Jesús Requena-Carrión

Sensitivity and Spatial Resolution of Transvenous Leads in Implantable Cardioverter Defibrillator

It has been previously documented that the main features and sensing performance of electrograms (EGMs) recorded in implantable cardioverter defibrillators (ICDs) depend on lead configuration. Although this dependence has been ascribed to differences in lead sensitivity and spatial resolution, the quantification of these two properties on ICD has not yet been attempted. In this paper, an operative framework to study the spatial resolution of ICD transvenous leads is presented. We propose to quantify the spatial resolution of ICD transvenous leads based on a new characterization called lead resolution volume (ResV). We analyzed the sensitivity distribution and the ResV of two unipolar (tip-can and coil-can ) and two bipolar (true or tip-ring and integrated or tip-coil) ICD transvenous lead configurations. A detailed 3-D model of the human thorax based on the visible human man dataset was used to compute the lead sensitivity and computer simulations of simple cardiac dynamics were used to quantify the ResV. Differences in the sensitivity distribution throughout the ventricular myocardium (VM) were observed for each lead configuration. In our computer model of the human thorax, the ResV was found to comprise 7%, 35%, 45%, and 70% of the VM for true bipolar, integrated bipolar, tip-can unipolar, and coil-can unipolar ICD leads, respectively. Furthermore, our analysis shows that the spatial resolution depends on both lead sensitivity and cardiac dynamics, and therefore, it can vary for different heart rhythms.

A student-centered collaborative learning environment for developing communication skills in engineering education

Communication skills development is one of the main goals of engineering education. We propose an integrated student-centered collaborative learning environment for developing communication skills, using project-based learning methods and peer assessment. In our learning environment, projects are assigned to small groups of students under teacher supervision, documented in a wiki-editing tool and presented during a public poster session. By combining wiki entries and poster presentations, we intend to facilitate students (1) to gain access to the project of their peers and share their results, (2) to analyze and comment critically the project of their peers and provide them with feedback, and (3) to enhance their writing and oral skills. Previous experiences encourage us to promote this integrated learning environment. Wiki environments allowed students to improve the quality of their projects and to develop a critical attitude towards their projects and the projects of their peers. The poster session was found to be more dynamic than traditional oral presentations. Students got engaged in a more open and critical manner with the project of their peers, and students presenting their project had the chance to improve the quality of their presentation on the fly, by presenting their work several times in the duration of the session. In future courses, we will implement a learning environment that combines both wiki-based and poster session approaches. We expect that the implementation of both approaches will help to develop the communication skills of engineering students.

Optimizing Average Performance of OFDM Systems Using Limited-Rate Feedback

Orthogonal frequency-division multiplexing (OFDM) is the most popular modulation in modern wireless communication systems. Among other features, OFDM has been able to successfully exploit channel state information (CSI) at the transmitter, allowing to implement dynamic resource allocation schemes that improve spectral efficiency and error resilience. Nevertheless, in most wireless communication systems achieving a perfect CSI at the transmitter is difficult. For this reason a limited-rate feedback mode has been proposed, in which only quantized CSI at the transmitter is available. In this paper we design joint channel quantization and resource allocation schemes for single-user OFDM systems, that use limited-rate feedback and do not assume any structure on the channel quantizer. The new schemes are obtained by solving an optimization problem that maximizes average ergodic rate subject to average power and bit error rate constraints. Necessary optimality conditions for the quantization and resource allocation schemes are derived and algorithms to find a solution satisfying such conditions are discussed. Since finding the overall global optimal solution is computationally cumbersome, suboptimal yet simple schemes are also explored. Three simplifications are investigated, namely: a worst-case robust design that reduces the dimensionality of the problem; optimal and provably convergent stochastic schemes that catch the average behavior of the system on-the-fly; and schemes that reduce the amount of feedback required from the receiver by exploiting the channel correlation among subcarriers. The signalling and computational costs associated to the implementation of the developed schemes and their extension to multiple user systems are also discussed. Numerical examples corroborate analytical claims and reveal that significant gains result even when suboptimal schemes based on affordable limited-rate feedback are used.

Effects of the Location of Myocardial Infarction on the Spectral Characteristics of Ventricular Fibrillation

Background: The location of the myocardial infarction (MI) might modify the spectral characteristics of ventricular fibrillation (VF) in humans.

Numerical analysis of the resolution of surface electrocardiographic lead systems

Non-invasive electrocardiographic (ECG) techniques for assessing the electrical activity of selected regions within the cardiac muscle can benefit from suitable positioning of surface electrodes. This positioning is usually guided heuristically and complemented by clinical and experimental studies, but there is a lack of general methods to characterize quantitatively the ability of a given electrode configuration to focus on selected regions of the heart. In this study we explore an approach to the characterization of the resolution of surface ECG systems based on the concept of Resolution Mass (RM). By integrating bioelectric signal modeling and numerical methods, we explore, in an application example, the location and size of the RM for a multielectrode ECG system. The concept of RM combined with bioelectric signal modeling and numerical methods constitutes a powerful tool to investigate the resolution properties of surface ECG systems.

Relating the spectrum of cardiac signals to the spatiotemporal dynamics of cardiac sources

An increasing number of studies use the spectrum of cardiac signals for analyzing the spatiotemporal dynamics of complex cardiac arrhythmias. However, the relationship between the spectrum of cardiac signals and the spatiotemporal dynamics of the underlying cardiac sources remains to date unclear. In this paper, we derive a mathematical expression relating the spectrum of cardiac signals to the spatiotemporal dynamics of cardiac sources and the measurement characteristics of the lead systems. Then, by using analytical methods and computer simulations we analyze the spectrum of cardiac signals measured by idealized lead systems during correlated and uncorrelated spatiotemporal dynamics. Our results show that lead systems can have distorting effects on the spectral envelope of cardiac signals, which depend on the spatial resolution of the lead systems and on the degree of spatiotemporal correlation of the underlying cardiac sources. In addition to this, our results indicate that the spectral features that do not depend on the spectral envelope, such as the dominant frequency, behave robustly against different choices of lead systems.

Effects of lead spatial resolution on the spectrum of cardiac signals: A simulation study

Spectral analysis is widely applied to bioelectric cardiac signals for quantifying the spatiotemporal organization of cardiac tissue. Nevertheless, to date it is not well understood how lead characteristics affect the spectrum of recorded cardiac signals and, as a consequence, the interpretation of cardiac spectrum is still controversial. In this paper we use simulation methods to investigate the effects of lead spatial resolution on the spectrum of cardiac signals. We simulate three cardiac rhythms of different degrees of spatiotemporal organization in a square sample of cardiac tissue. Then, by using a lead field approach, we synthesize the signals recorded by four idealized leads of different spatial resolution. Finally, we estimate the spectrum of simulated cardiac signals. Our simulations indicate that lead spatial resolution affects cardiac spectrum, although the effects depend on the organization of the underlying rhythm. Specifically, our simulations show that for highly organized rhythms, the smaller the lead resolution region, the broader the distribution of power in frequency. Since lead resolution can affect significantly cardiac spectrum, we conclude that caution should be used when quantifying cardiac spatiotemporal organization based on the spectrum of cardiac signals.

Work in progress - implantation of a collaborative student-centered learning environment in a wireless technology course

Engineering students face their professional future with uncertainty partly because of a poor understanding of the industry and careers opportunities. This problem can be tackled by designing an engineering curriculum that includes courses embodying both technical and professional contents. In this paper we report the preliminary results of the implantation of a lifelong, collaborative student-centered learning environment that emphasizes both technical and professional aspects of engineering. A semi-public Wiki database was used in a course on wireless technologies for senior electrical engineering students. Wiki-entries were created by students under teacher supervision, and were intended to provide an overview of a wireless technology, companies in the sector and careers opportunities. Students gain of knowledge and level of satisfaction, and the degree of utilization of the Wiki were analyzed based on students marks, student surveys, log files tracking the Wiki activity, and the quality of the Wiki entries. Results are positive and indicate that students have understood the importance of approaching their professional future in a collaborative way.

A Review on Recent Patents in Digital Processing for Cardiac Electric Signals (II): Advanced Systems and Applications

Digital signal processing algorithms for cardiac recordings have been paid much attention in recently disclosed patents. In this second part of our review of the state-of-art patents, systems for sudden cardiac death prediction, as well as for apnea analysis, are summarized. Advanced digital signal processing algorithms for cardiac electric signals are specifically reviewed, including independent component decompositions, and nonlinear methods (chaos, fractals, and entropies). Finally, systems aiming to solve the inverse problem in electrocardiography are presented. Concluding remarks on these systems and on the whole review are discussed. In the companion paper (1), a compilation of recent patents on digital processing algorithms for cardiac signals analysis has been presented. This review included systems for basic feature extraction of the electric signals recorded from electrodes in the skin (called electrocardiogram, ECG), or from electrode systems in catheters placed inside the heart (called electrograms, EGM). Systems for analyzing cardiac arrhythmias in different applications have also been assembled therein. In this paper, we include relevant higher-level systems that use digital processing algorithms in advanced applications related with electrocardiology and with cardiac electrophysiology environments. Specifically, the problem of accurate sudden cardiac death (SCD) prediction (2,3) has been targeted by a large number of disclosed systems. Also, apnea analysis (4) has been addressed in the recently disclosed patents. Special attention is paid here to advanced signal processing algorithms, such as blind source separation techniques or nonlinear analysis procedures including chaos, fractals and entropy principled algorithms, which have been disclosed as a part of advanced application systems. Finally, the inverse problem in electrocardiography (5), consisting of creating an anatomically detailed image of the electrical underlying cardiac activity from a diversity of electric signals, continues to be a field in which disclosed advanced systems aim to give to the clinician the best view of the cardiac electric activity. The structure of this second part of the review is as follows. Section 2 deals with patents devoted to SCD risk stratification and prediction. Section 3 briefly deals with

Action Potential Alternans in LQT3 Syndrome: A Simulation Study

The long QT syndrome type-3 (LQT3) is an inherited cardiac disorder caused by mutations in the sodium channel gene SCN5A. LQT3 has been associated with ventricular arrhythmias and sudden cardiac death, specially at low heart rates. Based on computer simulations and experimental investigations, analysis of the morphology of the action potential (AP) has shown that it undergoes early after depolarizations (EADs) and spontaneous discharges, which are thought to be the trigger for reentry like-activity. However, dynamic characteristics of cardiac tissue are also important factors of arrhythmia mechanisms. In this work, we propose a dynamical analysis of the LQT3 at cellular level. We use a detailed Markovian model of the DeltaKPQ mutation, which is associated with LQT3, and we study beat-to-beat AP Duration (APD) variations by using a long-term stimulation protocol. Compared to wild-type (WT) cells, DeltaKPQ mutant cells are found to develop APD alternans over a narrow range of stimulation frequencies. Moreover, the interval of frequency dependence of APD alternans is related to the degree of severity of the EADs present in the AP. In conclusion, dynamical analysis of paced cells is a useful approach to understand the mechanisms of rate dependent arrhythmias.