Evangelos B. Mazomenos

A Low-Complexity ECG Feature Extraction Algorithm for Mobile Healthcare Applications

This paper introduces a low-complexity algorithm for the extraction of the fiducial points from the electrocardiogram (ECG). The application area we consider is that of remote cardiovascular monitoring, where continuous sensing and processing takes place in low-power, computationally constrained devices, thus the power consumption and complexity of the processing algorithms should remain at a minimum level. Under this context, we choose to employ the discrete wavelet transform (DWT) with the Haar function being the mother wavelet, as our principal analysis method. From the modulus-maxima analysis on the DWT coefficients, an approximation of the ECG fiducial points is extracted. These initial findings are complimented with a refinement stage, based on the time-domain morphological properties of the ECG, which alleviates the decreased temporal resolution of the DWT. The resulting algorithm is a hybrid scheme of time- and frequency-domain signal processing. Feature extraction results from 27 ECG signals from QTDB were tested against manual annotations and used to compare our approach against the state-of-the art ECG delineators. In addition, 450 signals from the 15-lead PTBDB are used to evaluate the obtained performance against the CSE tolerance limits. Our findings indicate that all but one CSE limits are satisfied. This level of performance combined with a complexity analysis, where the upper bound of the proposed algorithm, in terms of arithmetic operations, is calculated as 2.423N+214 additions and 1.093N+12 multiplications for N ≤ 861 or 2.553N+102 additions and 1.093N+10 multiplications for N > 861 (N being the number of input samples), reveals that the proposed method achieves an ideal tradeoff between computational complexity and performance, a key requirement in remote cardiovascular disease monitoring systems.

A Time-Domain Morphology and Gradient based algorithm for ECG feature extraction

A Time Domain Morphology and Gradient (TDMG) based algorithm is presented in this paper for the extraction of all the fiducial time instances from a single PQRST complex. By estimating these characteristic points, all clinically important temporal ECG parameters can be calculated. The proposed algorithm is based on a combination of extrema detection and slope information, with the use of adaptive thresholding to achieve the extraction of 11 time instances. A pre-processing step removes any noise and artefacts from the captured ECG signal. Initially, the position of the R-wave and the QRS-complex boundaries are localized in time. Following, by focusing on the part of the signal that precedes and succeeds the QRS-complex, the remaining fiducial points from the P and T waves are estimated. The initial localisation of the wave boundaries is complimented by amendment steps which are introduced to cater for atypical wave morphologies, indicative of particular heart conditions. The proposed algorithm is evaluated on the QT and PTB databases against medically annotated ECG samples. The results demonstrate the ability of the proposed scheme, to estimate the ECG fiducial points with acceptable accuracy from a single-lead ECG signal. In addition, this investigation reveals the ability of the TDMG algorithm to perform accurately irrespective of the lead chosen, the different disease categories and the sampling frequency of the captured ECG signal.

An automated algorithm for online detection of fragmented QRS and identification of its various morphologies

Fragmented QRS (f-QRS) has been proven to be an efficient biomarker for several diseases, including remote and acute myocardial infarction, cardiac sarcoidosis, non-ischaemic cardiomyopathy, etc. It has also been shown to have higher sensitivity and/or specificity values than the conventional markers (e.g. Q-wave, ST-elevation, etc.) which may even regress or disappear with time. Patients with such diseases have to undergo expensive and sometimes invasive tests for diagnosis. Automated detection of f-QRS followed by identification of its various morphologies in addition to the conventional ECG feature (e.g. P, QRS, T amplitude and duration, etc.) extraction will lead to a more reliable diagnosis, therapy and disease prognosis than the state-of-the-art approaches and thereby will be of significant clinical importance for both hospital-based and emerging remote health monitoring environments as well as for implanted ICD devices. An automated algorithm for detection of f-QRS from the ECG and identification of its various morphologies is proposed in this work which, to the best of our knowledge, is the first work of its kind. Using our recently proposed time–domain morphology and gradient-based ECG feature extraction algorithm, the QRS complex is extracted and discrete wavelet transform (DWT) with one level of decomposition, using the ‘Haar’ wavelet, is applied on it to detect the presence of fragmentation. Detailed DWT coefficients were observed to hypothesize the postulates of detection of all types of morphologies as reported in the literature. To model and verify the algorithm, PhysioNets PTB database was used. Forty patients were randomly selected from the database and their ECG were examined by two experienced cardiologists and the results were compared with those obtained from the algorithm. Out of 40 patients, 31 were considered appropriate for comparison by two cardiologists, and it is shown that 334 out of 372 (89.8%) leads from the chosen 31 patients complied favourably with our proposed algorithm. The sensitivity and specificity values obtained for the detection of f-QRS were 0.897 and 0.899, respectively. Automation will speed up the detection of fragmentation, reducing the human error involved and will allow it to be implemented for hospital-based remote monitoring and ICD devices.

A two-way time of flight ranging scheme for wireless sensor networks

Relative ranging between Wireless Sensor Network (WSN) nodes is considered to be an important requirement for a number of distributed applications. This paper focuses on a two-way, time of flight (ToF) technique which achieves good accuracy in estimating the point-to-point distance between two wireless nodes. The underlying idea is to utilize a two-way time transfer approach in order to avoid the need for clock synchronization between the participating wireless nodes. Moreover, by employing multiple ToF measurements, sub-clock resolution is achieved. A calibration stage is used to estimate the various delays that occur during a message exchange and require subtraction from the initial timed value. The calculation of the range between the nodes takes place on-node making the proposed scheme suitable for distributed systems. Care has been taken to exclude the erroneous readings from the set of measurements that are used in the estimation of the desired range. The two-way ToF technique has been implemented on commercial off-the-self (COTS) devices without the need for additional hardware. The system has been deployed in various experimental locations both indoors and outdoors and the obtained results reveal that accuracy between 1 m RMS and 2.5 m RMS in line-of-sight conditions over a 42 m range can be achieved.

Design of a Low-Power On-Body ECG Classifier for Remote Cardiovascular Monitoring Systems

In this paper, we first present a detailed study on the trade-off between the computational complexity (directly related to the power consumption) and classification accuracy for a number of classifiers for classifying normal and abnormal electrocardiograms (ECGs). In our analysis, we consider the spectral energy of the constituent waves of the ECG as the discriminative feature. Starting with the exhaustive exploration of single heartbeat-based classification to ascertain the complexity-accuracy trade-off in different classification algorithms, we then extend our study for multiple heartbeat-based classification. We use data available in Physionet as well as samples from Southampton General Hospital Cardiology Department for our investigation. Our primary conclusion is that a classifier based on linear discriminant analysis (LDA) achieves comparable level of accuracy to the best performing support vector machine classifiers with advantage of significantly reduced computational complexity. Subsequently, we propose an ultra low-power circuit implementation of the LDA classifier that could be integrated with the ECG sensor node enabling on-body normal and abnormal ECG classification. The simulated circuit is synthesized at 130 nm technology and occupies 0.70 mm2 of silicon area (0.979 mm2 after place and route) while it consumes 182.94 nW @ 1.08 V, estimated with Synopsys PrimeTime when operating at 1 KHz. These results clearly demonstrate the potential for low-power implementation of the proposed design.

On the Detection of Myocadial Scar Based on ECG/VCG Analysis

In this paper, we address the problem of detecting the presence of a myocardial scar from the standard electrocardiogram (ECG)/vectorcardiogram (VCG) recordings, giving effort to develop a screening system for the early detection of the scar in the point-of-care. Based on the pathophysiological implications of scarred myocardium, which results in disordered electrical conduction, we have implemented four distinct ECG signal processing methodologies in order to obtain a set of features that can capture the presence of the myocardial scar. Two of these methodologies are: 1) the use of a template ECG heartbeat, from records with scar absence coupled with wavelet coherence analysis and 2) the utilization of the VCG are novel approaches for detecting scar presence. Following, the pool of extracted features is utilized to formulate a support vector machine classification model through supervised learning. Feature selection is also employed to remove redundant features and maximize the classifiers performance. The classification experiments using 260 records from three different databases reveal that the proposed system achieves 89.22% accuracy when applying tenfold cross validation, and 82.07% success rate when testing it on databases with different inherent characteristics with similar levels of sensitivity (76%) and specificity (87.5%).

Towards the development of next-generation remote healthcare system: Some practical considerations

In this paper first we present an overview of the state-of-the-art remote patient monitoring systems in the backdrop of real clinical needs. The paper establishes a clear guideline in terms of clinical expectations from such a system from the viewpoint of practicing clinicians. It provides in-depth analysis of the shortcomings of the existing architectures and paves a way towards developing a practical “patient-centric” architecture that could be useful in the day-to-day clinical practice for providing “continuum of care”. Subsequently, the restrictions imposed by the resource constrained nature of such a system on development of appropriate hardware for supporting information processing on the data acquired by body-worn sensors are analyzed.

Development of an Automated Updated Selvester QRS Scoring System Using SWT-Based QRS Fractionation Detection and Classification

The Selvester score is an effective means for estimating the extent of myocardial scar in a patient from low-cost ECG recordings. Automation of such a system is deemed to help implementing low-cost high-volume screening mechanisms of scar in the primary care. This paper describes, for the first time to the best of our knowledge, an automated implementation of the updated Selvester scoring system for that purpose, where fractionated QRS morphologies and patterns are identified and classified using a novel stationary wavelet transform (SWT)-based fractionation detection algorithm. This stage informs the two principal steps of the updated Selvester scoring scheme-the confounder classification and the point awarding rules. The complete system is validated on 51 ECG records of patients detected with ischemic heart disease. Validation has been carried out using manually detected confounder classes and computation of the actual score by expert cardiologists as the ground truth. Our results show that as a stand-alone system it is able to classify different confounders with 94.1% accuracy whereas it exhibits 94% accuracy in computing the actual score. When coupled with our previously proposed automated ECG delineation algorithm, that provides the input ECG parameters, the overall system shows 90% accuracy in confounder classification and 92% accuracy in computing the actual score and thereby showing comparable performance to the stand-alone system proposed here, with the added advantage of complete automated analysis without any human intervention.

A novel approach for the diagnosis of ventricular tachycardia based on phase space reconstruction of ECG

Ventricular arrhythmias comprise a group of disorders which manifest clinically in a variety of ways from ventricular premature beats (VPB) and no sustained ventricular tachycardia (in healthy subjects) to sudden cardiac death due to ventricular tachyarrhythmia in patients with and/or without structural heart disease. Ventricular fibrillation (VF) and ventricular tachycardia (VT) are the most common electrical mechanisms for cardiac arrest. Accurate and automatic recognition of these arrhythmias from electrocardiography (ECG) is a crucial task for medical professionals. The purpose of this research is to develop a new index for the differential diagnosis of normal sinus rhythm (SR) and ventricular arrhythmias, based on phase space reconstruction (PSR).

On the Trade-Off of Accuracy and Computational Complexity for Classifying Normal and Abnormal ECG in Remote CVD Monitoring Systems

Remote cardiovascular disease monitoring systems are characterised from a limited number of available leads and limited processing capabilities. In this paper, we investigate the trade-off between accuracy and computational complexity in order to derive the best strategy for classifying the ECG signal into normal or abnormal in such systems, with the spectral energy contained in the constituent waves of the ECG signal, as the primary feature for classification. Five established classifiers are considered and through exhaustive simulations the maximum accuracy is derived for each classifier. Based on 104 ECG records, we present a systematic analysis of the tradeoff between computational complexity and accuracy, which allow us to deduce the best classification strategy considering only a small number of available leads.