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 ( * liv.ac.uk ) C linical E ngineering Royal Liverpool University Hospital 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. e+e+ e-e- e-e- e+e+ ref  PIC control  PIC data flow ~ ~ 5kHz isolation barrier Visual B ASIC MATLAB Active X PC serial communication embedded microcontrollers filters, programmable gain & A-to-D converters instrum. amplifiers switching network & electrodes click to headphones C code CMEX impedance test signal 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 SE HzSE Hz 4.2k 3.8k6.4k4.7k Tony_Fisher_eyes_closed_1.cfm Figure 3 RL-CeFAM Monitor Display 4.2k 3.2k a: right channel EEG amplitude mode b: left channel AEP mode / OASS Propofol μg.ml -1 plasma (target) concn min LivLAS form ml AEP 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. 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 ) 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: 1. Awake 2. Slow response: slurred speech 3. Eyes closed: response to commands 4. Response to commands only after several attempts and mild prodding 5. No response to commands or shaking The performance of RL-CeFAM wrt OASS is illustrated below. Figure 6: Liv-LAS course during 30 minutes of Propofol TIVA. The OASS staging (here superimposed a postiori) is consistent with the Depth of Anaesthesia Index derived in real-time by the RL-CeFAM time base [samples/2] sb whole.cln CEFAM-AEP (LIV-LASs) Liv-LAS [arbitrary units] time base [samples/2] raw Liv-LAS low pass Liv-LAS Liv-LAS [arbitrary units] voted Liv-LAS smoothed voted Liv-LAS voted-voted Liv-LAS Figure 4 Liv-LAS first-order time course Figure 5 Liv-LAS non-linear recovery