Multistage electrotherapy delivered through chronically-implanted leads terminates atrial fibrillation with lower energy than a single biphasic shock

Efficient termination of cardiac arrhythmias using optogenetic resonant feedback pacing

Malignant cardiac tachyarrhythmias are associated with complex spatiotemporal excitation of the heart. The termination of these life-threatening arrhythmias requires high-energy electrical shocks that have significant side effects, including tissue damage, excruciating pain, and worsening prognosis. This significant medical need has motivated the search for alternative approaches that mitigate the side effects, based on a comprehensive understanding of the nonlinear dynamics of the heart. Cardiac optogenetics enables the manipulation of cellular function using light, enhancing our understanding of nonlinear cardiac function and control. Here, we investigate the efficacy of optically resonant feedback pacing (ORFP) to terminate ventricular tachyarrhythmias using numerical simulations and experiments in transgenic Langendorff-perfused mouse hearts. We show that ORFP outperforms the termination efficacy of the optical single-pulse (OSP) approach. When using ORFP, the total energy required for arrhythmia termination, i.e., the energy summed over all pulses in the sequence, is 1 mJ. With a success rate of 50%, the energy per pulse is 40 times lower than with OSP with a pulse duration of 10 ms. We demonstrate that even at light intensities below the excitation threshold, ORFP enables the termination of arrhythmias by spatiotemporal modulation of excitability inducing spiral wave drift.

Optimising low-energy defibrillation in 2D cardiac tissue with a genetic algorithm

Sequences of low-energy electrical pulses can effectively terminate ventricular fibrillation (VF) and avoid the side effects of conventional high-energy electrical defibrillation shocks, including tissue damage, traumatic pain, and worsening of prognosis. However, the systematic optimisation of sequences of low-energy pulses remains a major challenge. Using 2D simulations of homogeneous cardiac tissue and a genetic algorithm, we demonstrate the optimisation of sequences with non-uniform pulse energies and time intervals between consecutive pulses for efficient VF termination. We further identify model-dependent reductions of total pacing energy ranging from ∼4% to ∼80% compared to reference adaptive-deceleration pacing (ADP) protocols of equal success rate (100%).

Electrical Stimulation for Low-Energy Termination of Cardiac Arrhythmias: a Review

Cardiac arrhythmias are a leading cause of morbidity and mortality in the developed world, estimated to be responsible for hundreds of thousands of deaths annually. Our understanding of the electrophysiological mechanisms of such arrhythmias has grown since they were formally characterized in the late nineteenth century, and this has led to the development of numerous devices and therapies that have markedly improved outcomes for patients affected by such conditions. Despite these advancements, the application of a single large shock remains the clinical standard for treating deadly tachyarrhythmias. Such defibrillating shocks are undoubtedly effective in terminating such arrhythmias; however, they are applied without forewarning, contributing to the patient's stress and anxiety; they can be intensely painful; and they can have adverse psychological and physiological effects on patients. In recent years, there has been interest in developing defibrillation protocols that can terminate arrhythmias without crossing the human pain threshold for energy delivery, generally estimated to be between 0.1 and 1 J. In this article, we review existing literature on the development of such low-energy defibrillation methods and their underlying mechanisms, in an attempt to broadly describe the current landscape of these technologies.

Taming cardiac arrhythmias: Terminating spiral wave chaos by adaptive deceleration pacing

Sequences of weak electrical pulses are considered a promising alternative for terminating ventricular and atrial fibrillations while avoiding strong defibrillation shocks with adverse side effects. In this study, using numerical simulations of four different 2D excitable media, we show that pulse trains with increasing temporal intervals between successive pulses (deceleration pacing) provide high success rates at low energies. Furthermore, we propose a simple and robust approach to calculate inter-pulse spacing directly from the frequency spectrum of the dynamics (for instance, computed based on the electrocardiogram), which can be practically used in experiments and clinical applications.

Non-monotonous dose response function of the termination of spiral wave chaos

The conventional termination technique of life threatening cardiac arrhythmia like ventricular fibrillation is the application of a high-energy electrical defibrillation shock, coming along with severe side-effects. In order to improve the current treatment reducing these side-effects, the application of pulse sequences of lower energy instead of a single high-energy pulse are promising candidates. In this study, we show that in numerical simulations the dose-response function of pulse sequences applied to two-dimensional spiral wave chaos is not necessarily monotonously increasing, but exhibits a non-trivial frequency dependence. This insight into crucial phenomena appearing during termination attempts provides a deeper understanding of the governing termination mechanisms in general, and therefore may open up the path towards an efficient termination of cardiac arrhythmia in the future.

The role of pulse timing in cardiac defibrillation

Life-threatening cardiac arrhythmias require immediate defibrillation. For state-of-the-art shock treatments, a high field strength is required to achieve a sufficient success rate for terminating the complex spiral wave (rotor) dynamics underlying cardiac fibrillation. However, such high energy shocks have many adverse side effects due to the large electric currents applied. In this study, we show, using 2D simulations based on the Fenton-Karma model, that also pulses of relatively low energy may terminate the chaotic activity if applied at the right moment in time. In our simplified model for defibrillation, complex spiral waves are terminated by local perturbations corresponding to conductance heterogeneities acting as virtual electrodes in the presence of an external electric field. We demonstrate that time series of the success rate for low energy shocks exhibit pronounced peaks which correspond to short intervals in time during which perturbations aiming at terminating the chaotic fibrillation state are (much) more successful. Thus, the low energy shock regime, although yielding very low temporal average success rates, exhibits moments in time for which success rates are significantly higher than the average value shown in dose-response curves. This feature might be exploited in future defibrillation protocols for achieving high termination success rates with low or medium pulse energies.

Novel Low-Voltage MultiPulse Therapy to Terminate Atrial Fibrillation

OBJECTIVES

This first-in-human feasibility study was undertaken to translate the novel low-voltage MultiPulse Therapy (MPT) (Cardialen, Inc., Minneapolis, Minnesota), which was previously been shown to be effective in preclinical studies in terminating atrial fibrillation (AF), into clinical use.

BACKGROUND

Current treatment options for AF, the most common arrhythmia in clinical practice, have limited success. Previous attempts at treating AF by using implantable devices have been limited by the painful nature of high-voltage shocks.

METHODS

Forty-two patients undergoing AF ablation were recruited at 6 investigational centers worldwide. Before ablation, electrode catheters were placed in the coronary sinus, right and/or left atrium, for recording and stimulation. After the induction of AF, MPT, which consists of up to a 3-stage sequence of far- and near-field stimulation pulses of varied amplitude, duration, and interpulse timing, was delivered via temporary intracardiac leads. MPT parameters and delivery methods were iteratively optimized.

RESULTS

In the 14 patients from the efficacy phase, MPT terminated 37 of 52 (71%) of AF episodes, with the lowest median energy of 0.36 J (interquartile range [IQR]: 0.14 to 1.21 J) and voltage of 42.5 V (IQR: 25 to 75 V). Overall, 38% of AF terminations occurred within 2 seconds of MPT delivery (p < 0.0001). Shorter time between AF induction and MPT predicted success of MPT in terminating AF (p < 0.001).

CONCLUSIONS

MPT effectively terminated AF at voltages and energies known to be well tolerated or painless in some patients. Our results support further studies of the concept of implanted devices for early AF conversion to reduce AF burden, symptoms, and progression.

A shock-free approach for ambulatory cardioversion in atrial fibrillation

Atrial fibrillation (AF), the most common persistent arrhythmia, is terminated most effectively by electrical cardioversion. This therapy requires in-hospital sedation to relieve the pain caused by electric shocks. Recently, our research group showed how the heart itself could be enabled to detect and terminate arrhythmias, including AF, thereby revealing the discovery of fully biological, shock-free cardioversion. Because of its biological nature, neither electric shocks nor hardware/software is required for sinus rhythm (SR) restoration, which creates a new perspective for ambulatory AF termination. Increasing evidence suggests that patients may indeed benefit from such continuous real-time rhythm control.

Terminating transient chaos in spatially extended systems

In many real-life systems, transient chaotic dynamics plays a major role. For instance, the chaotic spiral or scroll wave dynamics of electrical excitation waves during life-threatening cardiac arrhythmias can terminate by itself. Epileptic seizures have recently been related to the collapse of transient chimera states. Controlling chaotic transients, either by maintaining the chaotic dynamics or by terminating it as quickly as possible, is often desired and sometimes even vital (as in the case of cardiac arrhythmias). We discuss in this study that the difference of the underlying structures in state space between a chaotic attractor (persistent chaos) and a chaotic saddle (transient chaos) may have significant implications for efficient control strategies in real life systems. In particular, we demonstrate that in the latter case, chaotic dynamics in spatially extended systems can be terminated via a relatively low number of (spatially and temporally) localized perturbations. We demonstrate as a proof of principle that control and targeting of high-dimensional systems exhibiting transient chaos can be achieved with exceptionally small interactions with the system. This insight may impact future control strategies in real-life systems like cardiac arrhythmias.

Conceptual Intra-Cardiac Electrode Configurations That Facilitate Directional Cardiac Stimulation for Optimal Electrotherapy

OBJECTIVE

Electrotherapy remains the most effective direct therapy against lethal cardiac arrhythmias. When an arrhythmic event is sensed, either strong electric shocks or controlled rapid pacing is automatically applied directly to the heart via an implanted cardioverter defibrillator (ICDs). Despite their success, ICDs remain a highly non-optimal therapy: the strong shocks required for defibrillation cause significant extra-cardiac stimulation, resulting in pain and long-term tissue damage, and can also limit battery life. When used in anti-tachycardia pacing mode, ICDs are also often ineffective, as the pacing electrode can be far away from the centre of the arrhythmia, making it hard for the paced wave to interrupt and terminate it.

METHODS

In this paper, we present two conceptual intra-cardiac directional electrode configurations in silico based on novel arrangements of pairs of positive-negative electrodes. Both configurations have the potential to cause preferential excitation on specific regions of the heart.

RESULTS

We demonstrate how the properties of the induced field varies spatially around the electrodes and how it depends upon the specific arrangements of dipole electrode pairs. The results show that when tested within anatomically-realistic rabbit ventricular models, both electrode configurations produce strong virtual electrodes on the targeted endocardial surfaces, with weaker virtual electrodes produced elsewhere.

CONCLUSIONS

The proposed electrode configurations may facilitate targeted far-field anti-tachycardia pacing and/or defibrillation, which may be useful in cases where conventional anti-tachycardia pacing fails. In addition, the conceptual electrode designs intrinsically confine the electric field to the immediate vicinity of the electrodes, and may, thus, minimize pain due to unnecessary extra-cardiac stimulation.

Optogenetic termination of atrial fibrillation in mice

Aims

The primary goal in the treatment of symptomatic atrial fibrillation/flutter (AF) is to restore sinus rhythm by cardioversion. Electrical shocks are highly effective, but have to be applied under analgo-sedation and can further harm the heart. In order to develop a novel pain-free and less harmful approach, we explored herein the optogenetic cardioversion by light-induced depolarization.

Methods and results

Hearts from mice expressing Channelrhodopsin-2 (ChR2) and the AF-promoting loss-of-function Connexin 40 Ala96Ser mutation were explanted and perfused with low K+ Tyrode's solution and an atrial KATP-channel activator. This new protocol shortened atrial refractoriness as well as slowed atrial conduction and thereby enabled the induction of sustained AF. AF episodes could be terminated by epicardial illumination of the atria with focussed blue light (470 nm, 0.4 mW/mm2) with an efficacy of ∼97% (n = 17 hearts). In > 80% of cases, light directly terminated the AF episode with onset of illumination. Because similar illumination intensity was able to locally inhibit atrial activity, we propose that a light-induced block of electrical activity is responsible for reliable AF termination. The success rate was strongly depending on the illuminated area, applied light intensity and duration of illumination. Importantly, we were also able to demonstrate optogenetic termination of AF in vivo, using epicardial illumination through the open chest (n = 3 hearts). To point towards a translational potential, we systemically injected an adeno-associated virus to express ChR2 in wild type hearts. After 6-8 months, we found robust ChR2 expression in the atria, enabling light-mediated AF termination in six of seven mice tested.

Conclusion

We provide the first evidence for optogenetic termination of atrial tachyarrhythmia in intact hearts from transgenic as well as wild type mice ex and in vivo. Thus, this report could lay the foundation for the development of implantable devices for pain-free termination of AF.

A Magnetic Resonance Imaging-Conditional External Cardiac Defibrillator for Resuscitation Within the Magnetic Resonance Imaging Scanner Bore

BACKGROUND

Subjects undergoing cardiac arrest within a magnetic resonance imaging (MRI) scanner are currently removed from the bore and then from the MRI suite, before the delivery of cardiopulmonary resuscitation and defibrillation, potentially increasing the risk of mortality. This precludes many higher-risk (acute ischemic and acute stroke) patients from undergoing MRI and MRI-guided intervention. An MRI-conditional cardiac defibrillator should enable scanning with defibrillation pads attached and the generator ON, enabling application of defibrillation within the seconds of MRI after a cardiac event. An MRI-conditional external defibrillator may improve patient acceptance for MRI procedures.

METHODS AND RESULTS

A commercial external defibrillator was rendered 1.5 Tesla MRI-conditional by the addition of novel radiofrequency filters between the generator and commercial disposable surface pads. The radiofrequency filters reduced emission into the MRI scanner and prevented cable/surface pad heating during imaging, while preserving all the defibrillator monitoring and delivery functions. Human volunteers were imaged using high specific absorption rate sequences to validate MRI image quality and lack of heating. Swine were electrically fibrillated (n=4) and thereafter defibrillated both outside and inside the MRI bore. MRI image quality was reduced by 0.8 or 1.6 dB, with the generator in monitoring mode and operating on battery or AC power, respectively. Commercial surface pads did not create artifacts deeper than 6 mm below the skin surface. Radiofrequency heating was within US Food and Drug Administration guidelines. Defibrillation was completely successful inside and outside the MRI bore.

CONCLUSIONS

A prototype MRI-conditional defibrillation system successfully defibrillated in the MRI without degrading the image quality or increasing the time needed for defibrillation. It can increase patient acceptance for MRI procedures.

Mechanisms of vortices termination in the cardiac muscle

We propose a solution to a long-standing problem: how to terminate multiple vortices in the heart, when the locations of their cores and their critical time windows are unknown. We scan the phases of all pinned vortices in parallel with electric field pulses (E-pulses). We specify a condition on pacing parameters that guarantees termination of one vortex. For more than one vortex with significantly different frequencies, the success of scanning depends on chance, and all vortices are terminated with a success rate of less than one. We found that a similar mechanism terminates also a free (not pinned) vortex. A series of about 500 experiments with termination of ventricular fibrillation by E-pulses in pig isolated hearts is evidence that pinned vortices, hidden from direct observation, are significant in fibrillation. These results form a physical basis needed for the creation of new effective low energy defibrillation methods based on the termination of vortices underlying fibrillation.

Intracardiac impedance response during acute AF internal cardioversion using novel rectilinear and capacitor-discharge waveforms

Intracardiac impedance (ICI) is a major determinant of success during internal cardioversion of atrial fibrillation (AF). However, there have been few studies that have examined the dynamic behaviour of atrial impedance during internal cardioversion in relation to clinical outcome. In this study, voltage and current waveforms captured during internal cardioversion of acute AF in ovine models using novel radiofrequency (RF) generated low-tilt rectilinear and conventional capacitor-discharge based shock waveforms were retrospectively analysed using a digital signal processing algorithm to investigate the dynamic behaviour of atrial impedance during cardioversion. The algorithm was specifically designed to facilitate the simultaneous analysis of multiple impedance parameters, including: mean intracardiac impedance (Z M), intracardiac impedance variance (ICIV) and impedance amplitude spectrum area (IAMSA) for each cardioversion event. A significant reduction in ICI was observed when comparing two successive shocks of increasing energy where cardioversion outcome was successful. In addition, ICIV and IAMSA variables were found to inversely correlate to the magnitude of energy delivered; with a stronger correlation found to the former parameter. In conclusion, ICIV and IAMSA have been evidenced as two key dynamic intracardiac impedance variables that may prove useful in better understanding of the cardioversion process and that could potentially act as prognostic markers with respect to clinical outcome.

Imaging of Ventricular Fibrillation and Defibrillation: The Virtual Electrode Hypothesis

Ventricular fibrillation is the major underlying cause of sudden cardiac death. Understanding the complex activation patterns that give rise to ventricular fibrillation requires high resolution mapping of localized activation. The use of multi-electrode mapping unraveled re-entrant activation patterns that underlie ventricular fibrillation. However, optical mapping contributed critically to understanding the mechanism of defibrillation, where multi-electrode recordings could not measure activation patterns during and immediately after a shock. In addition, optical mapping visualizes the virtual electrodes that are generated during stimulation and defibrillation pulses, which contributed to the formulation of the virtual electrode hypothesis. The generation of virtual electrode induced phase singularities during defibrillation is arrhythmogenic and may lead to the induction of fibrillation subsequent to defibrillation. Defibrillating with low energy may circumvent this problem. Therefore, the current challenge is to use the knowledge provided by optical mapping to develop a low energy approach of defibrillation, which may lead to more successful defibrillation.

Toward microendoscopy-inspired cardiac optogenetics in vivo: technical overview and perspective

The ability to perform precise, spatially localized actuation and measurements of electrical activity in the heart is crucial in understanding cardiac electrophysiology and devising new therapeutic solutions for control of cardiac arrhythmias. Current cardiac imaging techniques (i.e. optical mapping) employ voltage- or calcium-sensitive fluorescent dyes to visualize the electrical signal propagation through cardiac syncytium in vitro or in situ with very high-spatiotemporal resolution. The extension of optogenetics into the cardiac field, where cardiac tissue is genetically altered to express light-sensitive ion channels allowing electrical activity to be elicited or suppressed in a precise cell-specific way, has opened the possibility for all-optical interrogation of cardiac electrophysiology. In vivo application of cardiac optogenetics faces multiple challenges and necessitates suitable optical systems employing fiber optics to actuate and sense electrical signals. In this technical perspective, we present a compendium of clinically relevant access routes to different parts of the cardiac electrical conduction system based on currently employed catheter imaging systems and determine the quantitative size constraints for endoscopic cardiac optogenetics. We discuss the relevant technical advancements in microendoscopy, cardiac imaging, and optogenetics and outline the strategies for combining them to create a portable, miniaturized fiber-based system for all-optical interrogation of cardiac electrophysiology in vivo.

Light-induced termination of spiral wave arrhythmias by optogenetic engineering of atrial cardiomyocytes

AIMS

Atrial fibrillation (AF) is the most common cardiac arrhythmia and often involves reentrant electrical activation (e.g. spiral waves). Drug therapy for AF can have serious side effects including proarrhythmia, while electrical shock therapy is associated with discomfort and tissue damage. Hypothetically, forced expression and subsequent activation of light-gated cation channels in cardiomyocytes might deliver a depolarizing force sufficient for defibrillation, thereby circumventing the aforementioned drawbacks. We therefore investigated the feasibility of light-induced spiral wave termination through cardiac optogenetics.

METHODS AND RESULTS

Neonatal rat atrial cardiomyocyte monolayers were transduced with lentiviral vectors encoding light-activated Ca(2+)-translocating channelrhodopsin (CatCh; LV.CatCh∼eYFP↑) or eYFP (LV.eYFP↑) as control, and burst-paced to induce spiral waves rotating around functional cores. Effects of CatCh activation on reentry were investigated by optical and multi-electrode array (MEA) mapping. Western blot analyses and immunocytology confirmed transgene expression. Brief blue light pulses (10 ms/470 nm) triggered action potentials only in LV.CatCh∼eYFP↑-transduced cultures, confirming functional CatCh-mediated current. Prolonged light pulses (500 ms) resulted in reentry termination in 100% of LV.CatCh∼eYFP↑-transduced cultures (n = 31) vs. 0% of LV.eYFP↑-transduced cultures (n = 11). Here, CatCh activation caused uniform depolarization, thereby decreasing overall excitability (MEA peak-to-peak amplitude decreased 251.3 ± 217.1 vs. 9.2 ± 9.5 μV in controls). Consequently, functional coresize increased and phase singularities (PSs) drifted, leading to reentry termination by PS-PS or PS-boundary collisions.

CONCLUSION

This study shows that spiral waves in atrial cardiomyocyte monolayers can be terminated effectively by a light-induced depolarizing current, produced by the arrhythmogenic substrate itself, upon optogenetic engineering. These results provide proof-of-concept for shockless defibrillation.