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Vincent Jacquemet

Publications and source records attributed to Vincent Jacquemet.

10 recordsLinked to original sources
PubMed2006

Steady-state solutions in mathematical models of atrial cell electrophysiology and their stability.

The steady states of the Fenton-Karma, the Courtemanche and the Nygren cell models were studied by determining the fixed points of the dynamical system describing their cell kinetics. The linear stability of the fixed points was investigated, as well as their response to external stimuli. Symbolic calculations were carried out as far as possible in order to prove the existence of these fixed points. In the Fenton-Karma model, a unique stable fixed point was found, namely the resting state. In contrast, the Courtemanche model had an infinite number of fixed points. A bifurcation diagram was constructed by classifying these fixed points according to a conservation law. Initial conditions were identified, for which the dynamical behavior of the cell was auto-oscillatory. In its original formulation, the Nygren model had no fixed point. After having restored charge conservation, the system was found to have an infinite number of fixed points, resulting in a bifurcation diagram similar to that of the Courtemanche model. The approach proposed in this paper assists in the exploration of the high-dimensional parameter space of the cell models and the identification of the conditions leading to spontaneous pacemaker activity.

Algorithms↗
PubMed2006

Adaptation of the standard 12-lead electrocardiogram system dedicated to the analysis of atrial fibrillation.

OBJECTIVE: The objective of the study was to design a lead system aimed at studying atrial fibrillation (AF), while being anchored to the standard 12-lead system. METHODS: The location of 4 of the 6 precordial electrodes was optimized while leaving the remaining 5 of the 9 electrodes of the standard 12-lead system in place. The analysis was based on episodes of 11 different variants of AF simulated by a biophysical model of the atria positioned inside an inhomogeneous thorax. The optimization criterion used was derived from the singular value decomposition of the data matrices. RESULTS: While maintaining VR, VL, VF, V1 and V4, the 4 new electrode positions increased the ratio of the eighth and the first singular values of the data matrices of the new configuration about 5-fold compared with that of the conventional electrode positions. CONCLUSION: The adapted lead system produces a more complete view on AF compared with that of the standard 12-lead system.

Atrial Fibrillation↗
PubMed2006

Vectorcardiographic lead systems for the characterization of atrial fibrillation.

OBJECTIVE: The aim of the study was to design a vectorcardiographic lead system dedicated to the analysis of atrial fibrillation (AF). METHODS: Body surface potentials during AF were simulated by using a biophysical model of the human atria and thorax. The XYZ components of the equivalent dipole were derived from the Gabor-Nelson equations. These served as the gold standard while searching for an optimal orthogonal lead system for the estimation of the heart vector while using a limited number of electrode positions. Six electrode configurations and their dedicated transfer matrices were tested by using 10 different episodes of simulated AF and 25 different thorax geometries. RESULTS: Root-mean-square-based relative estimation error of the vectorcardiogram using the Frank electrodes was 0.39. An adaptation of 4 of the 9 electrode locations of the standard electrocardiogram, with 1 electrode moved to the back, reduced the error to 0.24. CONCLUSION: The Frank lead system is suboptimal for estimating the equivalent dipole components (VCG) during AF. Alternative electrode configurations should include at least 1 electrode on the back.

Action Potentials↗
PubMed2006

Pacemaker activity resulting from the coupling with nonexcitable cells.

Fibroblasts are nonexcitable cells that are sometimes coupled with excitable cells (cardiomyocytes). Due to a higher resting potential, these cells may act as a current source or sink and therefore disturb the electrical activity of the surrounding excitable cells. The possible occurrence of spontaneous pacemaker activity resulting from these electrotonic interactions was investigated in a theoretical model of two coupled cells as well as in a multicellular fiber model based on the Courtemanche kinetics. The results indicate that repeated spontaneous activations can be observed after an alteration in the activation and recovery properties of the sodium current (changes in excitability properties), provided that the difference in the resting potential as well as the coupling between the excitable and nonexcitable cells is sufficiently high. This may constitute a mechanism of focal sources triggering arrhythmias such as atrial fibrillation.

Action Potentials↗
PubMed2005

Wavelength and vulnerability to atrial fibrillation: Insights from a computer model of human atria.

AIM: To explain the contradictory results related to the concept of critical cardiac wavelength in the initiation and perpetuation of atrial fibrillation (AF). METHODS: A biophysically based computer model was used to: (1) study the relationship between wavelength and AF perpetuation in the presence of multiple re-entrant wavelets, (2) evaluate the performance of different existing methods for wavelength estimation in the presence of different arrhythmogenic substrates, and (3) document the impact of either heterogeneities in refractoriness or the presence of a mother rotor on wavelength estimation. RESULTS: The simulations confirmed that the wavelength must be below a critical value for AF to be sustained, when the perpetuation mechanism relies on multiple re-entrant wavelets. The estimated value of wavelength was not the same for all methods tested and depended in part on the nature of the spatio-temporal organization of the AF dynamics. CONCLUSION: A priori information about the underlying wavelet dynamics is needed for a correct interpretation of the cardiac wavelength as estimated by the current clinical methods.

Atrial Fibrillation↗
PubMed2005

Genesis of the P wave: atrial signals as generated by the equivalent double layer source model.

AIM: To assess the effectiveness of the equivalent surface source model in the simulation of atrial signals as observed in ECG leads. METHODS: P waves were extracted from 64-lead ECGs recorded in healthy subjects. The geometries of torso, lungs, heart, and blood cavities of a healthy subject, derived from magnetic resonance imaging, were used to position a detailed, thick-walled 3D model of the atria consisting of a set of 800,000 units representing the activity of all atrial myocytes. The ion-kinetics of the units was based on the formulation of Courtemanche et al. The simulated transmembrane potentials following a normal sinus beat, as well as those during atrial fibrillation, were projected on the 1297 nodes of the surface encapsulating all atrial myocytes (endocardium and epicardium). The transmembrane potentials at these nodes formed the source strengths of the elements of the equivalent generator, which were used to compute body surface potentials. RESULTS: After invoking slight adaptations of the timing of depolarization of the transmembrane potentials, the simulated signals during the P wave closely corresponded to recorded ones. The correspondence during the entire PR interval improved markedly after the inclusion of early repolarization effects in the interval between the end of the P wave and onset of QRS. This demanded a shortening of the mean action potential duration generated by the Courtemanche model. The simulated ECGs related to atrial fibrillation demonstrated the characteristic features of those clinically observed. CONCLUSIONS: The equivalent double layer is a useful source model for the genesis of atrial signals observed on the thorax. The interval from the end of the P wave to onset of QRS is not iso-electric. The Courtemanche model of the ion-kinetics of atrial cells needs to be adapted when applied to represent the activity of healthy, 'common' atrial myocytes.

Adult↗
PubMed2005

Finite volume stiffness matrix for solving anisotropic cardiac propagation in 2-D and 3-D unstructured meshes.

The finite volume method (FVM) has been shown recently to be an effective method for discretizing the reaction-diffusion equations that govern wavefront propagation in anisotropic cardiac tissue, as it can naturally handle both complex geometries and no flux boundary conditions without the use of ghost nodes. This communication presents an alternative formulation of FVM for triangle and tetrahedral meshes using the concept of dual basis. An algorithm based on this form is given that leads to an efficient computation of the stiffness matrix, facilitating the incorporation of space adaptive schemes and time varying material properties into numerical simulations of cardiac dynamics.

Action Potentials↗
PubMed2003

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. METHODS AND RESULTS: A computer model of entire human atria with a gross fiber architecture based on histology and membrane kinetics based on the Courtemanche et al. atrial model was developed to simulate paced activation and simulated AF. Unipolar electrograms were computed using a current source approximation at 256 sites in right atrium, to simulate a mapping array. The results show the following: (1) In a homogeneous and isotropic tissue, the presence of highly asymmetric electrograms is rare (<2%), although there is a marked variability in amplitude and symmetry. (2) The introduction of anisotropy increases this variability in symmetry and amplitude of the, electrograms especially for propagation across fibers. The percentage of highly asymmetric electrograms increases to 12% to 15% for anisotropy ratios greater than 3:1. (3) Multiphasic and fractionated electrograms are rarely seen in the model with uniform properties but are more common (15%-17%) in a model including regions with abrupt changes in conductivity. Beat-to-beat variations in the occurrence of multiphasic signals are possible with fixed anatomic heterogeneity, due to beat-to-beat variations in the direction of the wavefront relative to the heterogeneity. CONCLUSION: Analysis of the amplitude and symmetry of unipolar atrial electrograms can provide information about the electrophysiologic substrate maintaining AF.

Algorithms↗
PubMed2003

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.

Algorithms↗