Entrainment by an extracellular AC stimulus in a computational model of cardiac tissue

Computational rabbit models to investigate the initiation, perpetuation, and termination of ventricular arrhythmia

Current understanding of cardiac electrophysiology has been greatly aided by computational work performed using rabbit ventricular models. This article reviews the contributions of multiscale models of rabbit ventricles in understanding cardiac arrhythmia mechanisms. This review will provide an overview of multiscale modeling of the rabbit ventricles. It will then highlight works that provide insights into the role of the conduction system, complex geometric structures, and heterogeneous cellular electrophysiology in diseased and healthy rabbit hearts to the initiation and maintenance of ventricular arrhythmia. Finally, it will provide an overview on the contributions of rabbit ventricular modeling on understanding the mechanisms underlying shock-induced defibrillation.

High frequency stimulation of cardiac myocytes: a theoretical and computational study

High-frequency stimulation (HFS) has recently been identified as a novel approach for terminating life-threatening cardiac arrhythmias. HFS elevates myocyte membrane potential and blocks electrical conduction for the duration of the stimulus. However, low amplitude HFS can induce rapidly firing action potentials, which may reinitiate an arrhythmia. The cellular level mechanisms underlying HFS-induced electrical activity are not well understood. Using a multiscale method, we show that a minimal myocyte model qualitatively reproduces the influence of HFS on cardiac electrical activity. Theoretical analysis and simulations suggest that persistent activation and de-inactivation of ionic currents, in particular a fast inward window current, underlie HFS-induced action potentials and membrane potential elevation, providing hypotheses for future experiments. We derive analytical expressions to describe how HFS modifies ionic current amplitude and gating dynamics. We show how fast inward current parameters influence the parameter regimes for HFS-induced electrical activity, demonstrating how the efficacy of HFS as a therapy for terminating arrhythmias may depend on the presence of pathological conditions or pharmacological treatments. Finally, we demonstrate that HFS terminates cardiac arrhythmias in a one-dimensional ring of cardiac tissue. In this study, we demonstrate a novel approach to characterize the influence of HFS on ionic current gating dynamics, provide new insight into HFS of the myocardium, and suggest mechanisms underlying HFS-induced electrical activity.

Advances in modeling ventricular arrhythmias: from mechanisms to the clinic

Modern cardiovascular research has increasingly recognized that heart models and simulation can help interpret an array of experimental data and dissect important mechanisms and interrelationships, with developments rooted in the iterative interaction between modeling and experimentation. This article reviews the progress made in simulating cardiac electrical behavior at the level of the organ and, specifically, in the development of models of ventricular arrhythmias and fibrillation, as well as their termination (defibrillation). The ability to construct multiscale models of ventricular arrhythmias, representing integrative behavior from the molecule to the entire organ, has enabled mechanistic inquiry into the dynamics of ventricular arrhythmias in the diseased myocardium, in understanding drug-induced proarrhythmia, and in the development of new modalities for defibrillation, to name a few. In this article, we also review the initial use of ventricular models of arrhythmia in personalized diagnosis, treatment planning, and prevention of sudden cardiac death. Implementing individualized cardiac simulations at the patient bedside is poised to become one of the most thrilling examples of computational science and engineering approaches in translational medicine.

Spatiotemporally controlled cardiac conduction block using high-frequency electrical stimulation

Background

Methods for the electrical inhibition of cardiac excitation have long been sought to control excitability and conduction, but to date remain largely impractical. High-amplitude alternating current (AC) stimulation has been known to extend cardiac action potentials (APs), and has been recently exploited to terminate reentrant arrhythmias by producing reversible conduction blocks. Yet, low-amplitude currents at similar frequencies have been shown to entrain cardiac tissues by generation of repetitive APs, leading in some cases to ventricular fibrillation and hemodynamic collapse in vivo. Therefore, an inhibition method that does not lead to entrainment – irrespective of the stimulation amplitude (bound to fluctuate in an in vivo setting) – is highly desirable.

Methodology/Principal Findings

We investigated the effects of broader amplitude and frequency ranges on the inhibitory effects of extracellular AC stimulation on HL-1 cardiomyocytes cultured on microelectrode arrays, using both sinusoidal and square waveforms. Our results indicate that, at sufficiently high frequencies, cardiac tissue exhibits a binary response to stimulus amplitude with either prolonged APs or no effect, thereby effectively avoiding the risks of entrainment by repetitive firing observed at lower frequencies. We further demonstrate the ability to precisely define reversible local conduction blocks in beating cultures without influencing the propagation activity in non-blocked areas. The conduction blocks were spatiotemporally controlled by electrode geometry and stimuli duration, respectively, and sustainable for long durations (300 s).

Conclusion/Significance

Inhibition of cardiac excitation induced by high-frequency AC stimulation exhibits a binary response to amplitude above a threshold frequency, enabling the generation of reversible conduction blocks without the risks of entrainment. This inhibition method could yield novel approaches for arrhythmia modeling in vitro, as well as safer and more efficacious tools for in vivo cardiac mapping and radio-frequency ablation guidance applications.

Reversible cardiac conduction block and defibrillation with high-frequency electric field

Electrical impulse propagation is an essential function in cardiac, skeletal muscle, and nervous tissue. Abnormalities in cardiac impulse propagation underlie lethal reentrant arrhythmias, including ventricular fibrillation. Temporary propagation block throughout the ventricular myocardium could possibly terminate these arrhythmias. Electrical stimulation has been applied to nervous tissue to cause reversible conduction block, but has not been explored sufficiently in cardiac tissue. We show that reversible propagation block can be achieved in cardiac tissue by holding myocardial cells in a refractory state for a designated period of time by applying a sustained sinusoidal high-frequency alternating current (HFAC); in doing so, reentrant arrhythmias are terminated. We demonstrate proof of concept using several models, including optically mapped monolayers of neonatal rat ventricular cardiomyocytes, Langendorff-perfused guinea pig and rabbit hearts, intact anesthetized adult rabbits, and computer simulations of whole-heart impulse propagation. HFAC may be an effective and potentially safer alternative to direct current application, currently used to treat ventricular fibrillation.

Dofetilide effects on the inhibition by trains of subthreshold conditioning stimuli

We investigated the electrophysiological actions of dofetilide upon the ventricular myocardium to determine whether the drug modifies the inhibitory effects of subthreshold stimuli trains upon ventricular refractoriness. In nine Langendorff perfused rabbit hearts, ventricular epicardial electrodes were used to determine the following parameters at baseline and during dofetilide perfusion (0.5 micromolar): effective (ERP) and functional (FRP) refractory periods, conduction velocity (CV), wavelength (WL), and ERP prolongation (inhibitory effect) induced by subthreshold stimuli trains (STr) at pulse frequencies of 100, 300, and 600 Hz. Dofetilide significantly prolongs ventricular refractoriness and WL: ERP increment (Dofetilide-baseline, 300 ms cycle length) = 33 +/- 20 ms (24 +/- 12%, P < 0.01); FRP increment = 37 +/- 19 ms (23%+/- 10%, P < 0.01); WL increment = 4.1 +/- 3.2 cm (27%+/- 20%, P < 0.01), without modifying CV. These effects are diminished upon increasing the stimulation frequency: ERP increment (Dofetilide-baseline, 150 ms cycle length) = 18 +/- 10 ms (18%+/- 12%, P < 0.05); FRP increment = 15 +/- 4 ms (14%+/- 5%, P < 0.01); WL increment = 1.9 +/- 1.7 cm (18%+/- 10%, P < 0.01). The STr significantly prolong ERP, and the increments obtained at baseline and during dofetilide perfusion are similar. In both cases the inhibitory effect is slight for STr of 100 Hz (baseline = 5 +/- 3 ms, dofetilide = 6 +/- 5 ms, with nonsignificant (ns) differences between them) and highly manifest for STr of 300 Hz (baseline = 76 +/- 33 ms, dofetilide = 87 +/- 32 ms, ns) and 600 Hz (baseline = 109 +/- 39 ms, dofetilide = 89 +/- 34 ms, ns). Dofetilide prolongs ventricular refractoriness and WL, exerting a reverse-frequency dependent effect without modifying CV. The inhibitory effect of STr is greater when their pulse frequency is increased, and its magnitude is similar under the influence of dofetilide. During dofetilide perfusion the inhibitory effect of STr adds to the ERP prolongation induced by the drug.