The effect of spatial scale of resistive inhomogeneity on defibrillation of cardiac tissue

Alternans amplification following a two-stimulus protocol in a one-dimensional cardiac ionic model of reentry: from annihilation to double-wave quasiperiodic reentry

Electrical pacing is a common procedure in both experimental and clinical settings to study and/or annihilate anatomical reentry. A previous study [Comtois and Vinet, Chaos 12, 903 (2002)] has described new ways to terminate reentry in a one-dimensional loop model by a protocol consisting of only two stimulations. Annihilation of the reentrant activity was much more likely with these new scenarios than through a unidirectional block. This paper investigates the sensitivity of these scenarios of annihilation to the length of the pathway. It shows that double-pulse stimulation can stop the reentry if the circuit is shorter than a limiting length. Beyond this upper limit, stimulation rather yields sustained double-wave reentry. The same dynamical mechanism, labeled alternans amplification, is found to be responsible for these two types of post-stimulus dynamics.

Small random fiber angle variations as a mechanism for far-field stimulation of cardiac tissue

The mechanism by which an electric field stimulates cardiac tissue far from the stimulating electrode is not known. Current theories rely on the inherent heterogeneity of cardiac tissue as contributing to this phenomenon. This paper explores a possible mechanism for far-field stimulation in relation to fiber curvature. We incorporate a randomized fiber angle geometry into a two-dimensional active cardiac tissue model with unequal anisotropy ratios already exhibiting smooth, curving fibers. We simulate cross-field stimulation to initiate reentry in the tissue model, and compare the electric field thresholds at different S1-S2 intervals for tissue with randomized fiber angles and tissue with a smooth fiber geometry. The tissue with random fiber angles has a significantly lower threshold for reentry at certain intervals on the strength-interval curve. We conclude that a random fiber geometry may have an effect on the mechanism for far-field stimulation.

Simulated interactions between a Class III antiarrhythmic drug and a figure 8 reentry

Ventricular Fibrillation is responsible for a majority of sudden cardiac death, but little is known about how ventricular tachycardia (VT) degenerates into ventricular fibrillation. Several clinical studies focused only on preventing VT with a class III antiarrhythmic drug resulted in many deaths. Our simulations investigate the interactions between an antiarrhythmic drug likely to suppress a VT and a Figure 8 reentry. A parameter AAR is introduced to increase the action potential duration and therefore simulate various Class III drugs. Simulations are ran under several conditions (phases of the reentry, values of AAR, durations). They show that a VT can be suppressed whatever the phase of the reentry but it strongly depends on the duration of the effect. It confirms that a drug which can suppress a reentry can also worsen it. It also shows a great variety of activation patterns and thus the complexity of antiarrhythmic drugs effects. Simulations also demonstrate that suppressing VT is an increasing function of AAR.

How the spatial frequency of polarization influences the induction of reentry in cardiac tissue

Influences of spatial frequency of polarization.

INTRODUCTION

The mechanism by which an electric field induces a rotor during cross-field stimulation of cardiac tissue is not entirely known. Different heterogeneous aspects of cardiac tissue have been offered as possible theories, a prominent one being fiber curvature. The polarization produced when an electric field is applied to a sheet of tissue is varied over many spatial frequencies, depending upon the fiber angle. This article compares the effect of high and low spatial frequencies of polarization on reentry induction.

METHODS AND RESULTS

We incorporate a randomized fiber angle geometry into a two-dimensional active cardiac tissue model with unequal anisotropy ratios already exhibiting smooth, curving fibers. We simulate cross-field stimulation to initiate reentry in the tissue model, and compare the electric field thresholds at different S1-S2 intervals for tissue with randomized fiber angles, tissue with a smooth fiber geometry, and tissue with randomized fiber angles plus smooth, curving fibers. The tissue with both small, random fiber angles and curving fibers has a significantly lower threshold for reentry at certain intervals on the strength-interval curve than for the two cases individually.

CONCLUSION

Cardiac tissue exhibiting a random fiber geometry in conjunction with a smooth fiber geometry includes high and low spatial frequencies of polarization that may have an effect on the mechanism for reentry at certain S1-S2 intervals. Low spatial frequency regions of hyperpolarization carve out excitable pathways, and high spatial frequency regions provide the large gradient of transmembrane potential required to initiate break excitation.

Effect of plunge electrodes in active cardiac tissue with curving fibers

OBJECTIVES

Our goal is to determine if plunge electrodes change how the heart responds to electrical stimulation.

BACKGROUND

Several experiments designed to study the induction of a rotor in cardiac tissue have used plunge electrodes to measure the transmural potential. It is our hypothesis that these electrodes may have affected the electrical response of the tissue to a shock.

METHODS

We previously have shown that a single plunge electrode in two-dimensional, passive cardiac tissue induces a significant transmembrane potential when stimulated by a large shock. In this study, we expand our simulation to include an array of nine electrodes in active tissue with curving fibers. We compare the thresholds for rotor induction in tissue with and without electrodes by initiating a planar S1 wavefront and then stimulating the tissue at different intervals with a uniform S2 electric field perpendicular to S1. In tissue without plunge electrodes, virtual electrode polarization due to the curving fibers is generally widespread over the entire tissue, whereas polarization tends to be localized around the electrodes in tissue including them.

RESULTS

Our results show that at some S1-S2 intervals, the presence of plunge electrodes can result in reentry when it otherwise would not be possible. For other S1-S2 intervals, such as during the vulnerable period when the reentry threshold is at a minimum, the induction of reentry is unaffected by the presence of plunge electrodes.

CONCLUSIONS

Plunge electrodes can play an important role during the stimulation of cardiac tissue, but this is highly dependent on the chosen S1-S2 interval.

Computer simulations of successful defibrillation in decoupled and non-uniform cardiac tissue

Aim

The aim of the present study is to investigate the origin and effect of virtual electrode polarization in uniform, decoupled and non-uniform cardiac tissue during field stimulation.

Methods

A discrete bidomain model with active membrane behaviour was used to simulate normal cardiac tissue as well as cardiac tissue that is decoupled due to fibrosis and gap junction remodelling. Various uniform and non-uniform electric fields were applied to the external domain of uniform, decoupled and non-uniform resting cardiac tissue as well as cardiac tissue in which spiral waves were induced.

Results

Field stimulation applied on non-uniform tissue results in more virtual electrodes compared with uniform tissue. The spiral waves were terminated in decoupled tissue, but not in uniform, homogeneous tissue. By gradually increasing local differences in intracellular conductivities, the amount and spread of virtual electrodes increased and the spiral waves were terminated.

Conclusion

Fast depolarization of the tissue after field stimulation may be explained by intracellular decoupling and spatial heterogeneity present in normal and pathological cardiac tissue. We demonstrated that termination of spiral waves by means of field stimulation can be achieved when the tissue is modelled as a non-uniform, anisotropic bidomain with active membrane behaviour.

The topology of defibrillation

We describe how Art Winfree's ideas about phase singularities can be used to understand the response of cardiac tissue with a random preexisting pattern of reentrant waves (fibrillation) to a large brief current stimulus. This discussion is organized around spatial dimension, beginning with a discussion of reentry on a periodic ring, followed by reentry in a two-dimensional planar domain (spiral waves), and ending with consideration of three-dimensional reentrant patterns (scroll waves). In all cases, we show how reentrant activity is changed by the application of a shock, describing conditions under which defibrillation is successful or not. Using topological arguments we draw the general conclusion that with a generic placement of stimulating electrodes, large-scale virtual electrodes do not give an adequate explanation for the mechanism of defibrillation.