatening emergencies are risk factors for awareness?
- severe bleeding/hypovolaemia
- severe septic shock
- cardiac arrest/peri-arrest
What specialist surgeries are more associated with awareness?
- cardiac
- obstetric
- paediatric
- rigid bronchoscopy
- trauma
What does increasing the concentration of anaesthetic drugs in the brain do?
Progressive suppression of all CNS activity including
- cortical
- spinal cord
- autonomic responses
What is the gold standard technique for monitoring consciousness?
The isolated forearm technique which measures responsiveness as a surrogate for consciousness.
However, only 50% of patients who respond to command with this can later recall doing so.
What are the 2 types of monitor available to measure depth of anaesthesia?
- EEG (measures spontaneous cortical electrical activity)
- evoked potentials (measures stimulus-evoked electrical activity)
During anaesthesia, how does the EEG change?
From a high frequency, low amplitude signal (around 10 microVolts) to a more regular, lower frequency, higher amplitude signal.
What will not affect the BIS?
- ketamine
- nitrous oxide
- xenon
- opioids
What does the BIS do?
Depth of anaesthesia monitor.
Combines power spectral analysis with analysis of phase relationships (bispectral analysis) between the component frequencies of the EEG signal.
How does narcotrend work?
Uses power spectral analysis and automated pattern recognition algorithms to classify the EEG into stages (A-F) and generate an index of depth of anaesthesia
How does M-Entropy work?
Analyses the amount of disorder of the EEG signal (entropy) - usually decreased entropy during anaesthesia.
Also measures the irregularity of the frontalis electromyogram (FEMG) which diminishes as anaesthesia deepens.
What is the FEMG?
The frontalis electromyogram is an indication of analgesic adequacy (“response” entropy).
The state entropy and response entropy should be the same value during anaesthesia, but if the response entropy diverges more than 10 points from the state entropy the analgesia component of anaesthetic may be inadequate
What does the aepEX do?
Generates clicks at 7Hz through earphones and records EEG response.
The evoked responses are generated by synapses during the passage of signal from the cochlea, through the brainstem to the cortex, and are extracted from the EEG signal by digital averaging.
Anaesthesia has a dose dependent decrease of amplitude and increased latency of the early cortical responses following the auditory stimulus
How is the AEP derived?
By averaging the previous 256 EEG waveforms
What numbers should be aimed for on the BIS for adequate anaesthesia?
40 - 60
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Neuromuscular blocking agents97
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Applied epidural anatomy27
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Practical Gastric Physiology62
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Basics of electricity for anaesthetists64
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Electrical Safety in the Operating Theatre37
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Foetal Circulation40
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Cerebral oximetry20
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Arterial pressure waveform analysis16
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Unintended awareness25
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Point of care coagulation testing37
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Mathematical relationships in anaesthesia and ICU19
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Minimally invasive cardiac output monitors29
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Respiratory Gas Analysis28
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Blood Pressure Measurement41
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Monitoring of Neuromuscular Block50
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Conventional and near-patient tests of coagulation42
