Spatiotemporal correlation between phase singularities and wavebreaks during ventricular fibrillation

Rotor Localization and Phase Mapping of Cardiac Excitation Waves Using Deep Neural Networks

The analysis of electrical impulse phenomena in cardiac muscle tissue is important for the diagnosis of heart rhythm disorders and other cardiac pathophysiology. Cardiac mapping techniques acquire local temporal measurements and combine them to visualize the spread of electrophysiological wave phenomena across the heart surface. However, low spatial resolution, sparse measurement locations, noise and other artifacts make it challenging to accurately visualize spatio-temporal activity. For instance, electro-anatomical catheter mapping is severely limited by the sparsity of the measurements, and optical mapping is prone to noise and motion artifacts. In the past, several approaches have been proposed to create more reliable maps from noisy or sparse mapping data. Here, we demonstrate that deep learning can be used to compute phase maps and detect phase singularities in optical mapping videos of ventricular fibrillation, as well as in very noisy, low-resolution and extremely sparse simulated data of reentrant wave chaos mimicking catheter mapping data. The self-supervised deep learning approach is fundamentally different from classical phase mapping techniques. Rather than encoding a phase signal from time-series data, a deep neural network instead learns to directly associate phase maps and the positions of phase singularities with short spatio-temporal sequences of electrical data. We tested several neural network architectures, based on a convolutional neural network (CNN) with an encoding and decoding structure, to predict phase maps or rotor core positions either directly or indirectly via the prediction of phase maps and a subsequent classical calculation of phase singularities. Predictions can be performed across different data, with models being trained on one species and then successfully applied to another, or being trained solely on simulated data and then applied to experimental data. Neural networks provide a promising alternative to conventional phase mapping and rotor core localization methods. Future uses may include the analysis of optical mapping studies in basic cardiovascular research, as well as the mapping of atrial fibrillation in the clinical setting.

Ventricular divergence correlates with epicardial wavebreaks and predicts ventricular arrhythmia in isolated rabbit hearts during therapeutic hypothermia

INTRODUCTION

High beat-to-beat morphological variation (divergence) on the ventricular electrogram during programmed ventricular stimulation (PVS) is associated with increased risk of ventricular fibrillation (VF), with unclear mechanisms. We hypothesized that ventricular divergence is associated with epicardial wavebreaks during PVS, and that it predicts VF occurrence.

METHOD AND RESULTS

Langendorff-perfused rabbit hearts (n = 10) underwent 30-min therapeutic hypothermia (TH, 30°C), followed by a 20-min treatment with rotigaptide (300 nM), a gap junction modifier. VF inducibility was tested using burst ventricular pacing at the shortest pacing cycle length achieving 1:1 ventricular capture. Pseudo-ECG (p-ECG) and epicardial activation maps were simultaneously recorded for divergence and wavebreaks analysis, respectively. A total of 112 optical and p-ECG recordings (62 at TH, 50 at TH treated with rotigaptide) were analyzed. Adding rotigaptide reduced ventricular divergence, from 0.13±0.10 at TH to 0.09±0.07 (p = 0.018). Similarly, rotigaptide reduced the number of epicardial wavebreaks, from 0.59±0.73 at TH to 0.30±0.49 (p = 0.036). VF inducibility decreased, from 48±31% at TH to 22±32% after rotigaptide infusion (p = 0.032). Linear regression models showed that ventricular divergence correlated with epicardial wavebreaks during TH (p<0.001).

CONCLUSION

Ventricular divergence correlated with, and might be predictive of epicardial wavebreaks during PVS at TH. Rotigaptide decreased both the ventricular divergence and epicardial wavebreaks, and reduced the probability of pacing-induced VF during TH.

Dynamical mechanism of atrial fibrillation: A topological approach

While spiral wave breakup has been implicated in the emergence of atrial fibrillation, its role in maintaining this complex type of cardiac arrhythmia is less clear. We used the Karma model of cardiac excitation to investigate the dynamical mechanisms that sustain atrial fibrillation once it has been established. The results of our numerical study show that spatiotemporally chaotic dynamics in this regime can be described as a dynamical equilibrium between topologically distinct types of transitions that increase or decrease the number of wavelets, in general agreement with the multiple wavelets' hypothesis. Surprisingly, we found that the process of continuous excitation waves breaking up into discontinuous pieces plays no role whatsoever in maintaining spatiotemporal complexity. Instead, this complexity is maintained as a dynamical balance between wave coalescence-a unique, previously unidentified, topological process that increases the number of wavelets-and wave collapse-a different topological process that decreases their number.

A New Efficient Method for Detecting Phase Singularity in Cardiac Fibrillation

Background

The point of phase singularity (PS) is considered to represent a spiral wave core or a rotor in cardiac fibrillation. Computational efficiency is important for detection of PS in clinical electrophysiology. We developed a novel algorithm for highly efficient and robust detection of PS.

Methods

In contrast to the conventional method, which calculates PS based on the line integral of the phase around a PS point equal to ±2π (the Iyer-Gray method), the proposed algorithm (the location-centric method) looks for the phase discontinuity point at which PS actually occurs. We tested the efficiency and robustness of these two methods in a two-dimensional mathematical model of atrial fibrillation (AF), with and without remodeling of ionic currents.

Results

1. There was a significant association, in terms of the Hausdorff distance (3.30 ± 0.0 mm), between the PS points measured using the Iyer-Gray and location-centric methods, with almost identical PS trajectories generated by the two methods. 2. For the condition of electrical remodeling of AF (0.3 × I_CaL), the PS points calculated by the two methods were satisfactorily co-localized (with the Hausdorff distance of 1.64 ± 0.09 mm). 3. The proposed location-centric method was substantially more efficient than the Iyer-Gray method, with a 28.6-fold and 28.2-fold shorter run times for the control and remodeling scenarios, respectively.

Conclusion

We propose a new location-centric method for calculating PS, which is robust and more efficient compared with the conventionally used method.

The Contribution of Ionic Currents to Rate-Dependent Action Potential Duration and Pattern of Reentry in a Mathematical Model of Human Atrial Fibrillation

Persistent atrial fibrillation (PeAF) in humans is characterized by shortening of action potential duration (APD) and attenuation of APD rate-adaptation. However, the quantitative influences of particular ionic current alterations on rate-dependent APD changes, and effects on patterns of reentry in atrial tissue, have not been systematically investigated. Using mathematical models of human atrial cells and tissue and performing parameter sensitivity analysis, we evaluated the quantitative contributions to action potential (AP) shortening and APD rate-adaptation of ionic current remodeling seen with PeAF. Ionic remodeling in PeAF was simulated by reducing L-type Ca²⁺ channel current (I_CaL), increasing inward rectifier K⁺ current (I_K1) and modulating five other ionic currents. Parameter sensitivity analysis, which quantified how each ionic current influenced APD in control and PeAF conditions, identified interesting results, including a negative effect of Na⁺/Ca²⁺ exchange on APD only in the PeAF condition. At high pacing rate (2 Hz), electrical remodeling in I_K1 alone accounts for the APD reduction of PeAF, but at slow pacing rate (0.5 Hz) both electrical remodeling in I_CaL alone (-70%) and I_K1 alone (+100%) contribute equally to the APD reduction. Furthermore, AP rate-adaptation was affected by I_Kur in control and by I_NaCa in the PeAF condition. In a 2D tissue model, a large reduction (-70%) of I_CaL becomes a dominant factor leading to a stable spiral wave in PeAF. Our study provides a quantitative and unifying understanding of the roles of ionic current remodeling in determining rate-dependent APD changes at the cellular level and spatial reentry patterns in tissue.

Electrophysiological and structural remodeling in heart failure modulate arrhythmogenesis. 2D simulation study

BACKGROUND

Heart failure is operationally defined as the inability of the heart to maintain blood flow to meet the needs of the body and it is the final common pathway of various cardiac pathologies. Electrophysiological remodeling, intercellular uncoupling and a pro-fibrotic response have been identified as major arrhythmogenic factors in heart failure.

OBJECTIVE

In this study we investigate vulnerability to reentry under heart failure conditions by incorporating established electrophysiological and anatomical remodeling using computer simulations.

METHODS

The electrical activity of human transmural ventricular tissue (5 cm × 5 cm) was simulated using the human ventricular action potential model Grandi et al. under control and heart failure conditions. The MacCannell et al. model was used to model fibroblast electrical activity, and their electrotonic interactions with myocytes. Selected degrees of diffuse fibrosis and variations in intercellular coupling were considered and the vulnerable window (VW) for reentry was evaluated following cross-field stimulation.

RESULTS

No reentry was observed in normal conditions or in the presence of HF ionic remodeling. However, defined amount of fibrosis and/or cellular uncoupling were sufficient to elicit reentrant activity. Under conditions where reentry was generated, HF electrophysiological remodeling did not alter the width of the VW. However, intermediate fibrosis and cellular uncoupling significantly widened the VW. In addition, biphasic behavior was observed, as very high fibrotic content or very low tissue conductivity hampered the development of reentry. Detailed phase analysis of reentry dynamics revealed an increase of phase singularities with progressive fibrotic components.

CONCLUSION

Structural remodeling is a key factor in the genesis of vulnerability to reentry. A range of intermediate levels of fibrosis and intercellular uncoupling can combine to favor reentrant activity.

Pathogenesis of arrhythmias in a model of CKD

Patients with CKD have an increased risk of cardiovascular mortality from arrhythmias and sudden cardiac death. We used a rat model of CKD (Cy/+) to study potential mechanisms of increased ventricular arrhythmias. Rats with CKD showed normal ejection fraction but hypertrophic myocardium. Premature ventricular complexes occurred more frequently in CKD rats than normal rats (42% versus 11%, P=0.18). By optical mapping techniques, action potential duration (APD) at 80% of repolarization was longer in CKD rats (78±4ms) than normal rats (63±3 ms, P<0.05) at a 200-ms pacing cycle length. Calcium transient (CaT) duration was comparable. Pacing cycle length thresholds to induce CaT alternans or APD alternans were longer in CKD rats than normal rats (100±7 versus 80±3 ms and 93±6 versus 76±4 ms for CaT and APD alternans, respectively, P<0.05), suggesting increased vulnerability to ventricular arrhythmia. Ventricular fibrillation was induced in 9 of 12 CKD rats and 2 of 9 normal rats (P<0.05); early afterdepolarization occurred in two CKD rats but not normal rats. The mRNA levels of TGF-β, microRNA-21, and sodium calcium-exchanger type 1 were upregulated, whereas the levels of microRNA-29, L-type calcium channel, sarco/endoplasmic reticulum calcium-ATPase type 2a, Kv1.4, and Kv4.3 were downregulated in CKD rats. Cardiac fibrosis was mild and not different between groups. We conclude that cardiac ion channel and calcium handling are abnormal in CKD rats, leading to increased vulnerability to early afterdepolarization, triggered activity, and ventricular arrhythmias.

Interventricular heterogeneity as a substrate for arrhythmogenesis of decoupled mitochondria during ischemia in the whole heart

Myocardial ischemia results in metabolic changes, which collapse the mitochondrial network, that increase the vulnerability of the heart to ventricular fibrillation (VF). It has been demonstrated at the single cell level that uncoupling the mitochondria using carbonyl cyanide p-(tri-fluoromethoxy)phenyl-hydrazone (FCCP) at low concentrations can be cardioprotective. The aim of our study was to investigate the effect of FCCP on arrhythmogenesis during ischemia in the whole rabbit heart. We performed optical mapping of isolated rabbit hearts ( n = 33) during control and 20 min of global ischemia and reperfusion, both with and without pretreatment with the mitochondrial uncoupler FCCP at 100, 50, or 30 nM. No hearts went into VF during ischemia under the control condition, with or without the electromechanical uncoupler blebbistatin. We found that pretreatment with 100 ( n = 4) and 50 ( n = 6) nM FCCP, with or without blebbistatin, leads to VF during ischemia in all hearts, whereas pretreatment with 30 nM of FCCP led to three out of eight hearts going into VF during ischemia. We demonstrated that 30 nM of FCCP significantly increased interventricular (but not intraventricular) action potential duration and conduction velocity heterogeneity in the heart during ischemia, thus providing the substrate for VF. We showed that wavebreaks during VF occurred between the right and left ventricular junction. We also demonstrated that no VF occurred in the heart pretreated with 10 μM glibenclamide, which is known to abolish interventricular heterogeneity. Our results indicate that low concentrations of FCCP, although cardioprotective at the single cell level, are arrhythmogenic at the whole heart level. This is due to the fact that FCCP induces different electrophysiological changes to the right and left ventricle, thus increasing interventricular heterogeneity and providing the substrate for VF.

The role of fine-scale anatomical structure in the dynamics of reentry in computational models of the rabbit ventricles

Fine-scale anatomical structures in the heart may play an important role in sustaining cardiac arrhythmias. However, the extent of this role and how it may differ between species are not fully understood. In this study we used computational modelling to assess the impact of anatomy upon arrhythmia maintenance in the rabbit ventricles. Specifically, we quantified the dynamics of excitation wavefronts during episodes of simulated tachyarrhythmias and fibrillatory arrhythmias, defined as being respectively characterised by relatively low and high spatio-temporal disorganisation.Two computational models were used: a highly anatomically detailed MR-derived rabbit ventricular model (representing vasculature, endocardial structures) and a simplified equivalent model, constructed from the same MR-data but lacking such fine-scale anatomical features. During tachyarrhythmias, anatomically complex and simplified models showed very similar dynamics; however, during fibrillatory arrhythmias, as activation wavelength decreased, the presence of fine-scale anatomical details appeared to marginally increase disorganisation of wavefronts during arrhythmias in the complex model. Although a small amount of clustering of reentrant rotor centres (filaments) around endocardial structures was witnessed in follow-up analysis (which slightly increased during fibrillation as rotor size decreased), this was significantly less than previously reported in large animals. Importantly, no anchoring of reentrant rotors was visibly identifiable in arrhythmia movies. These differences between tachy- and fibrillatory arrhythmias suggest that the relative size of reentrant rotors with respect to anatomical obstacles governs the influence of fine-scale anatomy in the maintenance of ventricular arrhythmias in the rabbit. In conclusion, our simulations suggest that fine-scale anatomical features play little apparent role in the maintenance of tachyarrhythmias in the rabbit ventricles and, contrary to experimental reports in larger animals, appear to play only a minor role in the maintenance of fibrillatory arrhythmias. These findings also have important implications in optimising the level of detail required in anatomical computational meshes frequently used in arrhythmia investigations.

The effect of cardiac sympathetic denervation through bilateral stellate ganglionectomy on electrical properties of the heart

The role of the cardiac sympathetic nerve activity in various cardiac diseases is typically evaluated using β-adrenergic receptor antagonists. However, these antagonists induce global denervation effects not only in the cardiovascular system, but also in the brain and kidney. The objective of this study was to detect the electrophysiological property changes due to 8 days of cardiac sympathetic denervation and investigate the possible mechanisms underlying these changes using a more cardiac-specific bilateral stellate ganglionectomy (SGX) rat model. High-resolution optical mapping using a voltage-sensitive dye was performed in isolated Langendorff-perfused sham and SGX hearts, which were paced at progressively reduced basic cycle lengths under several different conditions: control, pretreatment with isoproterenol, and administration of atenolol and esmolol. Several electrophysiological parameters were recorded during periodic pacing and ventricular fibrillation (VF). Our results demonstrate that cardiac sympathetic denervation by bilateral SGX shortens action potential duration (APD) and flattens the APD restitution curve, but does not significantly affect spatial dispersion of APD. We found that, although the vulnerability of sham and SGX hearts to VF is similar, the dynamics of VF are different. The maximum dominant frequency is higher, and the spatial distribution of VF is more complex in the SGX heart, resulting in different mechanisms of VF. We demonstrated that β(1)-adrenergic receptors are downregulated in the SGX compared with sham hearts. In addition, our data suggest that the mechanism of cardiac sympathetic denervation by SGX surgery is more similar to the administration of β-blocker esmolol than atenolol.

Therapeutic hypothermia (30 degrees C) enhances arrhythmogenic substrates, including spatially discordant alternans, and facilitates pacing-induced ventricular fibrillation in isolated rabbit hearts

BACKGROUND

Therapeutic hypothermia (TH, 30 degrees C) protects the brain from hypoxic injury. However, TH may potentiate the occurrence of lethal ventricular fibrillation (VF), although the mechanism remains unclear. The present study explored the hypothesis that TH enhances wavebreaks during VF and S(1) pacing, facilitates pacing-induced spatially discordant alternans (SDA), and increases the vulnerability of pacing-induced VF.

METHODS AND RESULTS

Using an optical mapping system, epicardial activations of VF were studied in 7 Langendorff-perfused isolated rabbit hearts at baseline (37 degrees C), TH (30 degrees C), and rewarming (37 degrees C). Action potential duration (APD)/conduction velocity (CV) restitution and APD alternans (n=6 hearts) were determined by S(1) pacing at these 3 stages. During TH, there was a higher percentage of VF duration containing epicardial repetitive activities (spatiotemporal periodicity) (P<0.001). However, TH increased phase singularity number (wavebreaks) during VF (P<0.05) and S(1) pacing (P<0.05). TH resulted in earlier onset of APD alternans (P<0.001), which was predominantly SDA (P<0.05), and increased pacing-induced VF episodes (P<0.05). TH also decreased CV, shortened wavelength, and enhanced APD dispersion and the spatial heterogeneity of CV restitution.

CONCLUSIONS

TH (30 degrees C) increased the vulnerability of pacing-induced VF by (1)facilitating wavebreaks during VF and S(1) pacing, and (2)enhancing proarrhythmic electrophysiological parameters, including promoting earlier onset of APD alternans (predominantly SDA) during S(1) pacing.

Origin choice and petal loss in the flower garden of spiral wave tip trajectories

Rotating spiral waves have been observed in numerous biological and physical systems. These spiral waves can be stationary, meander, or even degenerate into multiple unstable rotating waves. The spatiotemporal behavior of spiral waves has been extensively quantified by tracking spiral wave tip trajectories. However, the precise methodology of identifying the spiral wave tip and its influence on the specific patterns of behavior remains a largely unexplored topic of research. Here we use a two-state variable FitzHugh-Nagumo model to simulate stationary and meandering spiral waves and examine the spatiotemporal representation of the system's state variables in both the real (i.e., physical) and state spaces. We show that mapping between these two spaces provides a method to demarcate the spiral wave tip as the center of rotation of the solution to the underlying nonlinear partial differential equations. This approach leads to the simplest tip trajectories by eliminating portions resulting from the rotational component of the spiral wave.

Repetitive endocardial focal discharges during ventricular fibrillation with prolonged global ischemia in isolated rabbit hearts

BACKGROUND

Ventricular fibrillation (VF) during prolonged (>5 min) global ischemia (GI) could be due to repetitive endocardial focal discharges (REFDs). This hypothesis was tested in isolated rabbit hearts.

METHODS AND RESULTS

With optical mapping, simultaneous endocardial (left ventricle, LV) and epicardial (both ventricles) activations during VF with prolonged GI were studied (protocol I, 8 hearts). Lugol solution was applied to the LV endocardium in additional 5 hearts after 5-min GI (protocol II). During prolonged GI, sustained VF (>30 s) was successfully induced in 7 protocol I hearts. The dominant frequency of summed optical signals at the LV endocardium was higher than at the epicardium (P<0.05). Mapping data showed that after 5-min GI, REFDs were present in >90% for recording time. There were 18 windows of optical recording showing spontaneous VF termination. In 10, once REFDs ceased, the VF episode terminated immediately. Electrical defibrillation was also performed on 3 hearts. Eight shocks showed early VF recurrence after successful defibrillation. REFDs were consistently involved in the initiation period of recurrence. In protocol II, Lugol subendocardial ablation diminished REFD genesis during re-induced VF. These VF episodes were all non-sustained.

CONCLUSIONS

REFDs at the LV endocardium were important for both VF maintenance and post-shock recurrence during prolonged GI in this model.

Reducing the cyclic variations of ultrasonic integrated backscatters and myocardial electrical synchronism by reversibly blocking intercellular communications with heptanol

The purpose of this study is to provide direct evidence for the role of intercellular communications in electrical synchronization and mechanical function of myocardium. We used heptanol, a reversible inhibitor of gap junctions, at low (0.16 mM) and high (0.5 mM) concentration as perfusate for 18 Langendorff-perfused rabbit hearts to study its effects on myocardial electrical and mechanical functions. Optical mapping was performed to measure conduction velocity (CV) and action potential duration (APD). Ultrasonic integrated backscatter and Doppler tissue imaging (DTI) were used to evaluate the intrinsic and global myocardial contractile performance. The CV decreased during low-dose heptanol infusion and became much slower at high dose (high dose vs. baseline, 50.8 +/- 10.2 cm/s vs. 69.3 +/- 8.8 cm/s, p < 0.001). After washout of heptanol, CV completely recovered. The alterations of APD by heptanol infusion were similar to CV. The APD dispersion, standard deviation of APD(80), was increased after heptanol infusion (low dose vs. baseline, 5.9 +/- 1.1 ms vs. 4.3 +/- 1.1 ms, p = 0.004; high dose, 6.0 +/- 1.3 ms, vs. baseline, p = 0.035). However, washout did not restore the APD dispersion which became even larger after washout (13.6 +/- 1.9 ms vs. high dose and baseline, both p < 0.001). Regarding contractile function, heptanol treatment resulted in a progressive decrease of cardiac cycle-dependent variations of integrated backscatter (CVIBS; low dose vs. baseline, 6.1 +/- 1.7 dB vs. 7.2 +/- 1.8 dB, p = 0.007; high dose 1.7 +/- 0.3 dB vs. baseline, p < 0.001) and peak systolic strain rate (low dose vs. baseline, -1.5 +/- 0.6 1/s vs. -1.9 +/- 0.6 1/s, p = 0.014; high dose -0.4 +/- 0.2 1/s; vs. baseline, p < 0.001). That both CVIBS and strain rate incompletely recovered after heptanol washout may be attributed to the increased APD dispersion. In conclusion, uncoupling of gap junctions resulted in slowing CV, increased repolarization heterogeneity, reduced CVIBS and impaired myocardial contractility. There was a reversible dose-response relationship between the myocardial electromechanical functions and gap junction coupling.

d,l-Sotalol at therapeutic concentrations facilitates the occurrence of long-lasting non-stationary reentry during ventricular fibrillation in isolated rabbit hearts

BACKGROUND

The effects of d,l-sotalol at therapeutic concentrations (

METHODS AND RESULTS

By using an optical mapping system, epicardial activation patterns of VF were studied in 6 Langendorff-perfused rabbit hearts at baseline, during 10 mg/L d,l-sotalol infusion, and after washout. In an additional 4 hearts, action potential duration (APD), conduction velocity, and wavelength (WL) restitutions were determined. During d,l-sotalol infusion, VF was terminated in 3 of the 6 hearts. Only 1 heart developed transient ventricular tachycardia (VT). d,l-Sotalol reduced the number of phase singularities (ie, wavebreak) during VF (P<0.05), and it also increased the occurrence frequency (P<0.05) and lifespan (P<0.05) of epicardial reentry during VF. These reentries were non-stationary in nature and did not anchor on anatomical structures. Restitution data showed that d,l-sotalol flattened APD restitution. Furthermore, APD dispersion and spatial heterogeneity of restitutions were not enhanced by d,l-sotalol.

CONCLUSIONS

d,l-Sotalol at therapeutic concentrations decreased wavebreak and facilitated the occurrence of long-lasting, non-stationary reentry during VF. However, VT rarely occurred. The related mechanisms include: (1) flattening of APD restitution without enhancement of spatial heterogeneity of electrophysiological properties, causing wavefront organization, and (2) WL prolongation, preventing steady anchoring of reentry.

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Key Words

• d,l-sotalol
• reentry
• ventricular fibrillation

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MESH Headings

• Action Potentials/drug effects
• Action Potentials/physiology
• Animals
• Anti-Arrhythmia Agents/pharmacology
• Disease Models, Animal
• Dose-Response Relationship, Drug
• Electrocardiography
• Electrophysiologic Techniques, Cardiac
• Heart Conduction System/drug effects
• Heart Conduction System/physiopathology
• Heart Ventricles/drug effects
• Heart Ventricles/physiopathology
• Rabbits
• Sotalol/pharmacology
• Tachycardia, Ventricular/physiopathology
• Ventricular Fibrillation/physiopathology

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Grants

• P01 HL078931 NHLBI NIH HHS
• P01 HL078931-03 NHLBI NIH HHS

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Affiliation(s)

Yu-Cheng Hsieh Cardiovascular Center, Taichung Veterans General Hospital and Department of Internal Medicine, Faculty of Medicine, Institute of Clinical Medicine, Cardiovascular Research Center, National Yang-Ming University School of Medicine, Taipei, Taiwan
Tzyy-Leng Horng Department of Applied Mathematics, Feng-Chia University, Taichung, Taiwan
Shien-Fong Lin Krannert Institute of Cardiology and the Division of Cardiology, Department of Medicine, Indiana University School of Medicine, Indianapolis, IN
Tung-Chao Lin Cardiovascular Center, Taichung Veterans General Hospital and Department of Internal Medicine, Faculty of Medicine, Institute of Clinical Medicine, Cardiovascular Research Center, National Yang-Ming University School of Medicine, Taipei, Taiwan
Chih-Tai Ting Cardiovascular Center, Taichung Veterans General Hospital and Department of Internal Medicine, Faculty of Medicine, Institute of Clinical Medicine, Cardiovascular Research Center, National Yang-Ming University School of Medicine, Taipei, Taiwan
Tsu-Juey Wu Cardiovascular Center, Taichung Veterans General Hospital and Department of Internal Medicine, Faculty of Medicine, Institute of Clinical Medicine, Cardiovascular Research Center, National Yang-Ming University School of Medicine, Taipei, Taiwan

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Termination of equine atrial fibrillation by quinidine: an optical mapping study

OBJECTIVE

To perform the first optical mapping studies of equine atrium to assess the spatiotemporal dynamics of atrial fibrillation (AF) and of its termination by quinidine.

ANIMALS

Intact, perfused atrial preparations obtained from four horses with normal cardiovascular examinations.

MATERIALS AND METHODS

AF was induced by a rapid pacing protocol with or without acetylcholine perfusion, and optical mapping was used to determine spatial dominant frequency distributions, electrical activity maps, and single-pixel optical signals. Following induction of AF, quinidine gluconate was perfused into the preparation and these parameters were monitored during quinidine-induced termination of AF.

RESULTS

Equine AF develops in the context of spatial gradients in action potential duration (APD) and diastolic interval (DI) that produce alternans, conduction block, and Wenckebach conduction in different regions at fast pacing rates. Quinidine terminates AF and prevents subsequent reinduction by reducing the maximal frequency and increasing frequency homogeneity.

CONCLUSIONS

Heterogeneity of APD and DI promote alternans and conduction block at fast pacing rates in the equine atrium, predisposing to the development of AF. Quinidine terminates AF by reducing maximum frequency and increasing frequency homogeneity. Our results are consistent with the hypothesis that quinidine increases effective refractory period, thereby decreasing frequency.

Spatiotemporal frequency analysis of ventricular fibrillation in explanted human hearts

Ventricular fibrillation (VF) is a medical condition that occurs due to rapid and irregular electrical activity of heart. If undiagnosed or untreated, VF leads to sudden cardiac death. VF has been studied by researchers for over 100 years to elucidate the mechanism that maintains VF, and thus to arrive at therapeutic options. VF is a nonstationary process, and it manifests into variations in the waveform morphology, phase, and frequency dynamics of the surface electrograms. Dominant frequency analysis (DF maps) and phase maps are two widely used complementary approaches in assessing the evolution of VF process. These techniques are applied to electrograms or fluorescence signals obtained with voltage-sensitive dyes. In spite of VF being a nonstationary process, most of the existing literature limits frequency analysis to a segmented, time-averaged spectral analysis, where valuable information on the instantaneous temporal evolution of the spectral characteristics is lost. In order to resolve this issue, in this paper, we present a joint time-frequency approach that is suited for VF analysis and demonstrate the application of instantaneous mean frequency (IMF) in interpreting VF episodes. Human VF sources are rarely anatomically stable and are migratory. Traditional DF techniques fail in tracking this migratory behavior. IMF, on the other hand, can deal with these migratory sources and conduction blocks better than DF approaches. Results of the analysis using the electrograms of 204 VF segments obtained from 13 isolated human hearts (explanted during cardiac transplantation) indicate that in 81% of the VF segments, there were significant changes in the spatiotemporal evolution of the frequency, suggesting that IMF provides better mechanistic insight of these signals. The IMF tool presented in this paper demonstrates potential for applications in tracking frequency patterns, conduction blocks, and arriving at newer therapies to modulate VF.

Epicardial wavefronts arise from widely distributed transient sources during ventricular fibrillation in the isolated swine heart

It has been proposed that VF waves emanate from stable localized sources, often called "mother rotors." However, evidence for the existence of these rotors is conflicting. Using a new panoramic optical mapping system that can image nearly the entire ventricular epicardium, we recently excluded epicardial mother rotors as the drivers of Wiggers' stage II VF in the isolated swine heart. Furthermore, we were unable to find evidence that VF requires sustained intramural sources. The present study was designed to test the following hypotheses: 1. VF is driven by a specific region, and 2. Rotors that are long-lived, though not necessarily permanent, are the primary generators of VF wavefronts. Using panoramic optical mapping, we mapped VF wavefronts from 6 isolated swine hearts. Wavefronts were tracked to characterize their activation pathways and to locate their originating sources. We found that the wavefronts that participate in epicardial reentry were not confined to a compact region; rather they activated the entire epicardial surface. New wavefronts feeding into the epicardial activation pattern were generated over the majority of the epicardium and almost all of them were associated with rotors or repetitive breakthrough patterns that lasted for less than 2 s. These findings indicate that epicardial wavefronts in this model are generated by many transitory epicardial sources distributed over the entire surface of the heart.

Spatially discordant voltage alternans cause wavebreaks in ventricular fibrillation

BACKGROUND

Ventricular fibrillation (VF) is characterized by complex ECG patterns emanating from multiple, short-lived, reentrant electrical waves. The incessant breakup and creation of new daughter waves (wavebreaks) perpetuate VF. Dispersion of refractoriness (static or dynamic) has been implicated as a mechanism underlying wavebreaks.

OBJECTIVE

The purpose of this study was to investigate the mechanisms underlying wavefront instability in VF by localizing wave fractionation sites (the appearance of multiple waves) and their relationship to local spatial dispersion of voltage (V(m)) oscillations.

METHODS

Wave fractionations were identified by tracking V(m) oscillations optically at unprecedented spatial (100 x 100 pixels) and temporal (2,000 frames per second) resolution using a CMOS camera viewing the surface (1 x 1 cm(2)) of perfused guinea pig hearts (n = 6). VF was induced by burst stimulation, and wavefront dynamics were highlighted using region-based image analysis to automatically detect wavebreaks. Direct detection of wavebreak locations by image analysis was more reliable than the phase reconstruction method because baseline noise obstructed the correct identification of phase singularities by detecting false-positives.

RESULTS

Wave fractionations (34 +/- 4 splits/s.cm(2)) fell into three categories: decremental conduction (49% +/- 7%), wave collisions (32% +/- 8%), and wavebreaks (17 +/- 2%). Wavebreaks occurred at a frequency of 5.8 +/- 1 splits/s.cm(2) and did not preferentially occur at anatomic obstacles (i.e., coronary vessels) but coincided with discordant alternans where V(m) amplitudes and durations shifted from high to low to from low to high on opposite sides of wavebreak sites.

CONCLUSION

Spatial discordant alternans cause wavebreaks most likely because they are sites of abrupt dispersion of refractoriness.

Remodelling of action potential and intracellular calcium cycling dynamics during subacute myocardial infarction promotes ventricular arrhythmias in Langendorff-perfused rabbit hearts

We hypothesize that remodelling of action potential and intracellular calcium (Ca(i)) dynamics in the peri-infarct zone contributes to ventricular arrhythmogenesis in the postmyocardial infarction setting. To test this hypothesis, we performed simultaneous optical mapping of Ca(i) and membrane potential (V(m)) in the left ventricle in 15 rabbit hearts with myocardial infarction for 1 week. Ventricular premature beats frequently originated from the peri-infarct zone, and 37% showed elevation of Ca(i) prior to V(m) depolarization, suggesting reverse excitation-contraction coupling as their aetiology. During electrically induced ventricular fibrillation, the highest dominant frequency was in the peri-infarct zone in 61 of 70 episodes. The site of highest dominant frequency had steeper action potential duration restitution and was more susceptible to pacing-induced Ca(i) alternans than sites remote from infarct. Wavebreaks during ventricular fibrillation tended to occur at sites of persistently elevated Ca(i). Infusion of propranolol flattened action potential duration restitution, reduced wavebreaks and converted ventricular fibrillation to ventricular tachycardia. We conclude that in the subacute phase of myocardial infarction, the peri-infarct zone exhibits regions with steep action potential duration restitution slope and unstable Ca(i) dynamics. These changes may promote ventricular extrasystoles and increase the incidence of wavebreaks during ventricular fibrillation. Whereas increased tissue heterogeneity after subacute myocardial infarction creates a highly arrhythmogenic substrate, dynamic action potential and Ca(i) cycling remodelling also contribute to the initiation and maintenance of ventricular fibrillation in this setting.

Panoramic optical mapping reveals continuous epicardial reentry during ventricular fibrillation in the isolated swine heart

During ventricular fibrillation (VF), activation waves are fragmented and the heart cannot contract synchronously. It has been proposed that VF waves emanate from stable sources ("mother rotors"). Previously, we used new optical mapping technology to image VF wavefronts from nearly the entire epicardial surface of six isolated swine hearts. We found that VF was not driven by epicardial rotors, but could not exclude the presence of stable rotors hidden within the ventricular walls. Here, we use graph theoretic analysis to show that, in all 17 VF episodes we analyzed, it was always possible to trace sequences of wavefronts through series of fragmentation and collision events from the beginning to the end of the episode. The set of wavefronts that were so related (the dominant component) consisted of 92%+/-1% of epicardial wavefronts. Because each such wavefront sequence constitutes a continuous activation front, this finding shows that complete reentrant pathways were always present on the epicardial surface and therefore, that wavefront infusion from nonepicardial sources was not strictly necessary for VF maintenance. These data suggest that VF in this model is not driven by localized sources; thus, new anti-VF treatments designed to target such sources may be less effective than global interventions.

Cellular basis for trigger and maintenance of ventricular fibrillation in the Brugada syndrome model: high-resolution optical mapping study

OBJECTIVES

We examined how repolarization and depolarization abnormalities contribute to the development of extrasystoles and subsequent ventricular fibrillation (VF) in a model of the Brugada syndrome.

BACKGROUND

Repolarization and depolarization abnormalities have been considered to be mechanisms of the coved-type ST-segment elevation (Brugada-electrocardiogram [ECG]) and development of VF in the Brugada syndrome.

METHODS

We used high-resolution (256 x 256) optical mapping techniques to study arterially perfused canine right ventricular wedges (n = 20) in baseline and in the Brugada-ECG produced by administration of terfenadine (5 micromol/l), pinacidil (2 micromol/l), and pilsicainide (5 micromol/l). We recorded spontaneous episodes of phase 2 re-entrant (P2R)-extrasystoles and subsequent self-terminating polymorphic ventricular tachycardia (PVT) or VF under the Brugada-ECG condition and analyzed the epicardial conduction velocity and action potential duration (APD) restitutions in each condition.

RESULTS

Forty-one episodes of spontaneous P2R-extrasystoles in the Brugada-ECG were successfully mapped in 9 of 10 preparations, and 33 of them were originated from the maximum gradient of repolarization (GR(max): 176 +/- 54 ms/mm) area in the epicardium, leading to PVT (n = 12) or VF (n = 5). The epicardial GR(max) was not different between PVT and VF. Wave-break during the first P2R-extrasystole produced multiple wavelets in all VF cases, whereas no wave-break or wave-break followed by wave collision and termination occurred in PVT cases. Moreover, conduction velocity restitution was shifted lower and APD restitution was more variable in VF cases than in PVT cases.

CONCLUSIONS

Steep repolarization gradient in the epicardium but not endocardium develops P2R-extrasystoles in the Brugada-ECG condition, which might degenerate into VF by further depolarization and repolarization abnormalities.

Intracellular Calcium Dynamics and Anisotropic Reentry in Isolated Canine Pulmonary Veins and Left Atrium

Background— Rapid activations due to either focal discharge or reentry are often present during atrial fibrillation (AF) in the pulmonary veins (PVs). The mechanisms of these rapid activations are unclear.

Methods and Results— We studied 7 isolated, Langendorff-perfused canine left atrial (LA) and PV preparations and used 2 cameras to map membrane potential alone (Vm, n=3) or Vm and intracellular calcium simultaneously (Ca i , n=4). Rapid atrial pacing induced 26 episodes of focal discharge from the proximal PVs in 5 dogs. The cycle lengths were 223±52 ms during ryanodine infusion (n=13) and 133±59 ms during ryanodine plus isoproterenol infusion (n=13). The rise of Ca i preceded Vm activation at the sites of focal discharge in 6 episodes of 2 preparations, compatible with voltage-independent spontaneous Ca i release. Phase singularities during pacing-induced reentry clustered specifically at the PV-LA junction. Periodic acid-Schiff (PAS) stain identified large cells with pale cytoplasm along the endocardium of PV muscle sleeves. There were abrupt changes in myocardial fiber orientation and increased interstitial fibrosis in the PV and at the PV-LA junction.

Conclusions— PV muscle sleeves may develop voltage-independent Ca i release, resulting in focal discharge. Focal discharge may also be facilitated by the presence of PAS-positive cells that are compatible with node-like cells. During reentry, phase singularities clustered preferentially at sites of increased anisotropy such as the PV-LA junction. These findings suggest that focal discharge caused by spontaneous calcium release and anisotropic reentry both contribute to rapid activations in the PVs during AF.