Nathalie Virag

Study of unipolar electrogram morphology in a computer model of atrial fibrillation.

Introduction: Electrograms exhibit a wide variety of morphologies during atrial fibrillation (AF). The basis of these time courses, however, is not completely understood. In this study, data from computer models were studied to relate features of the signals to the underlying dynamics and tissue substrate.

Study of atrial arrhythmias in a computer model based on magnetic resonance images of human atria.

The maintenance of multiple wavelets appears to be a consistent feature of atrial fibrillation (AF). In this paper, we investigate possible mechanisms of initiation and perpetuation of multiple wavelets in a computer model of AF. We developed a simplified model of human atria that uses an ionic-based membrane model and whose geometry is derived from a segmented magnetic resonance imaging data set. The three-dimensional surface has a realistic size and includes obstacles corresponding to the location of major vessels and valves, but it does not take into account anisotropy. The main advantage of this approach is its ability to simulate long duration arrhythmias (up to 40 s). Clinically relevant initiation protocols, such as single-site burst pacing, were used. The dynamics of simulated AF were investigated in models with different action potential durations and restitution properties, controlled by the conductance of the slow inward current in a modified Luo-Rudy model. The simulation studies show that (1) single-site burst pacing protocol can be used to induce wave breaks even in tissue with uniform membrane properties, (2) the restitution-based wave breaks in an atrial model with realistic size and conduction velocities are transient, and (3) a significant reduction in action potential duration (even with apparently flat restitution) increases the duration of AF. (c) 2002 American Institute of Physics.

Speech enhancement based on masking properties of the auditory system

This paper addresses the problem of the intelligibility enhancement of speech corrupted by additive background noise in a single channel system. The proposed algorithm uses a criterion based on the human perception. It is a variation of the well-known spectral subtraction method which is attractive because of its simplicity, but introduces an unnatural and unpleasant residual noise. The proposed approach incorporates in this method considerations about noise masking of the auditory system. It succeeds in finding the best trade-off between noise reduction and speech distortion in a perceptual sense. Simulations show perceptually very satisfactory results and objective measures indicate a quality improvement. The speech processed with this new algorithm sounds more pleasant to a human listener than those obtained by the classical methods. This shows the relevance to incorporate perceptual aspects in the enhancement process.

Evaluation of ablation patterns using a biophysical model of atrial fibrillation

Atrial fibrillation (AF) is the most common form of cardiac arrhythmia. Surgical/Radiofrequency (RF) ablation is a therapeutic procedure that consists of creating lines of conduction block to interrupt AF. The present study evaluated 13 different ablation patterns by means of a biophysical model of the human atria. In this model, ablation lines were abruptly applied transmurally during simulated sustained AF, and success rate, time to AF termination and average beat-to-beat interval were documented. The gold standard Cox’s Maze III procedure was taken as reference. The effectiveness of twelve less invasive patterns was compared to it. In some of these incomplete lines (entailing a gap) were simulated. Finally, the computer simulations were compared to clinical data. The results show that the model reproduces observations made in vivo: (1) the Maze III is the most efficient ablation procedure; (2) less invasive patterns should include lines in both right and left atrium; (3) incomplete ablation lines between the pulmonary veins and the mitral valve annulus lead to uncommon flutter; (4) computer simulations of incomplete lines are consistent with clinical results of non-transumural RF ablation. Biophysical modeling may therefore be considered as a useful tool for understanding the mechanisms underlying AF therapies.

A numerical scheme for modeling wavefront propagation on a monolayer of arbitrary geometry

The majority of models of wavefront propagation in cardiac tissue have assumed relatively simple geometries. Extensions to complicated three-dimensional (3-D) representations are computationally challenging due to issues related both to problem size and to the correct implementation of flux conservation. In this paper, we present a generalized finite difference scheme (GDFS) to simulate the reaction-diffusion system on a 3-D monolayer of arbitrary shape. GDFS is a vertex-centered variant of the finite-volume method that ensures local flux conservation. Owing to an effectively lower dimensionality, the overall computation time is reduced compared to full 3-D models at the same spatial resolution. We present the theoretical background to compute both the wavefront conduction and local electrograms using a matrix formulation. The same matrix is used for both these quantities. We then give some results of simulation for simple monolayers and complex monolayers resembling a human atria.

Observer of autonomic cardiac outflow based on blind source separation of ECG parameters

We present a novel method which provides an observer of the autonomic cardiac outflow using heartbeat intervals (RR) and QT intervals. The model of the observer is inferred from qualitative physiological knowledge. It consists in a problem of blind source separation of noisy mixtures which is resolved by a simple and robust algorithm. The robustness of the algorithm has been assessed by numerical simulations in adverse noisy environments. In clinical applications, we have validated the observer on subjects exposed to experimental conditions known to elicit sympathetic or parasympathetic response.

A computer model of cardiac electrical activity for the simulation of arrhythmias

Modern computer power allows development of models of the heart that may be helpful for the understanding of arrhythmia mechanisms if, based on realistic physiological parameters, such models can display phenomena difficult to study in nature. Therefore, a two‐dimensional model of the cardiac tissue has been implemented, where the modeling of each cell is based on membrane ionic channels (Beeler‐Reuter and Luo‐Rudy models). In addition, an ECG was computed based on the ionic currents simulated. This model allows us to observe the propagation of the action potentials Vm across the cardiac tissue, the evolution of Vm for any of the cardiac cells, and the underlying ionic currents. The computation of the ECG makes it possible to relate this information with an often‐used diagnostic tool. Simulations of normal and pathological phenomena such as functional and anatomic reentry have been performed. Our simulation results show that the applied computer model based on ionic currents seems accurate and realistic when compared with biological models and offers a new approach to study the origin, prevention, and termination of arrhythmias.

Impact of Varying Ablation Patterns in a Simulation Model of Persistent Atrial Fibrillation

Background: Several strategies of endovascular ablation with varying success rates and proarrhythmic effects have been proposed to treat persistent atrial fibrillation (AF). Evaluation of ablation patterns by computer simulation provides a tool for examination of its effectiveness and side effects.

Atrial septal pacing for the termination of atrial fibrillation: study in a biophysical model of human atria

AIMS While successful termination by pacing of organized atrial tachycardias has been observed in patients, single site rapid pacing has not yet led to conclusive results for the termination of atrial fibrillation (AF). The purpose of this study was to evaluate a novel atrial septal pacing algorithm for the termination of AF in a biophysical model of the human atria. METHODS AND RESULTS Sustained AF was generated in a model based on human magnetic resonance images and membrane kinetics. Rapid pacing was applied from the septal area following a dual-stage scheme: (i) rapid pacing for 10-30 s at pacing intervals 62-70% of AF cycle length (AFCL), (ii) slow pacing for 1.5 s at 180% AFCL, initiated by a single stimulus at 130% AFCL. Atrial fibrillation termination success rates were computed. A mean success rate for AF termination of 10.2% was obtained for rapid septal pacing only. The addition of the slow pacing phase increased this rate to 20.2%. At an optimal pacing cycle length (64% AFCL) up to 29% of AF termination was observed. CONCLUSION The proposed septal pacing algorithm could suppress AF reentries in a more robust way than classical single site rapid pacing. Experimental studies are now needed to determine whether similar termination mechanisms and rates can be observed in animals or humans, and in which types of AF this pacing strategy might be most effective.

Impact of defibrillation testing on predicted ICD shock efficacy: Implications for clinical practice

BACKGROUND Lack of consensus regarding defibrillation testing methods for implantable cardioverter-defibrillators relates to risks of repeated fibrillation episodes. OBJECTIVE To provide recommendations for testing protocols, repeating testing of patients with high defibrillation threshold (DFT), and interpreting testing after implantable cardioverter-defibrillator system revision. METHODS We constructed a computer model of defibrillation probability-of-success curves using data from 564 patients. Then, we compared 13 safety margin (SM) or DFT protocols in 50,000 simulated patients to identify those with the best balance of sensitivity and predictive value for detecting patients at high risk for failed defibrillation. Conditional retesting of patients with high DFT was simulated, both without and with revision that lowered defibrillation energy by one-third. RESULTS SM protocols were more efficient than DFT protocols; 2/2 successes at 20 J or 1/1 at 16 J performed best. Patients who failed testing had a mean probability of defibrillation of 94% at 35 J, but great uncertainty regarding that probability (range 67.0%-100%). When they repeated testing, 62% passed, with 48% owing to regression to the mean. If system revision was performed before retesting, 84% passed; the fraction of patients at high risk reduced (4.7% to 2.7%, with 43% relative reduction); but 3.5% underwent unnecessary revisions. Testing and revision of patients with high DFT benefitted 2.5% of the patients. CONCLUSIONS SM protocols are superior to DFT protocols for implant testing. For patients who fail testing, there is substantial uncertainty in defibrillation efficacy. After a system revision that does not alter defibrillation efficacy, 62% of these patients pass retesting.