Unintended awareness

Question 1

What is the incidence of unintended awareness?

Answer 1

1:660 anaesthetics

Question 2

What is the most commonly implicated risk factor for unintended awareness?

Answer 2

neuromuscular blocking drugs account for 85% of cases of awareness

Question 3

What is an “adequate” administration rate of volatile agents to prevent awareness?

Answer 3

MAC >0.7

Question 4

What is general anaesthesia?

Answer 4

A state of drug induced, reversible loss of consciousness whereby the patient is not conscious of their surgery or surroundings

Question 5

What is awareness?

Answer 5

After completion of anaesthesia, an individual has an explicit recall of intraoperative events with or without pain.

Question 6

What is the name of the structured interview that tries to detect awareness?

Answer 6

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?

Question 7

What is subconscious awareness?

Answer 7

“Implicit memory” It occurs when the patient develops memory for events during anaesthesia that they do not later recall

Question 8

How does awareness occur?

Answer 8

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

Question 9

What are risk factors for awareness in a patient with normal physiology?

Answer 9

• 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)

Question 10

What are the risk factors for awareness due to abnormal physiology?

Answer 10

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

Question 11

What life threatening emergencies are risk factors for awareness?

Answer 11

• severe bleeding/hypovolaemia
• severe septic shock
• cardiac arrest/peri-arrest

Question 12

What specialist surgeries are more associated with awareness?

Answer 12

• cardiac
• obstetric
• paediatric
• rigid bronchoscopy
• trauma

Question 13

What does increasing the concentration of anaesthetic drugs in the brain do?

Answer 13

Progressive suppression of all CNS activity including

• cortical
• spinal cord
• autonomic responses

Question 14

What is the gold standard technique for monitoring consciousness?

Answer 14

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.

Question 15

What are the 2 types of monitor available to measure depth of anaesthesia?

Answer 15

• EEG (measures spontaneous cortical electrical activity)
• evoked potentials (measures stimulus-evoked electrical activity)

Question 16

During anaesthesia, how does the EEG change?

Answer 16

From a high frequency, low amplitude signal (around 10 microVolts) to a more regular, lower frequency, higher amplitude signal.

Question 17

What will not affect the BIS?

Answer 17

• ketamine
• nitrous oxide
• xenon
• opioids

Question 18

What does the BIS do?

Answer 18

Depth of anaesthesia monitor.

Combines power spectral analysis with analysis of phase relationships (bispectral analysis) between the component frequencies of the EEG signal.

Question 19

How does narcotrend work?

Answer 19

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

Question 20

How does M-Entropy work?

Answer 20

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.

Question 21

What is the FEMG?

Answer 21

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

Question 22

What does the aepEX do?

Answer 22

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

Question 23

How is the AEP derived?

Answer 23

By averaging the previous 256 EEG waveforms

Question 24

What numbers should be aimed for on the BIS for adequate anaesthesia?

Answer 24

40 - 60

Question 25

What amplitudes do auditory evoked potentials detect?

Answer 25

Auditory evoked potentials have a low signal-to-noise ratio and are detected at amplitudes of <1 µ V.