Mother rotor anchoring in branching tissue with heterogeneous membrane properties

Modeling atrial arrhythmias: impact on clinical diagnosis and therapies

Atrial arrhythmias are the most frequent sustained rhythm disorders in humans and often lead to severe complications such as heart failure and stroke. Despite the important insights provided by animal models into the mechanisms of atrial arrhythmias, direct translation of experimental findings to new therapies in patients has not been straightforward. With the advances in computer technology, large-scale electroanatomical computer models of the atria that integrate information from the molecular to organ scale have reached a level of sophistication that they can be used to interpret the outcome of experimental and clinical studies and aid in the rational design of therapies. This paper reviews the state-of-the-art of computer models of the electrical dynamics of the atria and discusses the evolving role of simulation in assisting the clinical diagnosis and treatment of atrial arrhythmias.

Computational modeling of the human atrial anatomy and electrophysiology

This review article gives a comprehensive survey of the progress made in computational modeling of the human atria during the last 10 years. Modeling the anatomy has emerged from simple "peanut"-like structures to very detailed models including atrial wall and fiber direction. Electrophysiological models started with just two cellular models in 1998. Today, five models exist considering e.g. details of intracellular compartments and atrial heterogeneity. On the pathological side, modeling atrial remodeling and fibrotic tissue are the other important aspects. The bridge to data that are measured in the catheter laboratory and on the body surface (ECG) is under construction. Every measurement can be used either for model personalization or for validation. Potential clinical applications are briefly outlined and future research perspectives are suggested.

Vulnerability for reentry in a three dimensional model of human atria: a simulation study

Atrial tachycardias are the most common cardiac arrhythmias in clinical practice, which induce changes in atrial properties that help to perpetuate them. These changes are called "atrial remodeling". Recent studies have shown that rapid ectopic activity principally on the pulmonary veins can trigger reentrant mechanisms and lead to atrial tachycardias. However, the influences of ectopic foci location, the number of ectopic beats and its frequency on the likelihood of triggering reentries are not well known. In this work the effects of electrical remodeling were incorporated in an atrial cell model and integrated in a three-dimensional model of human atria, to develop a study of vulnerability for reentries. To carry out the study, an ectopic beat and a burst of six ectopic beats at two different frequencies were applied in six different locations in the atria. The results show greater vulnerability in the left pulmonary veins when we applied a single ectopic beat. When we increase the number of ectopic beats to six, a greater width of the vulnerable window was observed when ectopic focus frequency was high. The location, the number of ectopic beats and their frequency affect the vulnerability for reentry.

Arrhythmogenic mechanisms of the Purkinje system during electric shocks: a modeling study

BACKGROUND

The function of the Purkinje system (PS) is to ensure fast and uniform activation of the heart. Although this vital role during sinus rhythm is well understood, this is not the case when shocks are applied to the heart, especially in the case of failed defibrillation. The PS activates differently from the myocardium, has different electrophysiological properties, and provides alternate propagation pathways; thus, there are many ways in which it can contribute to postshock behavior.

OBJECTIVE

The purpose of this study was to elucidate the role of the PS in the initiation and maintenance of postshock arrhythmias.

METHODS

A computer model of the ventricles including the PS was subjected to different reentry induction protocols.

RESULTS

The PS facilitated reentry induction at relatively weaker shocks. Disconnecting the PS from the ventricles during the postshock interval revealed that the PS helps stabilize early-stage reentry by providing focal breakthroughs. During later stages, the PS contributed to reentry by leading to higher frequency rotors. The PS also promoted wave front splitting during reentry due to electrotonic coupling, which prolongs action potential durations at PS-myocyte junctions. The presence of a PS results in the anchoring of reentrant activations that propagate through the pathways provided by the PS.

CONCLUSIONS

The PS is proarrhythmic in that it provides pathways that prolong activity, and it plays a supplementary role in maintaining the later stages of reentry (>800 ms).

Spatial-temporal filter effect in a computer model study of ventricular fibrillation

Prediction of countershock success from ventricular fibrillation (VF) ECG is a major challenge in critical care medicine. Recent findings indicate that stable, high frequency mother rotors are one possible mechanism maintaining VF. A computer model study was performed to investigate how epicardiac sources are reflected in the ECG. In the cardiac tissues of two computer models - a model with cubic geometry and a simplified torso model with a left ventricle - a mother rotor was induced by increasing the potassium rectifier current. On the epicardium, the dominant frequency (DF) map revealed a constant DF of 23 Hz (cubic model) and 24.4 Hz (torso model) in the region of the mother rotor, respectively. A sharp drop of frequency (3-18 Hz in the cubic model and 12.4-18 Hz in the torso model) occurred in the surrounding epicardial tissue of chaotic fibrillatory conduction. While no organized pattern was observable on the body surface of the cubic model, the mother rotor frequency can be identified in the anterior surface of the torso model because of the chosen position of the mother rotor in the ventricle (shortest distance to the body surface). Nevertheless, the DFs were damped on the body surfaces of both models (4.6-8.5 Hz in the cubic model and 14.4-16.4 Hz in the torso model). Thus, it was shown in this computer model study that wave propagation transforms the spatial low pass filtering of the thorax into a temporal low pass. In contrast to the resistive-capacitive low pass filter formed by the tissue, this spatial-temporal low pass filter becomes effective at low frequencies (tens of Hertz). This effect damps the high frequency components arising from the heart and it hampers a direct observation of rapid, organized sources of VF in the ECGs, when in an emergency case an artifact-free recording is not possible.

Frequency distribution effects of anchored mother rotors--a computer model study

Recent findings indicate that major organized centers (mother rotors) can maintain ventricular fibrillation (VF). In computer models the mother rotors can be induced by local shortening of the action potential duration (APD) in the cardiac tissue. Because of the fact that these rotors tend to drift away towards regions with longer APD, an additional heterogeneity (e.g. bundle) has to be included in the model for stabilizing the activation. Thus, the rotor anchors on this bundle and yields to interesting frequency distribution effects. In the dominant frequency (DF) map of a simplified computer model of the left ventricle it can be observed that the anchoring site of the rotor produces a slightly lower DF than in the surrounding cardiac tissue. That means that due to the load effect of the bundle the frequency is decreased. Furthermore the meandering of the mother rotor around this anchor site is reflected in the spectra of signals taken randomly in the organized region. These effects are both detected with two different independent spectral estimators with different resolutions.