Biomedical subjects

Olivier Bernus

Publications and source records attributed to Olivier Bernus.

6 recordsLinked to original sources

PubMed2006

What can we learn from the optically recorded epicardial action potential?

Optical mapping using voltage-sensitive fluorescent dyes has become a major tool for studying excitation propagation in the heart. Computational and experimental studies have indicated that the optical upstroke morphology reflects the orientation of the subsurface excitation front. In a recent whole heart computational study performed by Bishop et al. (Bishop, M. J., B. Rodriguez, J. Eason, J. P. Whiteley, N. Trayanova, and D. J. Gavaghan. 2006. Synthesis of voltage-sensitive optical signals: application to panoramic optical mapping. Biophys. J. 90:2938-2945), an example was provided of two different directions of propagation having nevertheless very similar epicardial optical upstrokes. The goal of this comment is to clarify the interpretation of optical upstroke morphologies and reconcile the results obtained by Bishop et al. with previous computational and experimental studies.

Action Potentials↗

PubMed2006

Multiplicative optical tomography of cardiac electrical activity.

Cardiac electrical activity can be mapped today through the response of voltage-sensitive dyes; but poor transparency of muscle tissue has enforced shallow-depth imaging. We present a three-dimensional (3D) reconstruction method for electrical activity deep inside the myocardial wall. Our approach is nonlinear and differs substantially from standard diffusive optical tomography. It does not require matrix inversion, data regularization or a priori information concerning the original object. Opposite sides of a slab-shaped preparation are scanned in parallel by detection and illumination points with a constant vector offset between illumination and detection axes (biaxial scanning). Scanning is performed in two perpendicular directions. In each direction, a pair of 2D images is obtained under offsets of opposite signs. These two pairs are the input for a multiplicative reconstruction algorithm, whose output is a 3D image. The overall procedure was successfully tested on computer-generated sources that include points, lines and hemispheres, patterned after actual electrophysiological excitations. The algorithm is computationally efficient and stable with respect to varying noise levels in the raw data.

Algorithms↗

PubMed2005

Comparison of electrophysiological models for human ventricular cells and tissues.

In this paper we briefly review currently published models for human ventricular cells and tissues. We discuss the Priebe-Beuckelmann (PB) model and the reduced version of this model constructed by Bernus et al. (redPB), the Ten Tusscher-Noble-Noble-Panfilov (TNNP) model and the Iyer-Mazhari-Winslow (IMW) model. We compare several characteristics of these models such as: sources of experimental data the models are based on, action potential morphology, action potential duration (APD) and conduction velocity (CV) restitution and computational efficiency. Finally, we discuss the application of a subset of these models-the redPB and the TNNP model-to study simulated spiral wave dynamics in 2D tissue sheets and in the human ventricles. We discuss the suitability of the different models for particular research questions and their limitations.

Action Potentials↗

PubMed2005

Alternating conduction in the ischaemic border zone as precursor of reentrant arrhythmias: a simulation study.

AIMS: Here, we investigate the mechanisms underlying the onset of conduction-related arrhythmias in a three-dimensional (3D) computational model of acute regional ischaemia. METHODS: Ischaemia was introduced by realistic gradients of potassium, pH, oxygen and electrical coupling in a 3D slab of ventricular tissue using the LRd model. We focused on a specific stage (10-15 min after occlusion) at which an intramural non-conductive ischaemic core (IC) surrounded by a border zone (BZ) has formed. RESULTS: At pacing frequencies greater than 4.5 Hz, we observed narrow areas (0.5 mm wide) of 2:1 conduction blocks at the periphery of the IC. As the pacing frequency increased, the area of block widened to 9 mm and gave rise to reentry at the periphery of the BZ. Alternating conduction blocks produced discordant action potential duration (APD) alternans throughout the slab and T-wave alternans in pseudo-ECG. Slowing the recovery of the calcium current broadened the range of pacing frequencies at which blocks were observed. Hyperkalaemia alone was sufficient to induce the alternating blocks. CONCLUSION: Computer modelling predicts that ischaemia-related arrhythmias are triggered by calcium-mediated alternating conduction blocks in the ischaemic border zone. Alternating conduction blocks lead to intramural reentry and APD alternans.

Animals↗

PubMed2002

Modified ionic models of cardiac tissue for efficient large scale computations.

Recirculation of excitation, or re-entry, is one of the most important mechanisms of life-threatening cardiac arrhythmias and fibrillation. Modelling these phenomena requires large scale computations in two and three-dimensional slabs of cardiac tissue. Because of computational constraints, most of the studies use simplified (non-ionic) models of cardiac tissue, which are electrophysiologically less accurate than the detailed ionic models. In this paper, we propose a method to modify ionic models of cardiac tissue into an intermediate class of models, which are almost as efficient for computations as simplified models, and retain most of the properties of the original ionic models, such as the shape of the action potential, the restitution of action potential duration and of the conduction velocity, as well as unchanged description of most of the ionic currents.

Algorithms↗

PubMedDate not supplied

Method for the three-dimensional localization of intramyocardial excitation centers using optical imaging.

This study explores the possibility of localizing the excitation centers of electrical waves inside the heart wall using voltage-sensitive dyes (fluorescent or absorptive). In the present study, we propose a method for the 3-D localization of excitation centers from pairs of 2-D images obtained in two modes of observation: reflection and transillumination. Such images can be obtained using high-speed charge-coupled device (CCD) cameras and photodiode arrays with time resolution up to 0.5 ms. To test the method, we simulate optical signals produced by point sources and propagating ellipsoidal waves in 1-cm-thick slabs of myocardial tissue. Solutions of the optical diffusion equation are constructed by employing the method of images with Robin boundary conditions. The coordinates of point sources as well as of the centers of expanding waves can be accurately determined using the proposed algorithm. The method can be extended to depth estimations of the outer boundaries of the expanding wave. The depth estimates are based on ratios of spatially integrated images. The method shows high tolerance to noise and can give accurate results even at relatively low signal-to-noise ratios. In conclusion, we propose a novel and efficient algorithm for the localization of excitation centers in 3-D cardiac tissue.

Action Potentials↗