Dorian Krause

Similarities and differences between electrocardiogram signs of left bundle-branch block and left-ventricular uncoupling

AIMS A left bundle-branch block (LBBB) electrocardiogram (ECG) type may be caused by either a block in the left branch of the ventricular conduction system or by uncoupling in the working myocardium. We used a realistic large-scale computer model to evaluate the effects of uncoupling with and without left-sided block and in combination with biventricular pacing. METHODS AND RESULTS Action potential propagation was simulated using a reaction-diffusion model of the human ventricles. Electrocardiograms and cardiac electrograms were computed from the simulated action potentials by solving the bidomain equations. In all situations, diffuse uncoupling reduced QRS amplitude, prolonged QRS duration, and rotated the QRS axis leftward. Uncoupling by 50% increased QRS duration from 90 to 120 ms with a normal conduction system and from 140 to 190 ms when the left bundle branch was blocked. Biventricular pacing did not change QRS duration but reduced total ventricular activation time. CONCLUSION Uncoupling in the working myocardium can mimic left-sided block in the ventricular conduction system and can explain an LBBB ECG pattern with relatively low amplitude. Biventricular pacing improves ventricular activation in true LBBB with or without uncoupling but not in case of 50% uncoupling alone.

Patient-specific modelling of cardiac electrophysiology in heart-failure patients

Aims Left-ventricular (LV) conduction disturbances are common in heart-failure patients and a left bundle-branch block (LBBB) electrocardiogram (ECG) type is often seen. The precise cause of this pattern is uncertain and is probably variable between patients, ranging from proximal interruption of the left bundle branch to diffuse distal conduction disease in the working myocardium. Using realistic numerical simulation methods and patient-tailored model anatomies, we investigated different hypotheses to explain the observed activation order on the LV endocardium, electrogram morphologies, and ECG features in two patients with heart failure and LBBB ECG. Methods and results Ventricular electrical activity was simulated using reaction–diffusion models with patient-specific anatomies. From the simulated action potentials, ECGs and cardiac electrograms were computed by solving the bidomain equation. Model parameters such as earliest activation sites, tissue conductivity, and densities of ionic currents were tuned to reproduce the measured signals. Electrocardiogram morphology and activation order could be matched simultaneously. Local electrograms matched well at some sites, but overall the measured waveforms had deeper S-waves than the simulated waveforms. Conclusion Tuning a reaction–diffusion model of the human heart to reproduce measured ECGs and electrograms is feasible and may provide insights in individual disease characteristics that cannot be obtained by other means.

Coupling Molecular Dynamics and Continua with Weak Constraints

One of the most challenging problems in dynamic concurrent multiscale simulations is the reflectionless transfer of physical quantities between the different scales. In particular, when coupling molecular dynamics and finite element discretizations in solid body mechanics, often spurious wave reflections are introduced by the applied coupling technique. The reflected waves are typically of high frequency and are arguably of little importance in the domain where the finite element discretization drives the simulation. In this work, we provide an analysis of this phenomenon. Based on the gained insight, we derive a new coupling approach, which neatly separates high and low frequency waves. Whereas low frequency waves are permitted to bridge the scales, high frequency waves can be removed by applying damping techniques without affecting the coupled share of the solution. As a consequence, our new method almost completely eliminates unphysical wave reflections and deals in a consistent way with waves of arbit...

Design and Analysis of a Lightweight Parallel Adaptive Scheme for the Solution of the Monodomain Equation

Numerical simulation of the nonlinear reaction-diffusion equations in computational electrocardiology requires locally high spatial resolution to capture the multiscale effects related to the electrical activation of the heart accurately, namely the strongly varying transmembrane potential. Here, we propose a novel lightweight adaptive algorithm which aims at combining the plainness of structured meshes with the resolving capabilities of unstructered adaptive meshes. Our “patchwise adaptive” approach is based on locally structured mesh hierarchies which are glued along their interfaces by a nonconforming mortar element discretization. To further increase the overall efficiency, we keep the spatial meshes constant over suitable time windows in which error indicators are accumulated. This approach facilitates strongly varying mesh sizes in neighboring patches as well as in consecutive time steps. For the transfer of the dynamic variables between different spatial approximation spaces we compare the

Towards a large-scale scalable adaptive heart model using shallow tree meshes

L^2

Poster: hybrid parallelization of a realistic heart model

-pr...

A Parallel Multiscale Simulation Toolbox for Coupling Molecular Dynamics and Finite Elements

Electrophysiological heart models are sophisticated computational tools that place high demands on the computing hardware due to the high spatial resolution required to capture the steep depolarization front. To address this challenge, we present a novel adaptive scheme for resolving the deporalization front accurately using adaptivity in space. Our adaptive scheme is based on locally structured meshes. These tensor meshes in space are organized in a parallel forest of trees, which allows us to resolve complicated geometries and to realize high variations in the local mesh sizes with a minimal memory footprint in the adaptive scheme. We discuss both a non-conforming mortar element approximation and a conforming finite element space and present an efficient technique for the assembly of the respective stiffness matrices using matrix representations of the inclusion operators into the product space on the so-called shallow tree meshes.We analyzed the parallel performance and scalability for a two-dimensional ventricle slice as well as for a full large-scale heart model. Our results demonstrate that the method has good performance and high accuracy.

Numerical validation of a constraints-based multiscale simulation method for solids

Heart failure is a major health problem, not only for the number of people affected (about five million in Europe alone) but also because of the direct and indirect costs for its treatment. A thorough understanding of the complex electrical activation system that triggers the mechanical contraction is a prerequisite for developing effective treatment strategies. Full-heart simulations are an indispensable tool to study the effect of molecular-level or tissue-level changes on clinical measurements [2]. Cardiac electrical activity originates in the millions of ion channels and pumps that are located in the outer membrane of each cardiac muscle cell. We denote the macroscopic ionic current density by Iion.

Enabling local time stepping in the parallel implicit solution of reaction-diffusion equations via space-time finite elements on shallow tree meshes

It is the ultimate goal of concurrent multiscale methods to provide computational tools that allow to simulation physical processes with the accuracy of micro-scale and the computational speed of macro-scale models. As a matter of fact, the efficient and scalable implementation of concurrent multiscale methods on clusters and supercomputers is a complicated endeavor. In this article we present the parallel multiscale simulation tool Maci which has been designed for efficient coupling between molecular dynamics and finite element codes. We propose a specification for a thin yet versatile interface for the coupling of molecular dynamics and finite element codes in a modular fashion. Further we discuss the parallelization strategy pursued in Maci, in particular, focusing on the parallel assembly of transfer operators and their efficient execution.

Quadrature and Implementation of the Weak Coupling Method

We present numerical validation studies for a concurrent multiscale method designed to combine molecular dynamics and finite element analysis targeting the simulation of solids. The method is based on an overlapping domaindecomposition and uses weak matching constraints to enforce matching between the finite element displacement field and the projection of the molecular dynamics displacement field on the mesh. A comparison between our method and the well-known bridging domain method by Xiao and Belytschko [22] is presented. As part of our validation study we discuss applicability of the method to the simulation of fracture propagation and show results.