Welcome to ALARIS AEP session

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Welcome toALARIS AEP session

• Kaare Jevnaker
• Alaris Medical

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The patient

• The patient has 2 worries
• 1 Will he sleep during the operation?
• 2. Will he wake up after the operation?

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History

• 1965 First article about different levels of
  anaesthesia. (States of awareness during general
  anaesthesia)
• Explicit and implicit memory. Different cognitive
  stages.
• Awareness today is related to Explicit recall

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Incidence of explicit recall
Remember being awake and recall things that were
said or done during operation
Number of patients
Year
Incidence

• Hutchinson 1960 1.2 656
• Harris 1971 1.6 120
• McKenna 1973 1.5 200
• Wilson 1975 0.8 490
• Flier 1986 1.4 140
• Liu 1991 0.2 (0.3) 1000 (684)
• Nordström 1997 0.2 (0.2) 1000 (1000)
• Ranta 1998 0.4 - 0.7 2612
• Myles 2000 0.11 10811
• Sandin 2000 0.15 (0.18) 11785 (7757)

The first half is not relevant today because the
anaesthesia technique has changes a lot.
With kind permission from Dr Rolf Sandin, Kalmar,
Sweden
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Why monitor sleep?It is important to think about
this.

• The implications of undersedation
• Patient remains immobilized but feels pain
• Although it is occurs in only 0.1 - 0.2 of all
  surgeries, 23 million surgeries are performed in
  the U.S. each year
• Resulting in 35,000 cases of surgical awareness
• The implications of oversedation
• To avoid the possibility of surgical awareness
  too much hypnotics and analgesics may be
  administered
• The patients recovery time is extended higher
  room cost
• More drugs than necessary are used higher drug
  cost

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Auditory Evoked Potentials
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Basic basic basic basic basic basic

• The hearing is the last sense that leaves and the
  first that returns during anaesthesia.
• AEP is just the brain response to a click stimuli
  through the hearing nerve
• AEP is a very weak electrical signal wrapped in
  the EEG background actvity.
• Lets look at how tiny tiny this signal is.

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Burst Suppression
We look at spikes lt 3,5 uV. In contrast An
awake Pa amplitude is typically 0.7 uV. And, an
asleep amplitude is typically 0.4 uV
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400 x
40 x
ECG signal has approx. 400 x amplitude than the
AEP signals. EEG signal has approx. 40 x
amplitude than the AEP signal
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Extracting the evoked response Before A-Line it
took too long to detect and present (extract)
this weak signal, because it requires advanced
signal processing
1 click
128 clicks
256 clicks
1024 clicks
100 ms
click
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But, lets make this more visible

• Lets see what happens when we send a click
  through the ear.

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Position of electrodes
A deviation in the positioning of the electrodes
up to 2 cm does not have significant influence on
the ARX-index.
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Place Headphones
To Monitor
Some prefer to wait with the headphones until
electrodes are connected
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The auditory Pathway
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Medial geniculate and
primary auditory cortex
Acoustic nerve
and brainstem
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What does the AEP Look Like?
Pa
Pa latency

0.1µV
Pa amplitude
Nb

100 msec
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Basic knowledge

• The early cortical AEP waves called Pa and Nb,
  which occurs between 20 and 80 ms reflects the
  activity in the temporal lobe/primary auditory
  cortex ( the site of sound registration)
• Changes in the latency of these waves ( in
  particular the Nb wave) are highly correlated
  with a transition from awake to loss of
  consciousness
• Changes in the amplitude of these waves reflects
  the interplay of general anaesthetics,surgical
  stimulation and the obtunding of the latter by
  analgesics!

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And, this is what happens
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Medial geniculate and
primary auditory cortex
Acoustic nerve
and brainstem
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And, of the opposite during awakening
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N
N
N
N
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1
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1
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The AEP during Anaesthesia
With kind permission from Dr Christine Thornton,
Northwick Park, London, UK.
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Effect of intubation on the AEP
Pa

0.1µV
Nb
Post-intubation
Pre-intubation
100ms
With kind permission from Dr Christine Thornton,
Northwick Park, London, UK.
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The Effect of Midazolam on the AEP
Awake
Nb
Loss of eyelash response
0
100
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Time (ms)
With kind permission from Dr Christine Thornton,
Northwick Park, London, UK.
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The effect of propofol on the AEP
Response to command
Nb
No response to command
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100
50
Time (ms)
With kind permission from Dr Christine Thornton,
Northwick Park, London, UK.
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Effects of opioids on the AEP

• Controversy exists as to whether opioids affect
  the AEP directly or indirectly.
• A study of 2 groups
• 1 group was given an Opioid
• 1 group got normal saline prior to tracheal
  intubation under general anaesthesia

With kind permission from Dr Christine Thornton,
Northwick Park, London, UK.
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Opioids
With kind permission from Dr Christine Thornton,
Northwick Park, London, UK.
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Conclusions

• The saline group had statistically significant
  50 higher increase in Pa amplitude!
• But, is it a direct or indirect effect?
• The Opioid group results could be because the
  pain of intubation was blunted rather than a
  direct effect on AEP itself.
• However It demonstrates that the AEP meets the
  clinical expectations of a signal which monitors
  depth of anaesthesia.

With kind permission from Dr Christine Thornton,
Northwick Park, London, UK.
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Drawbacks of AER

• It will not work in deaf people. The extent to
  which the AEP changes are affected needs to be
  explored
• A large amount of information is produced and
  before the ALARIS AEP ? Monitor you got the
  feedback too late. It took 2-3 minutes to collect
  an average response.

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Conclusions

• Graded changes with depth of anaesthesia
• Similar changes for different anaesthetics
• Shows response to noxious stimulation
• AEP indicates level of consciousness
• Technology has been studied since early 1980s

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AEP signal processing?How can it be so fast?
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ALARIS AEP signal processing v. 1.4
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Moving time Averaging and ARX
ARX -model
MTA 256 sweeps
MTA 18 sweeps
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Index calculation?
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Index calculation?
AEP window 20-80 ms
xi xi1
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Index calculation

• So, then you have a real curve, the index is high

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• And, an almost flat curve gives a low index

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What it is

• AAI is typically higher than 60 when the patient
  is awake and decreases when the patient is
  anaesthetised loss of consciousness typically
  occurs when the AAI is below 30

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A typical case
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Fentanyl 0,15 Pentothal 250mg
Intubation. Sevo FI 0,2
Start surgery. Gyn. Lap. procedure . FI 1,0 MAC
1,0
Moved Patient on table
Tracrium 15mg
Index dropped and NMB was given to prepare
intubation
Intubation too soon. Fentanyl had not reached
peak effect.
TIVA with induction and Maintenance would have
prevented this
Patient still not deep enough and reacts.
Remember 50 sleep at 1 MAC
Induction started with normal doses
Penthotal dose was small for this patient. Gas
conc. too low
Patient was not deep enough to be moved on table.
Dose of gas too low.
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Put in trocar (insertion tube for scope) FI 1,8
MAC 1,4
Sevo stopped FI 0,7 MAC 0,9
At MAC 1,4 the patient is deep enough and all
problems stops
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Start Up
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Induction is given
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EMG starts to drop
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Burst Suppression appears
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Starting to wake up
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Observe Alarm and EMG
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Operation over
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Exit
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Check and transfer DATA
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Use me again soon
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A good case

• Just to illustrate how important it is.
• Customer couldnt understand why the index was
  high?
• Complained that something was wrong
• All details captured by our man
• After downloading and descriptions the clinicians
  agreed the anaesthesia was not optimal.
• They could actually see things they never seen
  before

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Questions?