Lifetimes of epicardial rotors in panoramic optical maps of fibrillating swine ventricles

Optical mapping of cardiac electromechanics in beating in vivo hearts

Optical mapping has been widely used in the study of cardiac electrophysiology in motion-arrested, ex vivo heart preparations. Recent developments in motion artifact mitigation techniques have made it possible to optically map beating ex vivo hearts, enabling the study of cardiac electromechanics using optical mapping. However, the ex vivo setting imposes limitations on optical mapping such as altered metabolic states, oversimplified mechanical loads, and the absence of neurohormonal regulation. In this study, we demonstrate optical electromechanical mapping in an in vivo heart preparation. Swine hearts were exposed via median sternotomy. Voltage-sensitive dye, either di-4-ANEQ(F)PTEA or di-5-ANEQ(F)PTEA, was injected into the left anterior descending artery. Fluorescence was excited by alternating green and amber light for excitation ratiometry. Cardiac motion during sinus and paced rhythm was tracked using a marker-based method. Motion tracking and excitation ratiometry successfully corrected most motion artifact in the membrane potential signal. Marker-based motion tracking also allowed simultaneous measurement of epicardial deformation. Reconstructed membrane potential and mechanical deformation measurements were validated using monophasic action potentials and sonomicrometry, respectively. Di-5-ANEQ(F)PTEA produced longer working time and higher signal/noise ratio than di-4-ANEQ(F)PTEA. In addition, we demonstrate potential applications of the new optical mapping system including electromechanical mapping during vagal nerve stimulation, fibrillation/defibrillation. and acute regional ischemia. In conclusion, although some technical limitations remain, optical mapping experiments that simultaneously image electrical and mechanical function can be conducted in beating, in vivo hearts.

A comprehensive framework for evaluation of high pacing frequency and arrhythmic optical mapping signals

Introduction: High pacing frequency or irregular activity due to arrhythmia produces complex optical mapping signals and challenges for processing. The objective is to establish an automated activation time-based analytical framework applicable to optical mapping images of complex electrical behavior. Methods: Optical mapping signals with varying complexity from sheep (N = 7) ventricular preparations were examined. Windows of activation centered on each action potential upstroke were derived using Hilbert transform phase. Upstroke morphology was evaluated for potential multiple activation components and peaks of upstroke signal derivatives defined activation time. Spatially and temporally clustered activation time points were grouped in to wave fronts for individual processing. Each activation time point was evaluated for corresponding repolarization times. Each wave front was subsequently classified based on repetitive or non-repetitive events. Wave fronts were evaluated for activation time minima defining sites of wave front origin. A visualization tool was further developed to probe dynamically the ensemble activation sequence. Results: Our framework facilitated activation time mapping during complex dynamic events including transitions to rotor-like reentry and ventricular fibrillation. We showed that using fixed AT windows to extract AT maps can impair interpretation of the activation sequence. However, the phase windowing of action potential upstrokes enabled accurate recapitulation of repetitive behavior, providing spatially coherent activation patterns. We further demonstrate that grouping the spatio-temporal distribution of AT points in to coherent wave fronts, facilitated interpretation of isolated conduction events, such as conduction slowing, and to derive dynamic changes in repolarization properties. Focal origins precisely detected sites of stimulation origin and breakthrough for individual wave fronts. Furthermore, a visualization tool to dynamically probe activation time windows during reentry revealed a critical single static line of conduction slowing associated with the rotation core. Conclusion: This comprehensive analytical framework enables detailed quantitative assessment and visualization of complex electrical behavior.

Mapping and ablation of ventricular fibrillation substrate

Ventricular fibrillation (VF) is a life-threatening arrhythmia and a common cause of sudden cardiac death (SCD). A basic understanding of its mechanistic underpinning is crucial for enhancing our knowledge to develop innovative mapping and ablation techniques for this lethal rhythm. Significant advances in our understanding of VF have been made especially in the basic science and pre-clinical experimental realms. However, these studies have not yet translated into a robust clinical approach to identify and successfully ablate both the structural and functional substrate of VF. In this review, we aim to (1) provide a conceptual framework of VF and an overview of the data supporting the spatiotemporal dynamics of VF, (2) review experimental approaches to mapping VF to elucidate drivers and substrate for maintenance with a focus on the His-Purkinje system, (3) discuss current approaches using catheter ablation to treat VF, and (4) highlight current unknowns and gaps in the field where future work is necessary to transform the clinical landscape.

Understanding the origins of the basic equations of statistical fibrillatory dynamics

The mechanisms governing cardiac fibrillation remain unclear; however, it most likely represents a form of spatiotemporal chaos with conservative system dynamics. Renewal theory has recently been suggested as a statistical formulation with governing equations to quantify the formation and destruction of wavelets and rotors in fibrillatory dynamics. In this perspective Review, we aim to explain the origin of the renewal theory paradigm in spatiotemporal chaos. The ergodic nature of pattern formation in spatiotemporal chaos is demonstrated through the use of three chaotic systems: two classical systems and a simulation of cardiac fibrillation. The logistic map and the baker's transformation are used to demonstrate how the apparently random appearance of patterns in classical chaotic systems has macroscopic parameters that are predictable in a statistical sense. We demonstrate that the renewal theory approach developed for cardiac fibrillation statistically predicts pattern formation in these classical chaotic systems. Renewal theory provides governing equations to describe the apparently random formation and destruction of wavelets and rotors in atrial fibrillation (AF) and ventricular fibrillation (VF). This statistical framework for fibrillatory dynamics provides a holistic understanding of observed rotor and wavelet dynamics and is of conceptual significance in informing the clinical and mechanistic research of the rotor and multiple-wavelet mechanisms of AF and VF.

The inspection paradox: An important consideration in the evaluation of rotor lifetimes in cardiac fibrillation

Background and Objective: Renewal theory is a statistical approach to model the formation and destruction of phase singularities (PS), which occur at the pivots of spiral waves. A common issue arising during observation of renewal processes is an inspection paradox, due to oversampling of longer events. The objective of this study was to characterise the effect of a potential inspection paradox on the perception of PS lifetimes in cardiac fibrillation. Methods: A multisystem, multi-modality study was performed, examining computational simulations (Aliev-Panfilov (APV) model, Courtmanche-Nattel model), experimentally acquired optical mapping Atrial and Ventricular Fibrillation (AF/VF) data, and clinically acquired human AF and VF. Distributions of all PS lifetimes across full epochs of AF, VF, or computational simulations, were compared with distributions formed from lifetimes of PS existing at 10,000 simulated commencement timepoints. Results: In all systems, an inspection paradox led towards oversampling of PS with longer lifetimes. In APV computational simulations there was a mean PS lifetime shift of +84.9% (95% CI, ± 0.3%) (p < 0.001 for observed vs overall), in Courtmanche-Nattel simulations of AF +692.9% (95% CI, ±57.7%) (p < 0.001), in optically mapped rat AF +374.6% (95% CI, ± 88.5%) (p = 0.052), in human AF mapped with basket catheters +129.2% (95% CI, ±4.1%) (p < 0.05), human AF-HD grid catheters 150.8% (95% CI, ± 9.0%) (p < 0.001), in optically mapped rat VF +171.3% (95% CI, ±15.6%) (p < 0.001), in human epicardial VF 153.5% (95% CI, ±15.7%) (p < 0.001). Conclusion: Visual inspection of phase movies has the potential to systematically oversample longer lasting PS, due to an inspection paradox. An inspection paradox is minimised by consideration of the overall distribution of PS lifetimes.

Stop the beat to see the rhythm: excitation-contraction uncoupling in cardiac research

Optical mapping is an imaging technique that is extensively used in cardiovascular research, wherein parameter-sensitive fluorescent indicators are used to study the electrophysiology and excitation-contraction coupling of cardiac tissues. Despite many benefits of optical mapping, eliminating motion artifacts within the optical signals is a major challenge, as myocardial contraction interferes with the faithful acquisition of action potentials and intracellular calcium transients. As such, excitation-contraction uncoupling agents are frequently used to reduce signal distortion by suppressing contraction. When compared with other uncoupling agents, blebbistatin is the most frequently used, as it offers increased potency with minimal direct effects on cardiac electrophysiology. Nevertheless, blebbistatin may exert secondary effects on electrical activity, metabolism, and coronary flow, and the incorrect administration of blebbistatin to cardiac tissue can prove detrimental, resulting in erroneous interpretation of optical mapping results. In this "Getting It Right" perspective, we briefly review the literature regarding the use of blebbistatin in cardiac optical mapping experiments, highlight potential secondary effects of blebbistatin on cardiac electrical activity and metabolic demand, and conclude with the consensus of the authors on best practices for effectively using blebbistatin in optical mapping studies of cardiac tissue.

Cardiac Conduction Velocity, Remodeling and Arrhythmogenesis

Cardiac electrophysiological disorders, in particular arrhythmias, are a key cause of morbidity and mortality throughout the world. There are two basic requirements for arrhythmogenesis: an underlying substrate and a trigger. Altered conduction velocity (CV) provides a key substrate for arrhythmogenesis, with slowed CV increasing the probability of re-entrant arrhythmias by reducing the length scale over which re-entry can occur. In this review, we examine methods to measure cardiac CV in vivo and ex vivo, discuss underlying determinants of CV, and address how pathological variations alter CV, potentially increasing arrhythmogenic risk. Finally, we will highlight future directions both for methodologies to measure CV and for possible treatments to restore normal CV.

Right ventricular insertion promotes reinitiation of ventricular fibrillation in defibrillation failure

BACKGROUND

Shocks near defibrillation threshold (nDFT) strength commonly extinguish all ventricular fibrillation (VF) wavefronts, but a train of rapid, well-organized postshock activations (PAs) typically appears before sinus rhythm ensues. If one of the PA waves undergoes partial propagation block (wavebreak), reentry may be induced, causing VF to reinitiate and the shock to fail.

OBJECTIVE

The purpose of this study was to determine whether wavebreak leading to VF reinititation following nDFT shocks occurs preferentially at the right ventricular insertion (RVI), which previous studies have identified as a key site for wavebreak.

METHODS

We used panoramic optical mapping to image the ventricular epicardium of 6 isolated swine hearts during nDFT defibrillation episodes. After each experiment, the hearts were fixed and their geometry scanned with magnetic resonance imaging (MRI). The MRI and mapping datasets were spatially coregistered. For failed shocks, we identified the site of the first wavebreak of a PA wave during VF reinitiation.

RESULTS

We recorded 59 nDFT failures. In 31 of these, the first wavebreak event occurred within 1 cm of the RVI centerline, most commonly on the anterior side of the right ventricular insertion (aRVI) (23/31). The aRVI region occupies 16.8% ± 2.5% of the epicardial surface and would be expected to account for only 10 wavebreaks if they were uniformly distributed. By χ2 analysis, aRVI wavebreaks were significantly overrepresented.

CONCLUSION

The anterior RVI is a key site in promoting nDFT failure. Targeting this site to prevent wavebreak could convert defibrillation failure to success and improve defibrillation efficacy.

The transient outward potassium current plays a key role in spiral wave breakup in ventricular tissue

Spiral wave reentry as a mechanism of lethal ventricular arrhythmias has been widely demonstrated in animal experiments and recordings from human hearts. It has been shown that in structurally normal hearts spiral waves are unstable, breaking up into multiple wavelets via dynamical instabilities. However, many of the second-generation action potential models give rise only to stable spiral waves, raising issues regarding the underlying mechanisms of spiral wave breakup. In this study, we carried out computer simulations of two-dimensional homogeneous tissues using five ventricular action potential models. We show that the transient outward potassium current (I_to), although it is not required, plays a key role in promoting spiral wave breakup in all five models. As the maximum conductance of I_to increases, it first promotes spiral wave breakup and then stabilizes the spiral waves. In the absence of I_to, speeding up the L-type calcium kinetics can prevent spiral wave breakup, however, with the same speedup kinetics, spiral wave breakup can be promoted by increasing I_to. Increasing I_to promotes single-cell dynamical instabilities, including action potential duration alternans and chaos, and increasing I_to further suppresses these action potential dynamics. These cellular properties agree with the observation that increasing I_to first promotes spiral wave breakup and then stabilizes spiral waves in tissue. Implications of our observations to spiral wave dynamics in the real hearts and action potential model improvements are discussed.NEW & NOTEWORTHY Spiral wave breakup manifesting as multiple wavelets is a mechanism of ventricular fibrillation. It has been known that spiral wave breakup in cardiac tissue can be caused by a steeply sloped action potential duration restitution curve, a property mainly determined by the recovery of L-type calcium current. Here, we show that the transient outward potassium current (I_to) is another current that plays a key role in spiral wave breakup, that is, spiral waves can be stable for low and high maximum I_to conductance but breakup occurs for intermediate maximum I_to conductance. Since I_to is present in normal hearts of many species and required for Brugada syndrome, it may play an important role in the spiral wave stability and arrhythmogenesis under both normal condition and Brugada syndrome.

Cardiac optogenetics: a decade of enlightenment

The electromechanical function of the heart involves complex, coordinated activity over time and space. Life-threatening cardiac arrhythmias arise from asynchrony in these space-time events; therefore, therapies for prevention and treatment require fundamental understanding and the ability to visualize, perturb and control cardiac activity. Optogenetics combines optical and molecular biology (genetic) approaches for light-enabled sensing and actuation of electrical activity with unprecedented spatiotemporal resolution and parallelism. The year 2020 marks a decade of developments in cardiac optogenetics since this technology was adopted from neuroscience and applied to the heart. In this Review, we appraise a decade of advances that define near-term (immediate) translation based on all-optical electrophysiology, including high-throughput screening, cardiotoxicity testing and personalized medicine assays, and long-term (aspirational) prospects for clinical translation of cardiac optogenetics, including new optical therapies for rhythm control. The main translational opportunities and challenges for optogenetics to be fully embraced in cardiology are also discussed.

Granger Causality-Based Analysis for Classification of Fibrillation Mechanisms and Localization of Rotational Drivers

BACKGROUND

The mechanisms sustaining myocardial fibrillation remain disputed, partly due to a lack of mapping tools that can accurately identify the mechanism with low spatial resolution clinical recordings. Granger causality (GC) analysis, an econometric tool for quantifying causal relationships between complex time-series, was developed as a novel fibrillation mapping tool and adapted to low spatial resolution sequentially acquired data.

METHODS

Ventricular fibrillation (VF) optical mapping was performed in Langendorff-perfused Sprague-Dawley rat hearts (n=18), where novel algorithms were developed using GC-based analysis to (1) quantify causal dependence of neighboring signals and plot GC vectors, (2) quantify global organization with the causality pairing index, a measure of neighboring causal signal pairs, and (3) localize rotational drivers (RDs) by quantifying the circular interdependence of neighboring signals with the circular interdependence value. GC-based mapping tools were optimized for low spatial resolution from downsampled optical mapping data, validated against high-resolution phase analysis and further tested in previous VF optical mapping recordings of coronary perfused donor heart left ventricular wedge preparations (n=12), and adapted for sequentially acquired intracardiac electrograms during human persistent atrial fibrillation mapping (n=16).

RESULTS

Global VF organization quantified by causality pairing index showed a negative correlation at progressively lower resolutions (50% resolution: P=0.006, R2=0.38, 12.5% resolution, P=0.004, R2=0.41) with a phase analysis derived measure of disorganization, locations occupied by phase singularities. In organized VF with high causality pairing index values, GC vector mapping characterized dominant propagating patterns and localized stable RDs, with the circular interdependence value showing a significant difference in driver versus nondriver regions (0.91±0.05 versus 0.35±0.06, P=0.0002). These findings were further confirmed in human VF. In persistent atrial fibrillation, a positive correlation was found between the causality pairing index and presence of stable RDs (P=0.0005,R2=0.56). Fifty percent of patients had RDs, with a low incidence of 0.9±0.3 RDs per patient.

CONCLUSIONS

GC-based fibrillation analysis can measure global fibrillation organization, characterize dominant propagating patterns, and map RDs using low spatial resolution sequentially acquired data.

Renewal Theory as a Universal Quantitative Framework to Characterize Phase Singularity Regeneration in Mammalian Cardiac Fibrillation

BACKGROUND

Despite a century of research, no clear quantitative framework exists to model the fundamental processes responsible for the continuous formation and destruction of phase singularities (PS) in cardiac fibrillation. We hypothesized PS formation/destruction in fibrillation could be modeled as self-regenerating Poisson renewal processes, producing exponential distributions of interevent times governed by constant rate parameters defined by the prevailing properties of each system.

METHODS

PS formation/destruction were studied in 5 systems: (1) human persistent atrial fibrillation (n=20), (2) tachypaced sheep atrial fibrillation (n=5), (3) rat atrial fibrillation (n=4), (5) rat ventricular fibrillation (n=11), and (5) computer-simulated fibrillation. PS time-to-event data were fitted by exponential probability distribution functions computed using maximum entropy theory, and rates of PS formation and destruction (λ_f/λ_d) determined. A systematic review was conducted to cross-validate with source data from literature.

RESULTS

In all systems, PS lifetime and interformation times were consistent with underlying Poisson renewal processes (human: λ_f, 4.2%/ms±1.1 [95% CI, 4.0-5.0], λ_d, 4.6%/ms±1.5 [95% CI, 4.3-4.9]; sheep: λ_f, 4.4%/ms [95% CI, 4.1-4.7], λ_d, 4.6%/ms±1.4 [95% CI, 4.3-4.8]; rat atrial fibrillation: λ_f, 33%/ms±8.8 [95% CI, 11-55], λ_d, 38%/ms [95% CI, 22-55]; rat ventricular fibrillation: λ_f, 38%/ms±24 [95% CI, 22-55], λ_f, 46%/ms±21 [95% CI, 31-60]; simulated fibrillation λ_d, 6.6-8.97%/ms [95% CI, 4.1-6.7]; R²≥0.90 in all cases). All PS distributions identified through systematic review were also consistent with an underlying Poisson renewal process.

CONCLUSIONS

Poisson renewal theory provides an evolutionarily preserved universal framework to quantify formation and destruction of rotational events in cardiac fibrillation.

Optocardiography and Electrophysiology Studies of Ex Vivo Langendorff-perfused Hearts

Small animal models are most commonly used in cardiovascular research due to the availability of genetically modified species and lower cost compared to larger animals. Yet, larger mammals are better suited for translational research questions related to normal cardiac physiology, pathophysiology, and preclinical testing of therapeutic agents. To overcome the technical barriers associated with employing a larger animal model in cardiac research, we describe an approach to measure physiological parameters in an isolated, Langendorff-perfused piglet heart. This approach combines two powerful experimental tools to evaluate the state of the heart: electrophysiology (EP) study and simultaneous optical mapping of transmembrane voltage and intracellular calcium using parameter sensitive dyes (RH237, Rhod2-AM). The described methodologies are well suited for translational studies investigating the cardiac conduction system, alterations in action potential morphology, calcium handling, excitation-contraction coupling and the incidence of cardiac alternans or arrhythmias.

Effect of Heart Structure on Ventricular Fibrillation in the Rabbit: A Simulation Study

Ventricular fibrillation (VF) is a lethal condition that affects millions worldwide. The mechanism underlying VF is unstable reentrant electrical waves rotating around lines called filaments. These complex spatio-temporal patterns can be studied using both experimental and numerical methods. Computer simulations provide unique insights including high resolution dynamics throughout the heart and systematic control of quantities such as fiber orientation and cellular kinetics that are not feasible experimentally. Here we study filament dynamics using two bi-ventricular 3-D high-resolution rabbit heart geometries, one with detailed fine structure and another without fine structure. We studied filament dynamics using anisotropic and isotropic conductivities, and with four cellular action potential models with different recovery kinetics. Spiral wave dynamics observed in isotropic two-dimensional sheets were not predictive of the behavior in the whole heart. In 2-D the four cell models exhibited stable reentry, meandering spiral waves, and spiral-wave breakup. In the whole heart with fine structure, all simulation results exhibited complex dynamics reminiscent of fibrillation observed experimentally. In the whole heart without fine structure, anisotropy acted to destabilize filament dynamics although the number of filaments was reduced compared to the heart with structure. In addition, in isotropic hearts without structure the two cell models that exhibited meandering spiral waves in 2-D, stabilized into figure-of-eight surface patterns. We also studied the sensitivity of filament dynamics to computer system configuration and initial conditions. After large simulation times, different macroscopic results sometimes occurred across different system configurations, likely due to a lack of bitwise reproducibility. The study conclusions were insensitive to initial condition perturbations, however, the exact number of filaments over time and their trends were altered by these changes. In summary, we present the following new results. First, we provide a new cell model that resembles the surface patterns of VF in the rabbit heart both qualitatively and quantitatively. Second, filament dynamics in the whole heart cannot be predicted from spiral wave dynamics in 2-D and we identified anisotropy as one destabilizing factor. Third, the exact dynamics of filaments are sensitive to a variety of factors, so we suggest caution in their interpretation and their quantitative analyses.

RHYTHM: An Open Source Imaging Toolkit for Cardiac Panoramic Optical Mapping

Fluorescence optical imaging techniques have revolutionized the field of cardiac electrophysiology and advanced our understanding of complex electrical activities such as arrhythmias. However, traditional monocular optical mapping systems, despite having high spatial resolution, are restricted to a two-dimensional (2D) field of view. Consequently, tracking complex three-dimensional (3D) electrical waves such as during ventricular fibrillation is challenging as the waves rapidly move in and out of the field of view. This problem has been solved by panoramic imaging which uses multiple cameras to measure the electrical activity from the entire epicardial surface. However, the diverse engineering skill set and substantial resource cost required to design and implement this solution have made it largely inaccessible to the biomedical research community at large. To address this barrier to entry, we present an open source toolkit for building panoramic optical mapping systems which includes the 3D printing of perfusion and imaging hardware, as well as software for data processing and analysis. In this paper, we describe the toolkit and demonstrate it on different mammalian hearts: mouse, rat, and rabbit.

Endocardial Activation Drives Activation Patterns During Long-Duration Ventricular Fibrillation and Defibrillation

BACKGROUND

Understanding the mechanisms that drive ventricular fibrillation is essential for developing improved defibrillation techniques to terminate ventricular fibrillation (VF). Distinct organization patterns of chaotic, regular, and synchronized activity were previously demonstrated in VF that persisted over 1 to 2 minutes (long-duration VF [LDVF]). We hypothesized that activity on the endocardium may be driving these activation patterns in LDVF and that unsuccessful defibrillation shocks may alter activation patterns.

METHODS AND RESULTS

The study was performed using a 64-electrode basket catheter on the left ventricle endocardium and 54 6-electrode plunge needles inserted into the left ventricles of 6 dogs. VF was induced electrically, and after short-duration VF (10 seconds) and LDVF (7 minutes), shocks of increasing strengths were delivered every 10 seconds until VF was terminated. Endocardial activation patterns were classified as chaotic (varying cycle lengths and nonsynchronous activations), regular (highly repeatable cycle lengths), and synchronized (activation that spreads rapidly over the endocardium with diastolic periods between activations).

CONCLUSIONS

The results showed that the chaotic pattern was predominant in early VF, but the regular pattern emerges as VF progressed. The synchronized pattern only emerged occasionally during late VF. Failed defibrillation shocks changed chaotic and regular activation patterns to synchronized patterns in LDVF but not in short-duration VF. The regular and synchronized patterns of activation were driven by rapid activations on the endocardial surface that blocked and broke up transmurally, leading to an endocardial to epicardial activation rate gradient as LDVF progressed.

Enzyme-dependent fluorescence recovery of NADH after photobleaching to assess dehydrogenase activity of isolated perfused hearts

Reduction of NAD⁺ by dehydrogenase enzymes to form NADH is a key component of cellular metabolism. In cellular preparations and isolated mitochondria suspensions, enzyme-dependent fluorescence recovery after photobleaching (ED-FRAP) of NADH has been shown to be an effective approach for measuring the rate of NADH production to assess dehydrogenase enzyme activity. Our objective was to demonstrate how dehydrogenase activity could be assessed within the myocardium of perfused hearts using NADH ED-FRAP. This was accomplished using a combination of high intensity UV pulses to photobleach epicardial NADH. Replenishment of epicardial NADH fluorescence was then imaged using low intensity UV illumination. NADH ED-FRAP parameters were optimized to deliver 23.8 mJ of photobleaching light energy at a pulse width of 6 msec and a duty cycle of 50%. These parameters provided repeatable measurements of NADH production rate during multiple metabolic perturbations, including changes in perfusate temperature, electromechanical uncoupling, and acute ischemia/reperfusion injury. NADH production rate was significantly higher in every perturbation where the energy demand was either higher or uncompromised. We also found that NADH production rate remained significantly impaired after 10 min of reperfusion after global ischemia. Overall, our results indicate that myocardial NADH ED-FRAP is a useful optical non-destructive approach for assessing dehydrogenase activity.

Technical advances in studying cardiac electrophysiology - Role of rabbit models

Cardiovascular research has made a major contribution to an unprecedented 10 year increase in life expectancy during the last 50 years: most of this increase due to a decline in mortality from heart disease and stroke. The majority of the basic cardiovascular science discoveries, which have led to this impressive extension of human life, came from investigations conducted in various small and large animal models, ranging from mouse to pig. The small animal models are currently popular because they are amenable to genetic engineering and are relatively inexpensive. The large animal models are favored at the translational stage of the investigation, as they are anatomically and physiologically more proximal to humans, and can be implanted with various devices; however, they are expensive and less amenable to genetic manipulations. With the advent of CRISPR genetic engineering technology and the advances in implantable bioelectronics, the large animal models will continue to advance. The rabbit model is particularly poised to become one of the most popular among the animal models that recapitulate human heart diseases. Here we review an array of the rabbit models of atrial and ventricular arrhythmias, as well as a range of the imaging and device technologies enabling these investigations.

Analysis of cardiac fibrillation using phase mapping

The sequence of myocardial electrical activation during fibrillation is complex and changes with each cycle. Phase analysis represents the electrical activation-recovery process as an angle. Lines of equal phase converge at a phase singularity at the center of rotation of a reentrant wave, and the identification of reentry and tracking of reentrant wavefronts can be automated. We examine the basic ideas behind phase analysis. With the exciting prospect of using phase analysis of atrial electrograms to guide ablation in the human heart, we highlight several recent developments in preprocessing electrograms so that phase can be estimated reliably.

Optical Mapping of Ventricular Fibrillation Dynamics

There is very limited information regarding the dynamic patterns of the electrical activity during ventricular fibrillation (VF) in humans. Most of the data used to generate and test hypotheses regarding the mechanisms of VF come from animal models and computer simulations and the quantification of VF patterns is non-trivial. Many of the experimental recordings of the dynamic spatial patterns of VF have been obtained from mammals using "optical mapping" or "video imaging" technology in which "phase maps" are derived from high-resolution transmembrane recordings from the heart surface. The surface manifestation of the unstable reentrant waves sustaining VF can be identified as "phase singularities" and their number and location provide one measure of VF complexity. After providing a brief history of optical mapping of VF, we compare and contrast a quantitative analysis of VF patterns from the heart surface for four different animal models, hence providing physiological insight into the variety of VF dynamics among species. We found that in all four animal models the action potential duration restitution slope was actually negative during VF and that the spatial dispersion of electrophysiological parameters were not different during the first second of VF compared to pacing immediately before VF initiation. Surprisingly, our results suggest that APD restitution and spatial dispersion may not be essential causes of VF dynamics. Analyses of electrophysiological quantities in the four animal models are consistent with the idea that VF is essentially a two-dimensional phenomenon in small rabbit hearts whose size are near the boundary of the "critical mass" required to sustain VF, while VF in large pig hearts is three-dimensional and exhibits the maximal theoretical phase singularity density, and thus will not terminate spontaneously.

Optogenetic release of norepinephrine from cardiac sympathetic neurons alters mechanical and electrical function

AIMS

Release of norepinephrine (NE) from sympathetic neurons enhances heart rate (HR) and developed force through activation of β-adrenergic receptors, and this sympathoexcitation is a key risk for the generation of cardiac arrhythmias. Studies of β-adrenergic modulation of cardiac function typically involve the administration of exogenous β-adrenergic receptor agonists to directly elicit global β-adrenergic receptor activation by bypassing the involvement of sympathetic nerve terminals. In this work, we use a novel method to activate sympathetic fibres within the myocardium of Langendorff-perfused hearts while measuring changes in electrical and mechanical function.

METHODS AND RESULTS

The light-activated optogenetic protein channelrhodopsin-2 (ChR2) was expressed in murine catecholaminergic sympathetic neurons. Sympathetic fibres were then photoactivated to examine changes in contractile force, HR, and cardiac electrical activity. Incidence of arrhythmia was measured with and without exposure to photoactivation of sympathetic fibres, and hearts were optically mapped to detect changes in action potential durations and conduction velocities. Results demonstrate facilitation of both developed force and HR after photostimulated release of NE, with increases in contractile force and HR of 34.5 ± 5.5 and 25.0 ± 9.3%, respectively. Photostimulation of sympathetic fibres also made hearts more susceptible to arrhythmia, with greater incidence and severity. In addition, optically mapped action potentials displayed a small but significant shortening of the plateau phase (-5.5 ± 1.0 ms) after photostimulation.

CONCLUSION

This study characterizes a powerful and clinically relevant new model for studies of cardiac arrhythmias generated by increasing the activity of sympathetic nerve terminals and the resulting activation of myocyte β-adrenergic receptors.

Toward panoramic in situ mapping of action potential propagation in transgenic hearts to investigate initiation and therapeutic control of arrhythmias

To investigate the dynamics and propensity for arrhythmias in intact transgenic hearts comprehensively, optical strategies for panoramic fluorescence imaging of action potential (AP) propagation are essential. In particular, mechanism-oriented molecular studies usually depend on transgenic mouse hearts of only a few millimeters in size. Furthermore, the temporal scales of the mouse heart remain a challenge for panoramic fluorescence imaging with heart rates ranging from 200 min⁻¹ (e.g., depressed sinus node function) to over 1200 min⁻¹ during fast arrhythmias. To meet these challenging demands, we and others developed physiologically relevant mouse models and characterized their hearts with planar AP mapping. Here, we summarize the progress toward panoramic fluorescence imaging and its prospects for the mouse heart. In general, several high-resolution cameras are synchronized and geometrically arranged for panoramic voltage mapping and the surface and blood vessel anatomy documented through image segmentation and heart surface reconstruction. We expect that panoramic voltage imaging will lead to novel insights about molecular arrhythmia mechanisms through quantitative strategies and organ-representative analysis of intact mouse hearts.

Relevance of cardiomyocyte mechano-electric coupling to stretch-induced arrhythmias: optical voltage/calcium measurement in mechanically stimulated cells, tissues and organs

Stretch-induced arrhythmias are multi-scale phenomena in which alterations in channel activities and/or calcium handling lead to the organ level derangement of the heart rhythm. To understand how cellular mechano-electric coupling (MEC) leads to stretch-induced arrhythmias at the organ level, we developed stretching devices and optical voltage/calcium measurement techniques optimized to each cardiac level. This review introduces these experimental techniques of (1) optical voltage measurement coupled with a carbon-fiber technique for single isolated cardiomyocytes, (2) optical voltage mapping combined with motion tracking technique for myocardial tissue/whole heart preparations and (3) real-time calcium imaging coupled with a laser optical trap technique for cardiomyocytes. Following the overview of each methodology, results are presented. We conclude that individual MEC in cardiomyocytes can be heterogeneous at the ventricular level, especially when moderate amplitude mechanical stretches are applied to the heart, and that this heterogeneous MEC can evoke focal excitation that develops into re-entrant arrhythmias.

Developing a novel comprehensive framework for the investigation of cellular and whole heart electrophysiology in the in situ human heart: historical perspectives, current progress and future prospects

Understanding the mechanisms of fatal ventricular arrhythmias is of great importance. In view of the many electrophysiological differences that exist between animal species and humans, the acquisition of basic electrophysiological data in the intact human heart is essential to drive and complement experimental work in animal and in-silico models. Over the years techniques have been developed to obtain basic electrophysiological signals directly from the patients by incorporating these measurements into routine clinical procedures which access the heart such as cardiac catheterisation and cardiac surgery. Early recordings with monophasic action potentials provided valuable information including normal values for the in vivo human heart, cycle length dependent properties, the effect of ischaemia, autonomic nervous system activity, and mechano-electric interaction. Transmural recordings addressed the controversial issue of the mid myocardial "M" cell. More recently, the technique of multielectrode mapping (256 electrodes) developed in animal models has been extended to humans, enabling mapping of activation and repolarisation on the entire left and right ventricular epicardium in patients during cardiac surgery. Studies have examined the issue of whether ventricular fibrillation was driven by a "mother" rotor with inhomogeneous and fragmented conduction as in some animal models, or by multiple wavelets as in other animal studies; results showed that both mechanisms are operative in humans. The simpler spatial organisation of human VF has important implications for treatment and prevention. To link in-vivo human electrophysiological mapping with cellular biophysics, multielectrode mapping is now being combined with myocardial biopsies. This technique enables region-specific electrophysiology changes to be related to underlying cellular biology, for example: APD alternans, which is a precursor of VF and sudden death. The mechanism is incompletely understood but related to calcium cycling and APD restitution. Multielectrode sock mapping during incremental pacing enables epicardial sites to be identified which exhibit marked APD alternans and sites where APD alternans is absent. Whole heart electrophysiology is assessed by activation repolarisation mapping and analysis is performed immediately on-site in order to guide biopsies to specific myocardial sites. Samples are analysed for ion channel expression, Ca(2+)-handling proteins, gap junctions and extracellular matrix. This new comprehensive approach to bridge cellular and whole heart electrophysiology allowed to identify 20 significant changes in mRNA for ion channels Ca(2+)-handling proteins, a gap junction channel, a Na(+)-K(+) pump subunit and receptors (particularly Kir 2.1) between the positive and negative alternans sites.

Bisphenol A exposure and cardiac electrical conduction in excised rat hearts

BACKGROUND

Bisphenol A (BPA) is used to produce polycarbonate plastics and epoxy resins that are widely used in everyday products, such as food and beverage containers, toys, and medical devices. Human biomonitoring studies have suggested that a large proportion of the population may be exposed to BPA. Recent epidemiological studies have reported correlations between increased urinary BPA concentrations and cardiovascular disease, yet the direct effects of BPA on the heart are unknown.

OBJECTIVES

The goal of our study was to measure the effect of BPA (0.1-100 μM) on cardiac impulse propagation ex vivo using excised whole hearts from adult female rats.

METHODS

We measured atrial and ventricular activation times during sinus and paced rhythms using epicardial electrodes and optical mapping of transmembrane potential in excised rat hearts exposed to BPA via perfusate media. Atrioventricular activation intervals and epicardial conduction velocities were computed using recorded activation times.

RESULTS

Cardiac BPA exposure resulted in prolonged PR segment and decreased epicardial conduction velocity (0.1-100 μM BPA), prolonged action potential duration (1-100 μM BPA), and delayed atrioventricular conduction (10-100 μM BPA). These effects were observed after acute exposure (≤ 15 min), underscoring the potential detrimental effects of continuous BPA exposure. The highest BPA concentration used (100 μM) resulted in prolonged QRS intervals and dropped ventricular beats, and eventually resulted in complete heart block.

CONCLUSIONS

Our results show that acute BPA exposure slowed electrical conduction in excised hearts from female rats. These findings emphasize the importance of examining BPA's effect on heart electrophysiology and determining whether chronic in vivo exposure can cause or exacerbate conduction abnormalities in patients with preexisting heart conditions and in other high-risk populations.

KATP channel opening accelerates and stabilizes rotors in a swine heart model of ventricular fibrillation

AIMS

The mechanisms underlying ventricular fibrillation (VF) are still disputed. Recent studies have highlighted the role of KATP-channels. We hypothesized that, under certain conditions, VF can be driven by stable and epicardially detectable rotors in large hearts. To test our hypothesis, we used a swine model of accelerated VF by opening KATP-channels with cromakalim.

METHODS AND RESULTS

Optical mapping, spectral analysis, and phase singularity tracking were performed in eight perfused swine hearts during VF. Pseudo-bipolar electrograms were computed. KATP-channel opening almost doubled the maximum dominant frequency (14.3 ± 2.2 vs. 26.5 ± 2.8 Hz, P < 0.001) and increased the maximum regularity index (0.82 ± 0.05 vs. 0.94 ± 0.04, P < 0.001), the density of rotors (2.0 ± 1.4 vs. 16.0 ± 7.0 rotors/cm²×s, P < 0.001), and their maximum lifespans (medians: 368 vs. ≥3410 ms, P < 0.001). Persistent rotors (≥1 movie = 3410 ms) were found in all hearts after cromakalim (mostly coinciding with the fastest and highest organized areas), but they were not epicardially visible at baseline VF. A 'beat phenomenon' ruled by inter-domain frequency gradients was observed in all hearts after cromakalim. Acceleration of VF did not reveal any significant regional preponderance. Complex fractionated electrograms were not found in areas near persistent rotors.

CONCLUSION

Upon KATP-channel opening, VF consisted of rapid and highly organized domains mainly due to stationary rotors, surrounded by poorly organized areas. A 'beat phenomenon' due to the quasi-periodic onset of drifting rotors was observed. These findings demonstrate the feasibility of a VF driven by stable rotors in hearts whose size is similar to the human heart. Our model also showed that complex fractionation does not seem to localize stationary rotors.

The design and use of an optical mapping system for the study of intracardiac electrical signaling

Fluorescent optical mapping of electrically active cardiac tissues provides a unique method to examine the excitation wave dynamics of underlying action potentials. Such mapping can be viewed as a bridge between cellular level and organ systems physiology, e.g., by facilitating the development of advanced theoretical concepts of arrhythmia. We present the design and use of a high-speed, high-resolution optical mapping system composed entirely of "off the shelf" components. The electrical design integrates a 256 element photodiode array with a 16 bit data acquisition system. Proper grounding and shielding at various stages of the design reduce electromagnetic interference. Our mechanical design provides flexibility in terms of mounting positions and applications (use for whole heart or tissue preparations), while maintaining precise alignment between all optical components. The system software incorporates a user friendly graphical user interface, e.g., spatially recorded action potentials can be represented as intensity graphs or in strip chart format. Thus, this system is capable of displaying cardiac action potentials with high spatiotemporal resolution. Results from cardiac action potential mapping with intact mouse hearts are provided. It should be noted that this system could be readily configured to study isolated myocardial biopsies (e.g., isolated ventricular trabeculae). We describe the details of a versatile, user-friendly system that could be employed for a magnitude of study protocols.

Panoramic optical mapping shows wavebreak at a consistent anatomical site at the onset of ventricular fibrillation

AIMS

The first seconds of ventricular fibrillation (VF) are well organized and can consist of just one to two rotating waves (rotors). New rotors are spawned when local propagation block causes wave fragmentation. We hypothesized that this process, which leads to fully developed VF, begins at a consistent anatomic site.

METHODS AND RESULTS

We initiated VF with a stimulus timed to the local T-wave in 10 isolated pig hearts. Hearts were stained with a voltage-sensitive dye and four video cameras recorded electrical propagation panoramically across the epicardium. In each VF episode, we identified the position of the first wavebreak event that produced new rotor(s) that persisted for at least one cycle. The first such wavebreak occurred along the anterior right ventricular insertion (ARVI) in 26 of 32 VF episodes. In these episodes, wavebreak sites were 6 ± 4 mm from the midline of the ARVI. In the remaining 6 episodes, wavebreak sites were 24 ± 5 mm from the midline on either the LV or RV. During rapid pacing, conduction speed was locally depressed at the ARVI when waves crossed parallel to the midline. Action potential duration (APD) was slightly longer (2.2 ± 2.1 ms) at the ARVI compared with other sites (P< 0.01). Temporal APD alternans were small and not unique to the break site, suggesting that dynamic APD properties were not the cause of wavebreak.

CONCLUSION

The ARVI is the dominant site for wavebreak at the onset of VF in normal myocardium. This may be due to the anatomic complexity of the region.

A novel approach to dual excitation ratiometric optical mapping of cardiac action potentials with di-4-ANEPPS using pulsed LED excitation

We developed a new method for ratiometric optical mapping of transmembrane potential (V(m)) in cardiac preparations stained with di-4-ANEPPS. V(m)-dependent shifts of excitation and emission spectra establish two excitation bands (<481 and >481 nm) that produce fluorescence changes of opposite polarity within a single emission band (575-620 nm). The ratio of these positive and negative fluorescence signals (excitation ratiometry) increases V(m) sensitivity and removes artifacts common to both signals. We pulsed blue (450 ± 10 nm) and cyan (505 ± 15 nm) light emitting diodes (LEDs) at 375 Hz in alternating phase synchronized to a camera (750 frames-per-second). Fluorescence was bandpass filtered (585 ± 20 nm). This produced signals with upright (blue) and inverted (cyan) action potentials (APs) interleaved in sequential frames. In four whole swine hearts with motion chemically arrested, fractional fluorescence for blue, cyan, and ratio signals was 1.2 ± 0.3%, 1.2 ± 0.3%, and 2.4 ± 0.6%, respectively. Signal-to-noise ratios were 4.3 ± 1.4, 4.0 ± 1.2, and 5.8 ± 1.9, respectively. After washing out the electromechanical uncoupling agent, we characterized motion artifact by cross-correlating blue, cyan, and ratio signals with a signal with normal AP morphology. Ratiometry improved cross-correlation coefficients from 0.50 ± 0.48 to 0.81 ± 0.25, but did not cancel all motion artifacts. These findings demonstrate the feasibility of pulsed LED excitation ratiometry in myocardium.

Rate-dependent shortening of action potential duration increases ventricular vulnerability in failing rabbit heart

Congestive heart failure (CHF) predisposes to ventricular fibrillation (VF) in association with electrical remodeling of the ventricle. However, much remains unknown about the rate-dependent electrophysiological properties in a failing heart. Action potential properties in the left ventricular subepicardial muscles during dynamic pacing were examined with optical mapping in pacing-induced CHF ( n = 18) and control ( n = 17) rabbit hearts perfused in vitro. Action potential durations (APDs) in CHF were significantly longer than those observed for controls at basic cycle lengths (BCLs) >1,000 ms but significantly shorter at BCLs <400 ms. Spatial APD dispersions were significantly increased in CHF versus control (by 17–81%), and conduction velocity was significantly decreased in CHF (by 6–20%). In both groups, high-frequency stimulation (BCLs <150 ms) always caused spatial APD alternans; spatially concordant alternans and spatially discordant alternans (SDA) were induced at 60% and 40% in control, respectively, whereas 18% and 82% in CHF. SDA in CHF caused wavebreaks followed by reentrant excitations, giving rise to VF. Incidence of ventricular tachycardia/VFs elicited by high-frequency dynamic pacing (BCLs <150 ms) was significantly higher in CHF versus control (93% vs. 20%). In CHF, left ventricular subepicardial muscles show significant APD shortenings at short BCLs favoring reentry formations following wavebreaks in association with SDA. High-frequency excitation itself may increase the vulnerability to VF in CHF.

A computational study of mother rotor VF in the human ventricles

Sudden cardiac death is one of the major causes of death in the industrialized world. It is most often caused by a cardiac arrhythmia called ventricular fibrillation (VF). Despite its large social and economical impact, the mechanisms for VF in the human heart yet remain to be identified. Two of the most frequently discussed mechanisms observed in experiments with animal hearts are the multiple wavelet and mother rotor hypotheses. Most recordings of VF in animal hearts are consistent with the multiple wavelet mechanism. However, in animal hearts, mother rotor fibrillation has also been observed. For both multiple wavelet and mother rotor VF, cardiac heterogeneity plays an important role. Clinical data of action potential restitution measured from the surface of human hearts have been recently published. These in vivo data show a substantial degree of spatial heterogeneity. Using these clinical restitution data, we studied the dynamics of VF in the human heart using a heterogeneous computational model of human ventricles. We hypothesized that this observed heterogeneity can serve as a substrate for mother rotor fibrillation. We found that, based on these data, mother rotor VF can occur in the human heart and that ablation of the mother rotor terminates VF. Furthermore, we found that both mother rotor and multiple wavelet VF can occur in the same heart depending on the initial conditions at the onset of VF. We studied the organization of these two types of VF in terms of filament numbers, excitation periods, and frequency domains. We conclude that mother rotor fibrillation is a possible mechanism in the human heart.

Organization of ventricular fibrillation in the human heart: experiments and models

Sudden cardiac death is a major health problem in the industrialized world. The lethal event is typically ventricular fibrillation (VF), during which the co-ordinated regular contraction of the heart is overthrown by a state of mechanical and electrical anarchy. Understanding the excitation patterns that sustain VF is important in order to identify potential therapeutic targets. In this paper, we studied the organization of human VF by combining clinical recordings of electrical excitation patterns on the epicardial surface during in vivo human VF with simulations of VF in an anatomically and electrophysiologically detailed computational model of the human ventricles. We find both in the computational studies and in the clinical recordings that epicardial surface excitation patterns during VF contain around six rotors. Based on results from the simulated three-dimensional excitation patterns during VF, which show that the total number of electrical sources is 1.4 +/- 0.12 times greater than the number of epicardial rotors, we estimate that the total number of sources present during clinically recorded VF is 9.0 +/- 2.6. This number is approximately fivefold fewer compared with that observed during VF in dog and pig hearts, which are of comparable size to human hearts. We explain this difference by considering differences in action potential duration dynamics across these species. The simpler spatial organization of human VF has important implications for treatment and prevention of this dangerous arrhythmia. Moreover, our findings underline the need for integrated research, in which human-based clinical and computational studies complement animal research.

Epicardial mapping of ventricular fibrillation over the posterior descending artery and left posterior papillary muscle of the swine heart

BACKGROUND

Recent studies suggest that during ventricular fibrillation (VF) epicardial vessels may be a site of conduction block and the posterior papillary muscle (PPM) in the left ventricle (LV) may be the location of a "mother rotor." The goal of this study was to obtain evidence to support or refute these possibilities.

METHODS

Epicardial activation over the posterior LV and right ventricle (RV) was mapped during the first 20 s of electrically induced VF in six open-chest pigs with a 504 electrode plaque covering a 20 cm(2) area centered over the posterior descending artery (PDA).

RESULTS

The locations of epicardial breakthrough as well as reentry clustered in time and space during VF. Spatially, reentry occurred significantly more frequently over the LV than the RV in all 48 episodes, and breakthrough clustered near the PPM (p < 0.001). Significant temporal clustering occurred in 79% of breakthrough episodes and 100% of reentry episodes. These temporal clusters occurred at different times so that there was significantly less breakthrough when reentry was present (p < 0.0001). Conduction block occurred significantly more frequently near the PDA than elsewhere.

CONCLUSIONS

The PDA is a site of epicardial block which may contribute to VF maintenance. Epicardial breakthrough clusters near the PPM. Reentry also clusters in space but at a separate site. The fact that breakthrough and reentry cluster at different locations and at different times supports the possibility of a drifting filament at the PPM so that at times reentry is present on the surface but at other times the reentrant wavefront breaks through to the epicardium.

High-resolution high-speed panoramic cardiac imaging system

A panoramic cardiac imaging system consisting of three high-speed CCD cameras has been developed to image the surface electrophysiology of a rabbit heart via fluorescence imaging using a voltage-sensitive fluorescent dye. A robust, unique mechanical system was designed to accommodate the three cameras and to adapt to the requirements of future experiments. A unified computer interface was created for this application - a single workstation controls all three CCD cameras, illumination, stimulation, and a stepping motor that rotates the heart. The geometric reconstruction algorithms were adapted from a previous cardiac imaging system. We demonstrate the system by imaging a polymorphic cardiac tachycardia.

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.

Effect of heterogeneous APD restitution on VF organization in a model of the human ventricles

The onset of ventricular fibrillation (VF) has been associated with steep action potential duration restitution in both clinical and computational studies. Recently, detailed clinical restitution properties in cardiac patients were reported showing a substantial degree of heterogeneity in restitution slopes at the epicardium of the ventricles. The aim of the present study was to investigate the effect of heterogeneous restitution properties in a three-dimensional model of the ventricles using these clinically measured restitution data. We used a realistic model of the human ventricles, including detailed descriptions of cell electrophysiology, ventricular anatomy, and fiber direction anisotropy. We extended this model by mapping the clinically observed epicardial restitution data to our anatomic representation using a diffusion-based algorithm. Restitution properties were then fitted by regionally varying parameters of the electrophysiological model. We studied the effects of restitution heterogeneity on the organization of VF by analyzing filaments and the distributions of excitation periods. We found that the number of filaments and the excitation periods were both dependent on the extent of heterogeneity. An increased level of heterogeneity leads to a greater number of filaments and a broader distribution of excitation periods, thereby increasing the complexity and dynamics of VF. Restitution heterogeneity may play an important role in providing a substrate for cardiac arrhythmias.

Organization of ventricular fibrillation in the human heart

Sudden cardiac death is a major cause of death in the industrialized world, claiming approximately 300,000 victims annually in the United States alone. In most cases, sudden cardiac death is caused by ventricular fibrillation (VF). Experimental studies in large animal hearts have shown that the uncoordinated contractions during VF are caused by large numbers of chaotically wandering reentrant waves of electrical activity. However, recent clinical data on VF in the human heart seem to suggest that human VF may have a markedly different organization. Here, we use a detailed model of the human ventricles, including a detailed description of cell electrophysiology, ventricular anatomy, and fiber direction anisotropy, to study the organization of human VF. We show that characteristics of our simulated VF are qualitatively similar to the clinical data. Furthermore, we find that human VF is driven by only approximately 10 reentrant sources and thus is much more organized than VF in animal hearts of comparable size, where VF is driven by approximately 50 sources. We investigate the influence of anisotropy ratio, tissue excitability, and restitution properties on the number of reentrant sources driving VF. We find that the number of rotors depends strongest on minimum action potential duration, a property that differs significantly between human and large animal hearts. Based on these findings, we suggest that the simpler spatial organization of human VF relative to VF in large animal hearts may be caused by differences in minimum action potential duration. Both the simpler spatial organization of human VF and its suggested cause may have important implications for treating and preventing this dangerous arrhythmia in humans.

Development of device therapy for ventricular arrhythmias

The past 25 years have seen the implantable cardioverter defibrillator emerge as the treatment of choice for ventricular arrhythmias with reduction in size but increased therapeutic options. Understanding the complex mechanisms of ventricular arrhythmias and defibrillation in normal and diseased hearts has been the focus of many research teams including that of John Uther at the Westmead Hospital Department of Cardiology. Marked improvements in capacitor and battery technologies, arrhythmia discrimination, pacing algorithms, shock waveforms and monitoring capabilities enable wider use and patient acceptance. Emergence of cardiac resynchronisation therapy and the implantable defibrillator for treatment of chronic heart failure is not only giving quality of life and extended survival for heart failure patients but has also cast new light on the evolution of heart failure.

Optical mapping of Langendorff-perfused human hearts: establishing a model for the study of ventricular fibrillation in humans

Our objective was to establish a novel model for the study of ventricular fibrillation (VF) in humans. We adopted the established techniques of optical mapping to human ventricles for the first time to determine whether human VF is the result of wave breaks and singularity point formation and is maintained by high-frequency rotors and fibrillatory conduction. We describe the technique of acquiring optical signals in human hearts during VF, their characteristics, and the feasibility of possible analyses that could be performed to elucidate mechanisms of human VF. We used explanted hearts from five cardiomyopathic patients who underwent transplantation. The hearts were Langendorff perfused with Tyrode solution (95% O(2)-5% CO(2)), and the potentiometric dye di-4-ANEPPS was injected as a bolus into the coronary circulation. Fluorescence was excited at 531 +/- 20 nm with a 150-W halogen light source; the emission signal was long-pass filtered at 610 nm and recorded with a mapping camera. Fractional change of fluorescence varied between 2% and 12%. Average signal-to-noise ratio was 40 dB. The mean velocity of VF wave fronts was 0.25 +/- 0.04 m/s. Submillimetric spatial resolution (0.65-0.85 mm), activation mapping, and transformation of the data to phase-based analysis revealed reentrant, colliding, and fractionating wave fronts in human VF. On many occasions the VF wave fronts were as large as the entire vertical length (8 cm) of the mapping field, suggesting that there are a limited number of wave fronts on the human heart during VF. Phase transformation of the optical signals allowed the first demonstration ever of phase singularity point, wave breaks, and rotor formation in human VF. This method provides opportunities for potential analyses toward elucidation of the mechanisms of VF and defibrillation in humans.

Ventricular fibrillation in myopathic human hearts: mechanistic insights from in vivo global endocardial and epicardial mapping

Ventricular fibrillation (VF) is an important cause of sudden cardiac death and cardiovascular mortality in patients with cardiomyopathy. Although it was generally believed that chaotic reentrant wavefronts underlie VF in humans, there is emerging evidence of spatiotemporal organization during early VF. The mechanism of this organization of electrical activity in early VF is unknown in myopathic hearts. We studied early VF in vivo, intraoperatively in five cardiomyopathic patients. Simultaneous electrograms were obtained from the epicardium and endocardium in left ventricular cardiomyopathy and from the endocardium in right ventricular myopathy. The Hilbert transform was used to derive the phase of the electrograms. Rotors were identified by isolating phase singularity points. Rotors were present in all of the myopathic hearts studied during VF and cumulatively lasted a mean of 3.2 +/- 2.0 s of the 7.0 +/- 4.0 s of the VF segments analyzed. For each surface mapped, 3.6 +/- 2.9 rotors were identified for the duration mapped. The average number of cycles completed by these rotors was 4.9 +/- 4.9. The longest rotor lasted 10.2 +/- 6.2 rotations and lasted 2.0 +/- 1.2 s. The rotors on the endocardium had a cycle length of 192 +/- 33 ms compared with 220 +/- 15 ms on the epicardium (P=0.08). There is centrifugal activation of electrical activity from these rotors, and they give rise to domains that activate at faster rates with evidence of conduction block at the border with slower domains. These rotors frequently localized to border regions of myocardium with bipolar electrogram amplitude of <0.5 mV. The organization of electrical activity during early VF in myopathic human hearts is characterized by wavefronts emanating from a few rotors.

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.