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BY-NC-ND 3.0 license Open Access Published by De Gruyter September 12, 2015

Automated classification of stages of anaesthesia by populations of evolutionary optimized fuzzy rules

  • C. Walther EMAIL logo , A. Wenzel , M. Schneider , M. Trommer , K.-P. Sturm and U. Jaeger

Abstract

The detection of stages of anaesthesia is mainly performed on evaluating the vital signs of the patient. In addition the frontal one-channel electroencephalogram can be evaluated to increase the correct detection of stages of anaesthesia. As a classification model fuzzy rules are used. These rules are able to classify the stages of anaesthesia automatically and were optimized by multiobjective evolutionary algorithms. As a result the performance of the generated population of fuzzy rule sets is presented. A concept of the construction of an autonomic embedded system is introduced. This system should use the generated rules to classify the stages of anaesthesia using the frontal one-channel electroencephalogram only.

1 Introduction

The detection of stages of anaesthesia is performed by using vital signs like heart rate, blood-pressure or oxygen saturation for example. The anaesthetist is reacting to the values of these signs to de- or increase the concentration of the narcotic drugs to control the stage of anaesthesia of the patient. But the regular used vital signs are not indicating the depth of consciousness directly. In addition the patterns of the electroencephalogram (EEG) indicate the stages of anaesthesia while performing a surgical operation [14]. By assessing these EEG patterns the anaesthetist is getting a meaningful value to estimate the state of the patient. The structure of the electroencephalographic patterns differs throughout the different stages of anaesthesia. So to react to the patterns of EEG directly could be a heavy task for the anaesthetist. A simple EEG-based value is needed to add a robust indicator for classification of stages of anaesthesia. Therefore an automatic approach of a classification-algorithm could be implemented into an embedded system. This system reacts only to the variation of the frontal one-channel pattern of EEG and is indicating three stages of anaesthesia: “Wake”, “Anaesthesia” and “Asphyxia”.

2 Methods

To establish an autonomic device which is able to classify the different stages of anaesthesia the design of a classifier system is necessary in a first step. The training of the classification model should be established before implementing this classification model to an embedded system or microcontroller-based system. So the training itself has to be performed on a standalone computer or computer grid. In a second step the hardware itself has to be constructed. This hardware will capture the frontal one-channel EEG and processes a feature extraction. These features are than presented to the implemented classification approach. At least the classification system will calculate the different stages of anaesthesia and present it to the anaesthetist in a simple way.

2.1 Design of the classification model

The overall database is defined by datasets of the frontal one-channel EEG collected from different patients and chirurgical operations. The training data set is defined by 18 records collected throughout different surgical operations. The validation data set includes 20 additional independent records. In a first step features were extracted and used to train different classification approaches. To enable a linguistic interpretation of the classification system fuzzy rules were selected to evaluate the extracted features sets. By the means of multiobjective evolutionary algorithms simple fuzzy rules were generated [5]. These rules are able to distinguish between the three anaesthetic stages “Wake”, “Anaesthesia” and “Asphyxia”. The number of rules is limited to five rules for each class. To enhance the robustness of the classification system there were populations of rule sets generated. These populations include up to 18 independent rule sets. The population itself was established by the applied training algorithm [4]. The constructed classification system has a very simple structure connected to an appropriate value of robustness. A linguistic interpretation of the rule sets is possible. So there were generated efficient classifiers to support the anaesthetist in defining the actual anaesthetic stage of the patient. In addition by using fuzzy rules a linguistic interpretation could be performed to estimate the importance of the extracted EEG features.

2.2 Construction of the embedded system

An embedded system should be used to analyse the frontal one-channel EEG in an autonomic way. The construction of this device has to be transposed in a simple and efficient way. The generated population of fuzzy rules should be implemented to work as an essential part of the firmware. Therefore some hardware system approaches were presented in [69].

3 Results

Populations of fuzzy rule sets were generated by applying the algorithms presented in [4]. In a first step the selection of the linguistic variables and the corresponding rule structure for each fuzzy rule set was optimized. Therefore different Multiobjective Evolutionary Algorithms (MOEA) were used [4]. The Non-dominated Sorting Genetic Algorithm II (NSGAII) presented in [10], the Strength-Pareto Evolutionary Algorithm 2 (SPEA2) and the Indicator-based Evolutionary Algorithm (IBEA) [11, 12] generated populations of fuzzy rule sets. In a second step the number of fuzzy rule sets was reduced by using a genetic algorithm applied for each population. As presented in [4] for each MOEA-specific population a subset of fuzzy rule sets was extracted. The genetic algorithm determined for the NSGAII-optimized population 3 fuzzy rule sets as an optimal classification model. Out of the SPEA2-optimized population also 3 fuzzy sets were specified. The IBEA-optimized population provides 5 fuzzy rule sets after applying the genetic selection algorithm.

In Addition to the results presented in [4] these 11 different fuzzy sets can be used to classify the stages of anaesthesia in common. To raise the efficiency the genetic selection algorithm can be applied to this new population of fuzzy rule sets. The generation count was limited to 500. As a main result of this selection procedure 7 fuzzy rule sets were found to classify the stages of anaesthesia of 20 independent features sets of validation data. The classifi-cation performance of these fuzzy rule sets is presented in table 1.

Table 1

Median performance of the optimized fuzzy rule sets in comparison to the manual classification.

Given ClassificationAutomatic Classification
WakeAnaesthesiaAsphyxia
Wake78.42%8.99%12.22%
Anaesthesia0.34%86.77%12.94%
Asphyxia0.00%12.63%85.65%

The results presented in table 1 illustrate the median values of the decision vectors generated for each cell of 20 performance matrices. The matrices itself correspond to the classification output of each fuzzy set of the produced population. Each line of the table matches the manual rated classification of the experts. Each column represents the median degree of concordance in percentage between the manual rated epochs of EEG and the results received by the population of fuzzy rule sets. The anaesthetic stages “Wake”, “Anaesthesia” and “Asphyxia” indicate an accordance of 78.42%, 86.77% and 85.65%. The given anaesthetic stage “Asphyxia” was not classified as “Wake” by the automatic approach as presented in table 1.

4 Conclusion

By applying the general training algorithm as presented in [4] populations of optimized fuzzy sets were generated. These fuzzy rules are able to classify stages of anaesthesia by evaluating the frontal one-channel electroencephalogram. The fuzzy rules were optimized using the multiobjective evolutionary algorithms NSGAII, SPEA2 and IBEA. For each MOEA an independent population of fuzzy sets was generated. The size of each population was reduced by a genetic algorithm. The remaining fuzzy sets of each population were used to create a new population of effi-cient fuzzy rules. To reduce the computation effort in an additional way the genetic search algorithm was applied again to this new population. The number of fuzzy sets was reduced from 11 to 7. In relation to the results as presented in [4] the performance was raised lightly by using this new reduced fuzzy rule set to classify the validation data set. In a next step this new population of fuzzy rule sets has to be implemented into an embedded system to enable an online classification of anaesthetic stages.

Author's Statement

  1. Conflict of interest: Authors state no conflict of interest. Material and Methods: Informed consent: Informed consent has been obtained from all individuals included in this study. Ethical approval: The research related to human use has been complied with all the relevant national regulations, institutional policies and in accordance the tenets of the Helsinki Declaration, and has been approved by the authors’ institutional review board or equivalent committee.

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Published Online: 2015-9-12
Published in Print: 2015-9-1

© 2015 by Walter de Gruyter GmbH, Berlin/Boston

This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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