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click to headphones. isolation barrier. impedance test signal. ~. C linical. 5kHz. ~. Tony_Fisher_eyes_closed_1.cfm. E ngineering. Visual B ASIC MATLAB. Royal Liverpool University Hospital.  PIC control. e +. SE90 17.5Hz. SE90 15.0Hz. 4.2k. 3.8k. 6.4k. 4.7k. e -. ref.

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click to headphones

isolation

barrier

impedance test signal

~

Clinical

5kHz

~

Tony_Fisher_eyes_closed_1.cfm

Engineering

Visual BASIC

MATLAB

Royal Liverpool University Hospital

PIC

control

e+

SE90 17.5Hz

SE90 15.0Hz

4.2k

3.8k

6.4k

4.7k

e-

ref

Active X

PIC

data flow

CMEX

ref

C code

e-

e+

0

0.5

2.0

1.5

1.0

4.2k

3.2k

switching network & electrodes

serial communication

embedded microcontrollers

filters, programmable gain & A-to-D converters

instrum. amplifiers

PC

Figure 4 Liv-LAS first-order time course

sb181201-whole.cln CEFAM-AEP (LIV-LASs)

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raw Liv-LAS

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low pass Liv-LAS

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Liv-LAS [arbitrary units]

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10

5

35

time base [samples/2]

voted Liv-LAS

0

Figure 5 Liv-LAS non-linear recovery

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smoothed voted Liv-LAS

time base [samples/2]

voted-voted Liv-LAS

Liv-LAS [arbitrary units]

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LivLAS form mlAEP

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OASS

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4/5

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0 5 15 20 min

3.0

Propofol μg.ml-1 plasma (target) concn.

A duplex cerebral function analyzer using the processed EEG and middle latency auditory evoked potential (ml-AEP) for anaesthesia and sedation monitoring

A.C.Fisher*, A.F.G.Taktak, A.G.Jones#, S.M.Mostafa#, G.Sidaras#

Depts. of Clinical Engineering and #Anaesthesia & Theatres (*[email protected])

  • How are data processed in RL-CeFAM?

  • At the lowest level, data-acquisition and first-order noise rejection are performed by algorithm implemented in C-language at the microcontroller level.

  • At the higher levels in the PC, data are processed using the mathematical language MatLab viz:

    • signal recovery by finite impulse response (FIR) bandpass filtering

    • mains noise elimination by adaptive filtering (LS Adaptive Cancellation)

    • non-linear artefact rejection using mathematical-morphology operations

    • frequency analysis (eg. S95, S50) via the Fast Fourier Transform

    • Graphical User Interfaces (GUI) constructed in MS Visual BASIC

      What exactly is the Liv-LAS Index for Depth of Anaesthesia?

      The mlAEP is the eeg response to a ‘click’ of sound presented to the patient via headphones. The Liv-LAS is the vector sum of the Laplacian difference vector of the recovered AEP over the period 20 to 125ms. This is a measure of ‘curviness’ (sic). See the AEP displayed in Figure 3b

      RL-CeFAM in action

      Figures 4 & 5 show the Liv-Las throughout 2hrs abdominal surgery with anaesthesia maintained by Propofol TIVA. The index is computed ~ 500 times per hour. The upper trace (Figure 4) shows the raw and first-order-filtered index. Figure 5 shows the index recovery after non-linear processing.

      Induction, maintenance and wake-up phases are clearly discernable.

  • Summary

  • Measuring the depth of anaesthesia during surgery is not as straight-forward as one would think

  • In addition to the main anaesthetic agent, the patient often receives other medication (for example: neuromuscular blockers) which hide the symptoms of any pain or discomfort if the anaesthesia is too weak

  • Similarly, the responses that might indicate if the anaesthesia is too strong can also be masked

  • The most common methods to be proposed analyse the electrical activity of the brain (the EEG: electroencephalogram)

  • In Liverpool, a novel system which uses two EEG analyses simultaneously has been designed and used to monitor patients undergoing a variety of surgical procedures

  • This approach is made possible by the availability of fast computers and recently-developed mathematical techniques.

RL-CeFAM:The Royal Liverpool Cerebral Function Anaesthesia Monitor

Figure 1: The headbox:

2 channels of EEG monitoring ... OR

1 channel of EEG and 1 channel of mlAEP

Figure 2: The headbox electronics schematic

Figure 3 RL-CeFAM Monitor Display

  • The acid test. How does RL-CeFAM relate to conventional scoring of anaesthetic depth?

  • There is no gold standard by which the performance of a new depth of anaesthesia monitor can be assessed. The frequently accepted comparative test uses the Observer's Assessment of Alertness/Sedation scale (OAAS) levels 1 - 5:

    • Awake

    • Slow response: slurred speech

    • Eyes closed: response to commands

    • Response to commands only after several attempts and mild prodding

    • No response to commands or shaking

    • The performance of RL-CeFAM wrt OASS 1 - 5 is illustrated below.

  • a: right channel EEG amplitude mode

    b: left channel AEP mode

    • What is measured in RL-CeFAM?

    • amplitude/time domain

      • continuous eeg (linear and logarithmic representation)

      • mean & median integrated amplitude

    • amplitude/frequency/time domain

      • compressed spectral array

      • S90- and S95-spectral edges

      • S50: 50th percentile (median) frequency

    • sampled time: evoked potential

      • auditory (AEP) middle latency 25 to 125ms

      • specialised indices

      • Liv-LAS: AEP-based depth of anaesthesia index

      • ... (modified Glasgow LAS level of arousal score)

      • continuous electrode impedance monitor

    • The 2 corner stones are the S50 median frequency and the

    • Liv-LAS index derived from the mlAEP (see above and Figure 4 & 5 )

    Figure 6: Liv-LAS course during 30 minutes of Propofol TIVA.

    The OASS staging (here superimposeda postiori) is consistent with the Depth of Anaesthesia Index derived in real-time by the RL-CeFAM


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