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Welcome to ALARIS AEP session. Kaare Jevnaker Alaris Medical. Incidence of explicit recall. Remember being awake and recall things that were said or done during operation. Year. Incidence. Number of patients. Hutchinson 1960 1.2% 656 Harris 1971 1.6% 120

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Welcome to alaris aep session

Welcome toALARIS AEP session

Kaare Jevnaker

Alaris Medical


Incidence of explicit recall
Incidence of explicit recall

Remember being awake and recall things that were said or done during operation

Year

Incidence

Number of patients

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


Basic basic basic basic basic basic
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.

  • Let’s look at how tiny tiny this signal is.


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


Extracting the evoked responseBefore 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

click

100 ms


But lets make this more visible

But, lets make this more visible

Let’s see what happens when we send a click through the ear.


Position of electrodes

A deviation in the positioning of the electrodes up to 2 cm does not have significant influence on the ARX-index.


Place Headphones

To Monitor

Some prefer to wait with the headphones until electrodes are connected



Frontal cortex and

association areas

Medial geniculate and

primary auditory cortex

Acoustic nerve

and brainstem


What does the aep look like
What does the AEP Look Like?

Pa

Pa latency

+

0.1µV

Pa amplitude

Nb

100 msec


Basic knowledge
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!



Frontal cortex and

association areas

Medial geniculate and

primary auditory cortex

Acoustic nerve

and brainstem


The aep during anaesthesia

Desflurane

Pa

1.5%

Nb

3%

6%

The AEP during Anaesthesia

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


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.


Conclusions
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 1980’s


Aep signal processing how can it be so fast
AEP signal processing?How can it be so fast?


A-line Electrodes

Signal OK?

AMP

MTA256

sweeps

AAI

Calc.

Bandpass filter

AEP

25-65 Hz

Yes

ARX

MODEL

No

MTA18

sweeps

A/D

Converter

900 x

Sec.

Reject

Signal OK?

EEG + AEP + Artifact

Bandpass filter

EMG

65-85 Hz

EMG

Calc.

Yes

No

Bandpass filter

Burst Suppr.

1-35 Hz

AEP MTA256

BS%

Calc.

Reject

ALARIS AEP ™signal processing v. 1.4


1

2

3

4

5

6

7

8

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239

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256

257

Moving time Averaging and ARX

ARX

-model

MTA 256

sweeps

MTA 18

sweeps



Index calculation
Index calculation

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

=

93

  • And, an almost flat curve gives a low index

=

16


What it is
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


A typical case

Induction

Awake

Burst Suppression

Utter boredom

EMG

Start of surgery

End of operation

Intubation

A typical case


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

TIVA with induction and Maintenance would have prevented this

Patient still not deep enough and reacts. Remember: 50% sleep at 1 MAC

Intubation too soon. Fentanyl had not reached peak effect.

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.


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











Use me

again soon


A good case
A good case

  • Just to illustrate how important it is.

  • Customer couldn’t 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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