Importance of location and timing of electrical stimuli in terminating sustained functional reentry in isolated swine ventricular tissues: evidence in support of a small reentrant circuit

Cardiac Macrophages and Their Effects on Arrhythmogenesis

Cardiac electrophysiology is a complex system established by a plethora of inward and outward ion currents in cardiomyocytes generating and conducting electrical signals in the heart. However, not only cardiomyocytes but also other cell types can modulate the heart rhythm. Recently, cardiac macrophages were demonstrated as important players in both electrophysiology and arrhythmogenesis. Cardiac macrophages are a heterogeneous group of immune cells including resident macrophages derived from embryonic and fetal precursors and recruited macrophages derived from circulating monocytes from the bone marrow. Recent studies suggest antiarrhythmic as well as proarrhythmic effects of cardiac macrophages. The proposed mechanisms of how cardiac macrophages affect electrophysiology vary and include both direct and indirect interactions with other cardiac cells. In this review, we provide an overview of the different subsets of macrophages in the heart and their possible interactions with cardiomyocytes under both physiologic conditions and heart disease. Furthermore, we elucidate similarities and differences between human, murine and porcine cardiac macrophages, thus providing detailed information for researchers investigating cardiac macrophages in important animal species for electrophysiologic research. Finally, we discuss the pros and cons of mice and pigs to investigate the role of cardiac macrophages in arrhythmogenesis from a translational perspective.

Ventricular Premature Complexes and Their Associated Factors in a General Population of Japanese Men

Increased ventricular premature complexes (VPCs) are associated with a higher risk of cardiac morbidities. However, little information is available on the risk factors of Western general populations. Therefore, we aimed to assess the frequency and associated factors of VPCs in healthy general Japanese men. We conducted a population-based cross-sectional study in 517 men, aged 40 to 79 years, using 24-hour Holter electrocardiography. Age, body mass index, height, low-density lipoprotein cholesterol, triglycerides, high-density lipoprotein cholesterol, resting heart rate, diabetes mellitus, hypertension, physical activity, smoking, alcohol consumption, lipid-lowering therapy were included in multivariable negative binomial regression to assess independent correlates for the number of VPCs per hour. We observed at least 1 VPC in 1 hour in 429 men (83%). In multivariable negative binomial regression adjusted for all covariates simultaneously, age (risk ratio [95% confidence interval] 1.91 [1.56 to 2.33] per 1-SD increment), height (1.17 [1.04 to 1.49] per 1-SD increment), resting heart rate(1.34 [1.02 to 1.77] per 1-SD increment), diabetes mellitus (2.36 [1.17 to 4.76] ), hypertension (1.90 [1.03 to 3.50]), physical activity (0.67 [0.47 to 0.97] ), current smoking (4.23 [1.86 to 9.60] ), past smoking (2.08 [1.03 to 4.19] ), current light alcohol consumption (0.16 [0.04 to 0.64] ), and lipid-lowering therapy (0.47 [0.23 to 0.96] ) were independently associated with VPCs frequency. In conclusion, VPCs frequency was independently associated with age, height, resting heart rate, diabetes mellitus, hypertension, physical activity, smoking, alcohol consumption, and lipid-lowering therapy.

Modifiable Predictors of Ventricular Ectopy in the Community

Background

Premature ventricular contractions (PVCs) predict heart failure and death. Data regarding modifiable risk factors for PVCs are scarce.

Methods and Results

We studied 1424 Cardiovascular Health Study participants randomly assigned to 24‐hour Holter monitoring. Demographics, comorbidities, habits, and echocardiographic measurements were examined as predictors of PVC frequency and, among 845 participants, change in PVC frequency 5 years later. Participants exhibited a median of 0.6 (interquartile range, 0.1–7.1) PVCs per hour. Of the more directly modifiable characteristics and after multivariable adjustment, every SD increase in systolic blood pressure was associated with 9% more PVCs (95% confidence interval [CI], 2%–17%; P=0.01), regularly performing no or low‐intensity exercise compared with more physical activity was associated with ≈15% more PVCs (95% CI, 3–25%; P=0.02), and those with a history of smoking exhibited an average of 18% more PVCs (95% CI, 3–36%; P=0.02) than did never smokers. After 5 years, PVC frequency increased from a median of 0.5 (IQR, 0.1–4.7) to 1.2 (IQR, 0.1–13.8) per hour (P<0.0001). Directly modifiable predictors of 5‐year increase in PVCs, described as the odds per each quintile increase in PVCs, included increased diastolic blood pressure (odds ratio per SD increase, 1.16; 95% CI, 1.02–1.31; P=0.02) and a history of smoking (OR, 1.31; 95% CI, 1.02–1.68; P=0.04).

Conclusions

Enhancing physical activity, smoking cessation, and aggressive control of blood pressure may represent fruitful strategies to mitigate PVC frequency and PVC‐associated adverse outcomes.

Optogenetics design of mechanistically-based stimulation patterns for cardiac defibrillation

Current rescue therapies for life-threatening arrhythmias ignore the pathological electro-anatomical substrate and base their efficacy on a generalized electrical discharge. Here, we developed an all-optical platform to examine less invasive defibrillation strategies. An ultrafast wide-field macroscope was developed to optically map action potential propagation with a red-shifted voltage sensitive dye in whole mouse hearts. The macroscope was implemented with a random-access scanning head capable of drawing arbitrarily-chosen stimulation patterns with sub-millisecond temporal resolution allowing precise epicardial activation of Channelrhodopsin2 (ChR2). We employed this optical system in the setting of ventricular tachycardia to optimize mechanistic, multi-barrier cardioversion/defibrillation patterns. Multiple regions of conduction block were created with a very high cardioversion efficiency but with lower energy requirements as compared to whole ventricle interventions to interrupt arrhythmias. This work demonstrates that defibrillation energies can be substantially reduced by applying discrete stimulation patterns and promotes the progress of current anti-arrhythmic strategies.

Arrhythmias during and after zoledronic acid infusion patients with bone metastasis

Zoledronic acid (ZA) is one of the important bisphosphonates which is widely used in bone metastatic cancer and osteoporotic patients. In a few studies, it has been reported that treatment with bisphosphonates was associated with an increased risk of atrial fibrillation. We aimed to evaluate the arrhythmias that developed during and immediately after infusion of the ZA. Fifty-two bone metastatic patients were included in the study group. All patients had 24-h Holter monitorization during the first dose ZA infusion day. All of the patients had 4-h basal cardiac rhythm records before ZA infusion and about 19 h after infusion. A short survey including demographic data and past medical history has been completed. None of patients had clinically important arrhythmias before ZA infusion. We divided arrhythmias into two groups as supraventricular and ventricular. We evaluated arrhythmias in pre-infusion, during infusion, and post-infusion periods. ZA was administered 4 mg intravenously (IV) in 15 min. Thirty-three of patients (63.5 %) were male and 19 (36.5 %) patients were female. Mean age of the patients was 53.9 ± 11.8 years. Most frequent cancers were breast (25 %) and lung cancer (15.3 %). Twelve (23 %) patients had history of mediastinal radiotherapy. In basal records, we detected that twenty-four (46 %) of patients had supraventricular premature complexes (SVPC) or ventricular premature complexes (VPC). Fifteen (28.8 %) of patients had SVPC and fourteen (26.9 %) had VPC during infusion period. After infusion period, 48 (92.3 %) of patients had SVPC and 41 (78.8 %) had VPC. Only 3 patients had no arrhythmia after infusion. Three patients had sinus arrhythmia and two had Mobitz type 2 atrioventricular blocks after infusion. One patient, who had no history of comorbidities and had SVPC in the basal records, developed atrial fibrillation that was refractory to medical cardioversion after 10 days of seventh dose of ZA infusion. In this study, we found that both SVPC and VPC increased in cancer patients treated with ZA. Furthermore, ZA may induce clinically important arrhythmias.

Post-shock synchronized pacing in isolated rabbit left ventricle: evaluation of a novel defibrillation strategy

INTRODUCTION

A failed near-threshold defibrillation shock is followed by an isoelectric window (IEW) and rapid repetitive responses that reinitiate ventricular fibrillation (VF). We hypothesized that properly timed (synchronized) postshock pacing stimuli (SyncP) may capture the recovered tissues during the repetitive responses and prevent postshock reinitiation of VF, resulting in improved defibrillation efficacy.

METHODS AND RESULTS

We explored the effect of postshock SyncP on defibrillation efficacy in isolated rabbit hearts (n = 12). Optical recording-guided real-time detection and electrical stimulation (5 mA) of recovered tissues in anterior/posterior left ventricle (LV) were performed following IEW. The IEW duration was found to be 69 +/- 13 ms. With the same shock strength, successful and failed defibrillation episodes were associated with 50% and 15% of the myocardium, respectively, captured by the SyncP (P < 0.001). Electrical stimulation from the posterior LV resulted in 75% of episodes capturing myocardium, as compared with anterior LV stimulation (55%; P < 0.01) and higher successful defibrillation rate (14%, posterior vs. 3%, anterior LV). The overall success in terminating VF by postshock SyncP was approximately 10%. The causes for failed myocardium capture by postshock SyncP included lack of IEW after low-strength shock (42.9%), incorrect locations of reference site (25.7%) and pacing electrodes (17.9%), and others, such as wave breakthroughs (13.5%).

CONCLUSION

Postshock SyncP was feasible and the larger the myocardium captured area, the more likely was the successful defibrillation. Postshock SyncP delivered to the posterior LV was more effective than anterior LV to terminate VF.

Interaction between spiral and paced waves in cardiac tissue

For prevention of lethal arrhythmias, patients at risk receive implantable cardioverter-defibrillators, which use high-frequency antitachycardia pacing (ATP) to convert tachycardias to a normal rhythm. One of the suggested ATP mechanisms involves paced-induced drift of rotating waves followed by their collision with the boundary of excitable tissue. This study provides direct experimental evidence of this mechanism. In monolayers of neonatal rat cardiomyocytes in which rotating waves of activity were initiated by premature stimuli, we used the Ca(2+)-sensitive indicator fluo 4 to observe propagating wave patterns. The interaction of the spiral tip with a paced wave was then monitored at a high spatial resolution. In the course of the experiments, we observed spiral wave pinning to local heterogeneities within the myocyte layer. High-frequency pacing led, in a majority of cases, to successful termination of spiral activity. Our data show that 1) stable spiral waves in cardiac monolayers tend to be pinned to local heterogeneities or areas of altered conduction, 2) overdrive pacing can shift a rotating wave from its original site, and 3) the wave break, formed as a result of interaction between the spiral tip and a paced wave front, moves by a paced-induced drift mechanism to an area where it may become unstable or collide with a boundary. The data were complemented by numerical simulations, which was used to further analyze experimentally observed behavior.

Optical recording-guided pacing to create functional line of block during ventricular fibrillation

Low-energy defibrillation is very desirable in cardiac rhythm management. We previously reported that ventricular fibrillation (VF) can be synchronized with a novel synchronized pacing technique (SyncP) using low-energy pacing pulses. This study sought to create a line of block during VF using SyncP. SyncP was performed in six isolated rabbit hearts during VF using optical recording to control the delivery of pacing pulses in real time. Four pacing electrodes with interelectrode distances of 5 mm were configured in a line along and across the myocardial fiber direction. The electrodes were controlled independently (independent mode) or fired together (simultaneous mode). Significant wavefront synchronization was observed along the electrode line as indicated by a decrease in variance. With the independent SyncP protocol, the decrease in the variance was 19.3 and 13.7% (P<0.001) for the along-, and across-fiber configurations, respectively. With the simultaneous SyncP protocol, the variance was reduced by 24.2 and 10.7% (P<0.001) in the along- and across-fiber configurations. The effect of synchronization dropped off with distance from the line of pacing. We conclude that SyncP can effectively create a line of functional block that isolates regions of VF propagation. Further optimization of this technique may prove useful for low-energy ventricular defibrillation.

Method of post-shock synchronized pacing in the excitable gaps

Ventricular fibrillation (VF) can be synchronized with a novel synchronized pacing technique (SyncP) using low-energy pacing pulses, which causes pace termination of VF. Synchronized pacing (SyncP) is defined as optical recording guided real-time detection and stimulation of spatiotemporal excitable gaps. In this paper, we investigate the effect of post-shock SyncP strategy on improvement of defibrillation efficacy. After a near-threshold defibrillation shock, when the reference site detected the earliest activation of the reinitiated VF, a 5-mA electric stimulus was delivered from the post-shock pacing electrode to depolarize the excitable gap. This area of wavefront synchronization may lead to a change in the timing of VF propagation, which is important for VF termination. Here, we implemented the concept of post-shock synchronized pacing by a real-time feedback mechanism and demonstrated a successful VF termination by the post-shock SyncP strategy. Further optimization of this technique may prove effective in improving the defibrillation efficacy for low-energy ventricular defibrillation.

Spiral wave control by a localized stimulus: a bidomain model study

INTRODUCTION

It has been reported that electrical stimulation can control spiral wave (SW) reentry. However, previous research does not account for the effects of stimulus-induced virtual electrode polarization (VEP) and the ensuing cathode-break (CB) excitation. The aim of the present study was to examine the interaction of VEP with SW reentry in a bidomain model of electrical stimulation and thus provide insight into the mechanistic basis of SW control.

METHODS AND RESULTS

We conducted 3,168 simulations of localized stimulation during SW reentry in an anisotropic bidomain sheet. Unipolar cathodal 2-ms stimuli of strengths 4, 8, 16, and 24 mA were delivered at 99 locations in the sheet. The interaction between stimulus-induced VEP and SW reentry resulted in 1 of 3 possible outcomes: SW shift, SW breakup, or no effect. SW shift, which could be instrumental in SW termination at an anatomic or functional line of block, resulted from CB rather than cathode-make excitation. Stimulus timing, site, and strength all were important factors in VEP-mediated SW control. Furthermore, we found that the number of episodes of SW shift across the fibers was more sensitive to stimulus strength than that of SW shift along the fibers. SW shift can be explained by the interaction between the four VEP-induced wavebreaks and the wavebreak of the SW, ultimately resulting in termination of the original SW and the survival of one of the VEP-induced wavebreaks. This establishes a new SW reentry.

CONCLUSION

This study provides new mechanistic insight into SW control.

Characterization of fibrillatory rhythms by ensemble vector directional analysis

Recent studies have demonstrated that fibrillatory rhythms are not random phenomena but have definable patterns. However, standard mapping techniques may have limitations in their ability to identify the organization of fibrillation. The purpose of this study was to develop and apply a method, "ensemble vector mapping," for characterizing the spatiotemporal organization of fibrillation. Ventricular fibrillation was induced by burst pacing in normal mongrel dogs. In a separate protocol, atrial fibrillation was induced by epicardial aconitine application. Epicardial electrograms were recorded from a 112-electrode plaque array using a computerized mapping system. Vectors were created by summing orthogonal bipolar electrograms. The magnitude of the vectors was transformed using a logarithmic function, integrated over time, and normalized for local electrogram amplitude to produce an "ensemble vector" index whose magnitude is high when beat-to-beat activation direction is consistent and low when activation direction is variable. The mean index was 137 +/- 36 mV/s during ventricular pacing at a cycle length of 300 ms but only 39 +/- 23 mV/s during ventricular fibrillation (P < 0.001). The ensemble vector index was also lower during atrial fibrillation (60 +/- 54 mV/s) than during atrial pacing (115 +/- 27 mV/s, P < 0.01 vs. atrial fibrillation) but not as low as during ventricular fibrillation (P < 0.05, atrial vs. ventricular fibrillation). The index was also capable of distinguishing atrial tachycardia from atrial fibrillation. Ensemble vector mapping produces an objective assessment of the consistency of myocardial activation during fibrillation. The consistency of activation direction differs in different models of fibrillation and is higher during atrial than ventricular fibrillation. This technique has the potential to rapidly characterize repetitive activation patterns in fibrillatory rhythms and may help distinguish among different characteristics of fibrillatory rhythms.

Interactions Between Extracellular Stimuli and Excitation Waves in an Atrial Reentrant Loop

Extracellular Stimuli in an Atrial Reentrant Loop.

INTRODUCTION

The interactions between extracellular stimuli and excitation waves propagating in a reentrant loop are a complex function of stimulus parameters, structural properties, membrane state, and timing. Here the goal was a comprehensive understanding of the mechanisms and frequencies of the major interactions between the advancing excitation wave and a single extracellular stimulus, separated from issues of anatomic or geometric complexity.

METHODS AND RESULTS

A modernized computer model of a thin ring of uniform tissue that included a pair of extracellular stimulus electrodes (anode/cathode) was used to model one-dimensional cardiac reentry. Questions and results included the following: (1) What are the major interactions between a stimulus and the reentrant propagation wave, and are they induced near the cathode or near the anode; and, for each interaction, what are the initiating amplitude range and timing interval? At the cathode, the well-known mechanism of retrograde excitation terminated reentry; changes in timing or amplitude produced double-wave reentry or phase reset. At the anode, termination occurred at different cells depending on stimulus amplitude. (2) Relatively how often did termination occur at the anode? For most stimulus amplitudes, termination occurred more often at the anode than at the cathode, although not always at the same cell. (3) With random timing, what is the probability of terminating reentry? Stimulation for 5 msec terminated reentry with a probability from 0% to approximately 10%, as a function of increasing stimulus amplitude.

CONCLUSION

A single extracellular stimulus can initiate major changes in reentrant excitation via multiple mechanisms, even in a simple geometry. Termination of reentry, phase shifts, or double-wave reentry each occurs over well-defined ranges of stimulus amplitude and timing.

Contact fluorescence imaging of reentry in monolayers of cultured neonatal rat ventricular myocytes

INTRODUCTION

We present a novel contact fluorescence imaging (CFI) approach to monitor transmembrane potentials in monolayers of cultured neonatal rat ventricular cells. We apply CFI to demonstrate, for the first time, long-term recordings as well as electrical induction and termination of reentrant activity in this in vitro model.

METHODS AND RESULTS

CFI was performed in confluent cell monolayers stained with di-8-ANEPPS. An anatomic obstacle (6 x 0.5 mm) was created in the center of the monolayers. Reentry was induced with a premature stimulus after pacing at 2 Hz (both via field stimulation). Seven sustained (>3 min) reentrant episodes, anchored to the anatomic obstacle, were observed in three monolayers. Field stimulation (30 V/cm) was applied to successfully terminate 6 of the 7 reentries. Analysis of reentrant activity showed similarities with anatomic reentry in tissue preparations, such as reduced conduction velocity around the core, variable conduction velocity along the reentrant pathway due to wavefront curvature effects, and field-induced activation at the obstacle borders leading to reentry termination (cardioversion).

CONCLUSION

This study demonstrates the feasibility of CFI for macroscopic optical mapping of transmembrane potentials in a single layer of cultured cells. Our results suggest that the monolayer cell culture model is an attractive complement to tissue models of reentry and cardioversion.

Relation between cellular repolarization characteristics and critical mass for human ventricular fibrillation

INTRODUCTION

The critical mass for human ventricular fibrillation (VF) and its electrical determinants are unclear. The goal of this study was to evaluate the relationship between repolarization characteristics and critical mass for VF in diseased human cardiac tissues.

METHODS AND RESULTS

Eight native hearts from transplant recipients were studied. The right ventricle was immediately excised, then perfused (n = 6) or superfused (n = 2) with Tyrode's solution at 36 degrees C. The action potential duration (APD) restitution curve was determined by an S1-S2 method. Programmed stimulation and burst pacing were used to induce VF. In 3 of 8 tissues, 10 microM cromakalim, an ATP-sensitive potassium channel opener, was added to the perfusate and the stimulation protocol repeated. Results show that, at baseline, VF did not occur either spontaneously or during rewarming, and it could not be induced by aggressive electrical stimulation in any tissue. The mean APD at 90% depolarization (APD90) at a cycle length of 600 msec was 227+/-49 msec, and the mean slope of the APD restitution curve was 0.22+/-0.08. Among the six tissues perfused, five were not treated with any antiarrhythmic agent. The weight of these five heart samples averaged 111+/-23 g (range 85 to 138). However, after cromakalim infusion, sustained VF (> 30 min in duration) was consistently induced. As compared with baseline in the same tissues, cromakalim shortened the APD90 from 243+/-32 msec to 55+/-18 msec (P < 0.001) and increased the maximum slope of the APD restitution curve from 0.24+/-0.11 to 1.43+/-0.10 (P < 0.01).

CONCLUSION

At baseline, the critical mass for VF in diseased human hearts in vitro is > 111 g. However, the critical mass for VF can vary, as it can be reduced by shortening APD and increasing the slope of the APD restitution curve.

Evolving era of multidimensional medical imaging

Currently, computer-assisted imaging can visualize very fast or very slow nonvisible motion events. We can create measurable geometric representations of physiology, including transformation, blood flow velocity, perfusion, pressure, contractility, image features, electricity, metabolism, and a vast number of other constantly changing parameters. The greatest attribute is the ability to present physiologic phenomena as easily understood geometric images more suited to the human's four-dimensional comprehension of reality. The key research challenges are to discover new visual metaphors for representing information, understand the analysis tasks that they support, and associate relevant information to create new information.

Computerized mapping of fibrillation in normal ventricular myocardium

It is well known that the ability to fibrillate is intrinsic to a normal ventricle that exceeds a critical mass. The questions we address are how is ventricular fibrillation (VF) initiated and perpetuated in normal myocardium, and why is VF not seen more often in the general population if all ventricles have the ability to fibrillate. To study the mechanisms of VF, we used computerized mapping techniques with up to 512 channels of simultaneous multisite recordings for data acquisition. The data were then processed for dynamic display of the activation patterns and for mathematical analyses of the activation intervals. The results show that in normal ventricles, VF can be initiated by a single strong premature stimulus given during the vulnerable period of the cardiac cycle. The initial activations form a figure-eight pattern. Afterward, VF will perpetuate itself without any outside help. The self-perpetuation itself is due to at least two factors. One is that single wave fronts spontaneously break up into two or more wavelets. The second is that when two wavelets intersect perpendicular to each other, the second wavelet is broken by the residual refractoriness left over from the first wavelet. Mathematical analyses of the patterns of activation during VF revealed that VF is a form of chaos, and that transition from ventricular tachycardia (VT) to VF occurs via the quasiperiodic route. In separate experiments, we found that we can convert VF to VT by tissue size reduction. The physiological mechanism associated with the latter transition appears to be the reduction of the number of reentrant wave fronts and wandering wavelets. Based on these findings, we propose that the reentrant wave fronts and the wandering wavelets serve as the physiological equivalent of coupled oscillators. A minimal number of oscillators is needed for VF to perpetuate itself, and to generate chaotic dynamics; hence a critical mass is required to perpetuate VF. We conclude that VF in normal myocardium is a form of reentrant cardiac arrhythmia. A strong electrical stimulus initiates single or dual reentrant wave fronts that break up into multiple wavelets. Sometimes short-lived reentry is also generated during the course of VF. These organized reentrant and broken wavelets serve as coupled oscillators that perpetuate VF and maintain chaos. Although the ability to support these oscillators exists in a normal ventricle, the triggers required to generate them are nonexistent in the normal heart. Therefore, VF and sudden death do not happen to most people with normal ventricular myocardium. (c) 1998 American Institute of Physics.