High-rate pacing suppresses Torsade de Pointes arrhythmias and reduces spatial dispersion of repolarization in the chronic AV-block dog model

Background: An electrical storm of Torsade de Pointes arrhythmias (TdP) can be reproducibly induced in the anesthetized chronic AV-block (CAVB) dog by infusion of the IKr-blocker dofetilide. Earlier studies showed that these arrhythmias 1) arise from locations with high spatial dispersion in repolarization (SDR) and 2) can be suppressed by high-rate pacing. We examined whether suppression of TdP by high-rate pacing is established through a decrease in SDR in the CAVB dog. Methods: Dofetilide (25 μg/kg in 5 min) was administered to 5 anesthetized CAVB dogs to induce TdP arrhythmias. During the experiments, animals were continuously paced from the right ventricular apex at 50 beats/minute (RVA50). Upon TdP occurrence and conversion, RVA pacing was consecutively set to 100, 80 and 60 beats/minute for 2 min, referred to as pacing blocks. To determine the additional anti-arrhythmic effects of HRP over defibrillation alone, the number of arrhythmic events and SDR at RVA100 were compared to data from three previously conducted experiments, in which dogs underwent the same experimental protocol but were paced at RVA60 upon TdP occurrence (RVA60retro). In all experiments, recordings included surface electrocardiogram and mapping by 56 intramural needles, each recording four electrograms, evenly inserted into the ventricular walls and septum. For each pacing block, the number of ectopic beats (EB), and TdP severity were scored. SDR was quantified as the average difference in repolarization time within four squared needles (SDRcubic). Results: In 4 out of 5 animals, pacing at RVA100 suppressed TdP occurrence. One dog could not be converted by defibrillation after the initial TdP. Compared to RVA50, pacing at RVA100, but not RVA80 and RVA60, significantly reduced the TdP score (78 ± 33 vs. 0 ± 0, p < 0.05 and vs. 12.5 ± 25 and 25 ± 50, both p > 0.05). The reduction in TdP score was reflected by a significant decrease in SDRcubic (125 ± 46 ms before TdP vs. 49 ± 18 ms during RVA100, p < 0.05), and SDR was smaller than in the RVA60retro animals (101 ± 52 ms, p < 0.05 vs. RVA100). Conclusion: In CAVB dogs, high-rate pacing effectively suppresses TdP, which, at least in part, results from a spatial homogenization of cardiac repolarization, as reflected by a decrease in SDR.

The Role of TRPM4 in Cardiac Electrophysiology and Arrhythmogenesis

The transient receptor potential melastatin 4 (TRPM4) channel is a non-selective cation channel that activates in response to increased intracellular Ca²⁺ levels but does not allow Ca²⁺ to pass through directly. It plays a crucial role in regulating diverse cellular functions associated with intracellular Ca²⁺ homeostasis/dynamics. TRPM4 is widely expressed in the heart and is involved in various physiological and pathological processes therein. Specifically, it has a significant impact on the electrical activity of cardiomyocytes by depolarizing the membrane, presumably via Na⁺ loading. The TRPM4 channel likely contributes to the development of cardiac arrhythmias associated with specific genetic backgrounds and cardiac remodeling. This short review aims to overview what is known so far about the TRPM4 channel in cardiac electrophysiology and arrhythmogenesis, highlighting its potential as a novel therapeutic target to effectively prevent and treat cardiac arrhythmias.

Pulsed field ablation of superior vena cava in paroxysmal atrial fibrillation: a case report

Paroxysmal atrial fibrillation originates most commonly in the pulmonary veins. However, the superior vena cava has proved to be arrhythmogenic in some cases. Pulsed field ablation, an emerging ablation technology, selectively affects myocardial tissue. Herein, we present a case of paroxysmal atrial fibrillation in a 64-year-old man who was admitted to our hospital for pulsed field ablation. The tachycardia was recurrent despite four successful pulmonary vein isolations. The superior vena cava was determined to be involved in arrhythmogenesis. The atrial fibrillation terminated immediately after the pulsed field ablation discharge at the superior vena cava.

Secretoneurin as a Novel Biomarker of Cardiovascular Episodes: Are We There Yet? A Narrative Review

Secretoneurin (SN) is a 33 amino-acid evolutionary conserved neuropeptide from the chromogranin peptide family. SN's main effects may be cardioprotective and are believed to be mediated through its inhibition of calmodulin-dependent kinase II (CaMKII), which influences intracellular calcium handling. SN inhibition of CaMKII suppresses calcium leakage from the sarcoplasmic reticulum through the ryanodine receptor. This action may reduce the risk of ventricular arrhythmias and calcium-dependent remodelling in heart failure. SN is also involved in reducing the intracellular reactive oxygen species concentration, modulating the immune response, and regulating the cell cycle, including apoptosis. SN can predict mortality in different disease states, beyond the classical risk factors and markers of myocardial injury. Plasma SN levels are elevated soon after an arrhythmogenic episode. In summary, SN is a novel biomarker with potential in cardiovascular medicine, and probably beyond.

Atrial Fibrillation and Stress: A 2-Way Street?

The accumulating literature linking stress with negative health outcomes, including cardiovascular disease (CVD), is extensively reported yet poorly defined. Stress is associated with a higher risk of hypertension, acute myocardial infarction, arrhythmogenesis, and heart failure. Stress mediates its effect through direct neuronal, endocrine, autonomic, and immune processes and indirectly by modifying lifestyle behaviors that promote CVD progression. Stress occurs when an individual perceives that internal or external demands exceed the capacity for an adaptive response. Psychologic stress is increasingly recognized in the atrial fibrillation (AF) population, although the pathophysiology remains unclear. There appears to be a bidirectional relationship between AF and stress with a complex interplay between the 2 entities. Stress modulates the immune and autonomic nervous systems, key drivers in AF initiation and potentiation. AF leads to increasing anxiety, psychologic distress, and suicidal ideation. Recently, lifestyle modification has emerged as the fourth pillar of AF management, with stress reduction a potential reversible risk factor and future target for intervention. This review examines proposed mechanisms linking AF and stress and explores stress reduction as an adjunct to the AF management armamentarium.

Febuxostat increases ventricular arrhythmogenesis through calcium handling dysregulation in human induced pluripotent stem cell-derived cardiomyocytes

Febuxostat is a xanthine oxidase inhibitor used to reduce the formation of uric acid and prevent gout attacks. Previous studies have suggested that febuxostat was associated with a higher risk of cardiovascular events, including atrial fibrillation, compared with allopurinol, another anti-hyperuricemia drug. Whereas in our clinical practice, we identified two cases of febuxostat-associated ventricular tachycardia events. The proarrhythmogenic effects of febuxostat on human cardiomyocytes and underlined mechanisms remain poorly understood. In this study, we employed real time cell analysis (RTCA) and calcium transient to investigate the effects of febuxostat on the cytotoxicity and electrophysiology properties of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs). Up to 10 μM febuxostat treatment did not show toxicity to cell viability. However, 48-hour febuxostat exposure generated dose-dependent increased irregular calcium transients and decreased calcium transient amplitude. Furthermore, RNA-seq analysis indicated that the MAPK signaling pathway was enriched in the febuxostat-treated group, especially the protein kinases JNK. Western blotting of three main protein kinases demonstrated that JNK activation is related to febuxostat-induced arrythmia rather than ERK or p38. The dysfunctional calcium dynamics of febuxostat-treated hiPSC-CMs could be ameliorated by SP600125, the inhibitor of JNK. In conclusion, our study demonstrated that febuxostat increases the predisposition to ventricular arrythmia by dysregulating calcium dynamics.

Epicardial Adipose Tissue and Cardiac Arrhythmias: Focus on Atrial Fibrillation

Atrial Fibrillation (AF) is the most frequent cardiac arrhythmia and its prevalence increases with age. AF is strongly associated with an increased risk of stroke, heart failure and cardiovascular mortality. Among the risk factors associated with AF onset and severity, obesity and inflammation play a prominent role. Numerous recent evidence suggested a role of epicardial adipose tissue (EAT), the visceral fat depot of the heart, in the development of AF. Several potential arrhythmogenic mechanisms have been attributed to EAT, including myocardial inflammation, fibrosis, oxidative stress, and fat infiltration. EAT is a local source of inflammatory mediators which potentially contribute to atrial collagen deposition and fibrosis, the anatomical substrate for AF. Moreover, the close proximity between EAT and myocardium allows the EAT to penetrate and generate atrial myocardium fat infiltrates that can alter atrial electrophysiological properties. These observations support the hypothesis of a strong implication of EAT in structural and electrical atrial remodeling, which underlies AF onset and burden. The measure of EAT, through different imaging methods, such as echocardiography, computed tomography and cardiac magnetic resonance, has been proposed as a useful prognostic tool to predict the presence, severity and recurrence of AF. Furthermore, EAT is increasingly emerging as a promising potential therapeutic target. This review aims to summarize the recent evidence exploring the potential role of EAT in the pathogenesis of AF, the main mechanisms by which EAT can promote structural and electrical atrial remodeling and the potential therapeutic strategies targeting the cardiac visceral fat.

Profibrillatory Structural and Functional Properties of the Atrial-Pulmonary Junction in the Absence of Remodeling

Background: Sole pulmonary vein (PV) isolation by ablation therapy prevents atrial fibrillation (AF) in patients with short episodes of AF and without comorbidities. Since incomplete PV isolation can be curative, we tested the hypothesis that the PV in the absence of remodeling and comorbidities contains structural and functional properties that are proarrhythmic for AF initiation by reentry.

Methods: We performed percutaneous transvenous in vivo endocardial electrophysiological studies and quantitative histological analysis of PV from healthy sheep.

Results: The proximal PV contained more myocytes than the distal PV and a higher percentage of collagen and fat tissue relative to myocytes than the left atrium. Local fractionated electrograms occurred in both the distal and proximal PVs, but a large local activation (>0.75 mV) was more often present in the proximal PV than in the distal PV (86 vs. 50% of electrograms, respectively, p = 0.017). Atrial arrhythmias (run of premature atrial complexes) occurred more often following the premature stimulation in the proximal PV than in the distal PV (p = 0.004). The diastolic stimulation threshold was higher in the proximal PV than in the distal PV (0.7 [0.3] vs. 0.4 [0.2] mA, (median [interquartile range]), p = 0.004). The refractory period was shorter in the proximal PV than in the distal PV (170 [50] vs. 248 [52] ms, p < 0.001). A linear relation existed between the gradient in refractoriness (distal-proximal) and atrial arrhythmia inducibility in the proximal PV.

Conclusion: The structural and functional properties of the native atrial-PV junction differ from those of the distal PV. Atrial arrhythmias in the absence of arrhythmia-induced remodeling are caused by reentry in the atrial-PV junction. Ablative treatment of early paroxysmal AF, rather than complete isolation of focal arrhythmia, may be limited to inhibition of reentry.

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.

Extreme hyperthermia-induced arrhythmogenesis

Hyperthermia is defined as an elevated body temperature above the normal range due to a failure of heat regulatory mechanisms. In addition to its effects on other organ systems, hyperthermia is associated with profound cardiovascular effects. We report the sentinel case of a 6-year-old girl with structurally and electrically normal heart, who presented with life-threatening hyperpyrexia-induced ventricular tachycardia, which was refractory to cardioversion and anti-arrhythmics but responded promptly to cooling. We emphasise the lifesaving role of immediate and aggressive cooling in such patients.

Influence of Fibrosis Amount and Patterns on Ventricular Arrhythmogenesis and Pumping Efficacy: Computational Study

Myocardial fibrosis is an integral component of most forms of heart failure. Clinical and computational studies have reported that spatial fibrosis pattern and fibrosis amount play a significant role in ventricular arrhythmogenicity. This study investigated the effect of the spatial distribution of fibrosis and fibrosis amount on the electrophysiology and mechanical performance of the human ventricles. Seventy-five fibrosis distributions comprising diffuse, patchy, and compact fibrosis types that contain 10-50% fibrosis amount were generated. The spatial fibrosis distribution was quantified using the fibrosis entropy (FE) metric. Electrical simulations under reentry conditions induced using the S1-S2 protocol were conducted to investigate the fibrosis arrhythmogenicity. We also performed mechanical simulations to examine the influence of the fibrosis amount and the spatial distribution of fibrosis on the pumping efficacy of the LV. We observed that the mean FE of the compact type is the largest among the three types. The electrical simulation results revealed that the ventricular arrhythmogenicity of diffuse fibrosis depends on the fibrosis amount and marginally on the spatial distribution of fibrosis. Meanwhile, the ventricular arrhythmogenicity of the compact and patchy fibrosis pattern is more reliant on the spatial distribution of fibrosis than on the fibrosis amount. The average number of phase singularities (PSs) in the compact fibrosis pattern was the highest among the three patterns of fibrosis. The diffuse type of fibrosis has the lowest average number of PSs than that in the patchy and compact fibrosis. The reduction in the stroke volume (SV) showed high influence from the electrical instabilities induced by the fibrosis amount and pattern. The compact fibrosis exhibited the lowest SV among the three patterns except in the 40% fibrosis amount. In conclusion, the fibrosis pattern is as crucial as the fibrosis amount for sustaining and aggravating ventricular arrhythmogenesis.

Macrophage activation in stellate ganglia contributes to lung injury-induced arrhythmogenesis in male rats

AIM

Patients suffering from acute lung injury (ALI) are at high risk of developing cardiac arrhythmias. We hypothesized that stellate ganglia (SG) neural inflammation contributes to ALI-induced arrhythmia.

METHODS

We created an ALI rat model using a single tracheal instillation of bleomycin (2.5 mg/kg), with saline as a sham control. We recorded ECGs by implanted radiotelemetry in male bleomycin and sham rats treated with and without oral minocycline (20 mg/kg/d), an anti-inflammatory drug that inhibits microglia/macrophage activation. The SG neuronal excitability was assessed by electrophysiology experiments.

RESULTS

ECG data showed that bleomycin-exposed rats exhibited significantly more spontaneous premature ventricular contractions (PVCs) from 1- to 3-week post-induction compared with sham rats, which was mitigated by chronic oral administration of minocycline. The bleomycin-exposed rats displayed a robust increase in both the number of Iba1-positive macrophages and protein expression of interferon regulatory factor 8 in the SG starting as early at 1-week post-exposure and lasted for at least 4 weeks, which was largely attenuated by minocycline. Heart rate variability analysis indicated autonomic imbalance during the first 2-week post-bleomycin, which was significantly attenuated by minocycline. Electrical stimulation of the decentralized SG triggered more PVCs in bleomycin-exposed rats than sham and bleomycin + minocycline rats. Patch-clamp data demonstrated enhanced SG neuronal excitability in the bleomycin-exposed rats, which was attenuated by minocycline. Co-culture of lipopolysaccharide (LPS)-pretreated macrophages with normal SG neurons enhanced SG neuronal excitability.

CONCLUSION

Macrophage activation in the SG contributes to arrhythmogenesis in bleomycin-induced ALI in male rats.

Severe Bradycardia Increases the Incidence and Severity of Torsade de Pointes Arrhythmias by Augmenting Preexistent Spatial Dispersion of Repolarization in the CAVB Dog Model

INTRODUCTION

Torsade de pointes arrhythmias (TdP) in the chronic atrioventricular block (CAVB) dog model result from proarrhythmic factors, which trigger TdP and/or reinforce the arrhythmic substrate. This study investigated electrophysiological and arrhythmogenic consequences of severe bradycardia for TdP.

METHODS

Dofetilide (25 μg/kg per 5 min) was administered to eight anesthetized, idioventricular rhythm (IVR) remodeled CAVB dogs in two serial experiments: once under 60 beats per minute (bpm), right ventricular apex paced (RVA60) conditions, once under more bradycardic IVR conditions. Recordings included surface electrocardiogram and short-term variability (STV) of repolarization from endocardial unipolar electrograms. TdP inducibility (three or more episodes within 10 min after start of dofetilide) and arrhythmic activity scores (AS) were established. Mapping experiments in 10 additional dogs determined the effect of lowering rate on STV and spatial dispersion of repolarization (SDR) in baseline.

RESULTS

IVR-tested animals had longer baseline RR-interval (1,403 ± 271 ms) and repolarization intervals than RVA60 animals. Dofetilide increased STV similarly under both rhythm strategies. Nevertheless, TdP inducibility and AS were higher under IVR conditions (6/8 and 37 ± 27 vs. 1/8 and 8 ± 12 in RVA60, respectively, both p < 0.05). Mapping: Pacing from high (128 ± 10 bpm) to middle (88 ± 10 bpm) to experimental rate (61 ± 3 bpm) increased all electrophysiological parameters, including interventricular dispersion, due to steeper left ventricular restitution curves, and intraventricular SDR: maximal cubic dispersion from 60 ± 14 (high) to 69 ± 17 (middle) to 84 ± 22 ms (p < 0.05 vs. high and middle rate).

CONCLUSION

In CAVB dogs, severe bradycardia increases the probability and severity of arrhythmic events by heterogeneously causing electrophysiological instability, which is mainly reflected in an increased spatial, and to a lesser extent temporal, dispersion of repolarization.

Chronic Thromboembolic Pulmonary Hypertension - What Have We Learned From Large Animal Models

Chronic thrombo-embolic pulmonary hypertension (CTEPH) develops in a subset of patients after acute pulmonary embolism. In CTEPH, pulmonary vascular resistance, which is initially elevated due to the obstructions in the larger pulmonary arteries, is further increased by pulmonary microvascular remodeling. The increased afterload of the right ventricle (RV) leads to RV dilation and hypertrophy. This RV remodeling predisposes to arrhythmogenesis and RV failure. Yet, mechanisms involved in pulmonary microvascular remodeling, processes underlying the RV structural and functional adaptability in CTEPH as well as determinants of the susceptibility to arrhythmias such as atrial fibrillation in the context of CTEPH remain incompletely understood. Several large animal models with critical clinical features of human CTEPH and subsequent RV remodeling have relatively recently been developed in swine, sheep, and dogs. In this review we will discuss the current knowledge on the processes underlying development and progression of CTEPH, and on how animal models can help enlarge understanding of these processes.

Cardiac Adiposity and Arrhythmias: The Role of Imaging

Increased cardiac fat depots are metabolically active tissues that have a pronounced pro-inflammatory nature. Increasing evidence supports a potential role of cardiac adiposity as a determinant of the substrate of atrial fibrillation and ventricular arrhythmias. The underlying mechanism appears to be multifactorial with local inflammation, fibrosis, adipocyte infiltration, electrical remodeling, autonomic nervous system modulation, oxidative stress and gene expression playing interrelating roles. Current imaging modalities, such as echocardiography, computed tomography and cardiac magnetic resonance, have provided valuable insight into the relationship between cardiac adiposity and arrhythmogenesis, in order to better understand the pathophysiology and improve risk prediction of the patients, over the presence of obesity and traditional risk factors. However, at present, given the insufficient data for the additive value of imaging biomarkers on commonly used risk algorithms, the use of different screening modalities currently is indicated for personalized risk stratification and prognostication in this setting.

The effect of Sirt1 deficiency on Ca2+ and Na+ regulation in mouse ventricular myocytes

This study addressed the hypothesis that cardiac Sirtuin 1 (Sirt1) deficiency alters cardiomyocyte Ca²⁺ and Na⁺ regulation, leading to cardiac dysfunction and arrhythmogenesis. We used mice with cardiac‐specific Sirt1 knockout (Sirt1^−/−). Sirt1^flox/flox mice were served as control. Sirt1^−/− mice showed impaired cardiac ejection fraction with increased ventricular spontaneous activity and burst firing compared with those in control mice. The arrhythmic events were suppressed by KN93 and ranolazine. Reduction in Ca²⁺ transient amplitudes and sarcoplasmic reticulum (SR) Ca²⁺ stores, and increased SR Ca²⁺ leak were shown in the Sirt1^−/− mice. Electrophysiological measurements were performed using patch‐clamp method. While L‐type Ca²⁺ current (I_{Ca, L}) was smaller in Sirt1^−/− myocytes, reverse‐mode Na⁺/Ca²⁺ exchanger (NCX) current was larger compared with those in control myocytes. Late Na⁺ current (I_{Na, L}) was enhanced in the Sirt1^−/− mice, alongside with elevated cytosolic Na⁺ level. Increased cytosolic and mitochondrial reactive oxygen species (ROS) were shown in Sirt1^−/− mice. Sirt1^−/− cardiomyocytes showed down‐regulation of L‐type Ca²⁺ channel α1c subunit (Cav1.2) and sarcoplasmic/endoplasmic reticulum Ca²⁺ ATPase 2a (SERCA2a), but up‐regulation of Ca²⁺/calmodulin‐dependent protein kinase II and NCX. In conclusions, these findings suggest that deficiency of Sirt1 impairs the regulation of intracellular Ca²⁺ and Na⁺ in cardiomyocytes, thereby provoking cardiac dysfunction and arrhythmogenesis.

Autonomic regulation of the heart and arrhythmogenesis in trained breath-hold divers

AbstractBreath-hold divers are known to develop cardiac autonomic changes and brady-arrthymias during prolonged breath-holding (BH). The effects of BH-induced hypoxemia were investigated upon both cardiac autonomic status and arrhythmogenesis by comparing breath-hold divers (BHDs) to non-divers (NDs). Eighteen participants (9 BHDs, 9 NDs) performed a maximal voluntary BH with face immersion. BHDs were asked to perform an additional BH at water surface to increase the degree of hypoxemia. Beat-to-beat changes in heart rate (HR), short-term fractal scaling exponent (DFAα1), the number of arrhythmic events [premature ventricular contractions (PVCs), premature atrial contractions (PACs)] and peripheral oxygen saturation (SpO₂) were recorded during and immediately following BH. The corrected QT-intervals (QTc) were analyzed pre- and post-acute BH. A regression-based model was used to split BH into a normoxic (NX) and a hypoxemic phase (HX). During the HX phase of BH, BHDs showed a progressive decrease in DFAα1 during BH with face immersion (p < 0.01) and BH with whole-body immersion (p < 0.01) whereas NDs did not (p > 0.05). In addition, BHDs had more arrhythmic events during the HX of BH with whole-body immersion when compared to the corresponding NX phase (5.9 ± 6.7 vs 0.4 ± 1.3; p < 0.05; respectively). The number of PVCs was negatively correlated with SpO₂ during BH with whole-body immersion (r = -0.72; p < 0.05). The hypoxemic stage of voluntary BH is concomitant with significant cardiac autonomic changes toward a synergistic sympathetic and parasympathetic stimulation. Co-activation led ultimately to increased bradycardic response and cardiac electrophysiological disturbances.

Time Course of Low-Frequency Oscillatory Behavior in Human Ventricular Repolarization Following Enhanced Sympathetic Activity and Relation to Arrhythmogenesis

Background and Objectives: Recent studies in humans and dogs have shown that ventricular repolarization exhibits a low-frequency (LF) oscillatory pattern following enhanced sympathetic activity, which has been related to arrhythmic risk. The appearance of LF oscillations in ventricular repolarization is, however, not immediate, but it may take up to some minutes. This study seeks to characterize the time course of the action potential (AP) duration (APD) oscillatory behavior in response to sympathetic provocations, unveil its underlying mechanisms and establish a potential link to arrhythmogenesis under disease conditions.

Materials and Methods: A representative set of human ventricular computational models coupling cellular electrophysiology, calcium dynamics, β-adrenergic signaling, and mechanics was built. Sympathetic provocation was modeled via phasic changes in β-adrenergic stimulation (β-AS) and mechanical stretch at Mayer wave frequencies within the 0.03–0.15 Hz band.

Results: Our results show that there are large inter-individual differences in the time lapse for the development of LF oscillations in APD following sympathetic provocation, with some cells requiring just a few seconds and other cells needing more than 3 min. Whereas, the oscillatory response to phasic mechanical stretch is almost immediate, the response to β-AS is much more prolonged, in line with experimentally reported evidences, thus being this component the one driving the slow development of APD oscillations following enhanced sympathetic activity. If β-adrenoceptors are priorly stimulated, the time for APD oscillations to become apparent is remarkably reduced, with the oscillation time lapse being an exponential function of the pre-stimulation level. The major mechanism underlying the delay in APD oscillations appearance is related to the slow I_Ks phosphorylation kinetics, with its relevance being modulated by the I_Ks conductance of each individual cell. Cells presenting short oscillation time lapses are commonly associated with large APD oscillation magnitudes, which facilitate the occurrence of pro-arrhythmic events under disease conditions involving calcium overload and reduced repolarization reserve.

Conclusions: The time course of LF oscillatory behavior of APD in response to increased sympathetic activity presents high inter-individual variability, which is associated with different expression and PKA phosphorylation kinetics of the I_Ks current. Short time lapses in the development of APD oscillations are associated with large oscillatory magnitudes and pro-arrhythmic risk under disease conditions.

Functional Atrial Endocardial-Epicardial Dissociation in Patients With Structural Heart Disease Undergoing Cardiac Surgery

OBJECTIVES

The goal of this study was to describe functional endocardial-epicardial dissociation (FEED), signal complexities, and three-dimensional activation dynamics of the human atrium with structural heart disease (SHD).

BACKGROUND

SHD commonly predisposes to arrhythmias. Although progressive remodeling is implicated, direct demonstration of FEED in the human atrium has not been reported previously.

METHODS

Simultaneous intraoperative mapping of the endocardial and epicardial lateral right atrial wall was performed by using 2 high-density grid catheters during sinus rhythm, pacing drive (600 ms and 400 ms cycle length), and premature extrastimulation (PES). Unipolar electrograms (EGMs) were exported into custom-made software for activation and phase mapping. Difference of ≥20 ms between paired endocardial and epicardial electrodes defined dissociation. EGMs with ≥3 deflections were classified as fractionated.

RESULTS

Sixteen patients (mean age 60.5 ± 4.1 years; 18.7% with a history of atrial fibrillation) with SHD (43% ischemia, 57% valvular disease) were included. A total of 9,218 EGMs were analyzed. Compared with sinus rhythm, phase and activation analyses showed significant FEED during pacing at 600 ms and 400 ms (phase mapping 22.4% vs. 10% [p < 0.0001] and 25.8% vs. 10% [p < 0.0001], respectively; activation mapping 25.4% vs. 7.8% [p < 0.0001] and 27.7% vs. 7.8% [p < 0.0001]) and PES (phase mapping 34% vs. 10% [p < 0.0001]; activation mapping 29.5% vs. 7.8% [p < 0.0001]). Fractionated EGMs occurred significantly more during PES compared with sinus rhythm (50.2% vs. 39.5%; p < 0.0001). Activation patterns differed significantly during pacing drive and PES, with preferential epicardial exit during the latter (15.9% vs. 13.8%; p = 0.046).

CONCLUSIONS

Simultaneous endocardial-epicardial mapping revealed significant FEED with signal fractionation and preferential epicardial breakthroughs with PES. Such complex three-dimensional interaction in electrical activation provides mechanistic insights into atrial arrhythmogenesis with SHD.

Central Sleep Apnoea and Arrhythmogenesis After Myocardial Infarction-The CESAAR Study

The prevalence of sleep disordered breathing (SDB) after acute myocardial infarction (AMI) is high. However, little is known about predominant SDB type and the impact of SDB severity on arrhythmogenesis. We conducted a prospective single-center observational study and performed an unattended sleep study and Holter monitoring within 10 days after AMI, and an unattended sleep study 11.3 months after AMI. All patients were included from the Department of Cardiology at the University Hospital Schleswig-Holstein, Lübeck, Germany. A total of 202 subjects with AMI (73.8% with ST-elevation; 59.8 years; 73.8% male) were included. The mean BMI was 27.8 kg/m² and the mean neck/waist circumference was 41.7/103.3 cm. The mean left ventricular ejection fraction was 56.6%. The SDB prevalence defined as apnoea-hypopnea-index (AHI) ≥ 5/h was 66.7% with 44.9% having central (CSA), and 21.8% obstructive sleep apnoea (OSA). The mean AHI was 13.8 1/h. In 10.2% nsVT was detected in the Holter monitoring. AI >23/h was independently associated with higher risk of nsVT in the subacute AMI period. SDB is highly prevalent and CSA a predominant type of SDB in the subacute phase after uncomplicated AMI treated with modern revascularization procedures and evidence-based pharmacological therapy. Severe SDB is independently associated with higher risk for nsVT in the subacute AMI period and its course should be monitored as it can potentially have a negative impact on relevant outcomes of AMI patients. Further prospective studies are needed to assess long-term follow up of SDB after AMI and its impact on mortality and morbidity.

Mechanisms Underlying Interactions Between Low-Frequency Oscillations and Beat-to-Beat Variability of Celullar Ventricular Repolarization in Response to Sympathetic Stimulation: Implications for Arrhythmogenesis

Background and Objectives: Enhanced beat-to-beat variability of ventricular repolarization (BVR) has been linked to arrhythmias and sudden cardiac death. Recent experimental studies on human left ventricular epicardial electrograms have shown that BVR closely interacts with low-frequency (LF) oscillations of activation recovery interval during sympathetic provocation. In this work human ventricular computational cell models are developed to reproduce the experimentally observed interactions between BVR and its LF oscillations, to assess underlying mechanisms and to establish a relationship with arrhythmic risk.

Materials and Methods: A set of human ventricular action potential (AP) models covering a range of experimental electrophysiological characteristics was constructed. These models incorporated stochasticity in major ionic currents as well as descriptions of β-adrenergic stimulation and mechanical effects to investigate the AP response to enhanced sympathetic activity. Statistical methods based on Automatic Relevance Determination and Canonical Correlation Analysis were developed to unravel individual and common factors contributing to BVR and LF patterning of APD in response to sympathetic provocation.

Results: Simulated results reproduced experimental evidences on the interactions between BVR and LF oscillations of AP duration (APD), with replication of the high inter-individual variability observed in both phenomena. I_CaL, I_Kr and I_K1 currents were identified as common ionic modulators of the inter-individual differences in BVR and LF oscillatory behavior and were shown to be crucial in determining susceptibility to arrhythmogenic events.

Conclusions: The calibrated family of human ventricular cell models proposed in this study allows reproducing experimentally reported interactions between BVR and LF oscillations of APD. Ionic factors involving I_CaL, I_Kr and I_K1 currents are found to underlie correlated increments in both phenomena in response to sympathetic provocation. A link to arrhythmogenesis is established for concomitantly elevated levels of BVR and its LF oscillations.

Potential Arrhythmogenic Role of TRPC Channels and Store-Operated Calcium Entry Mechanism in Mouse Ventricular Myocytes

Background and Purpose: Store-operated calcium entry (SOCE) is an important physiological phenomenon that extensively mediates intracellular calcium ion (Ca²⁺) load. It has been previously found in myocytes isolated from neonatal or diseased hearts. We aimed to determine its existence, molecular nature in undiseased hearts and its potential arrhythmogenic implications under hyperactive conditions.

Experimental Approach: Ventricular myocytes isolated from adult FVB mice were studied by using Ca²⁺ imaging and whole-cell perforated patch-clamp recording. In addition, lead II ECGs were recorded in isolated Langendorff-perfused mice hearts. Functional TRPC channel antibodies and inhibitors, and TRPC6 activator hyperforin were used.

Key Results: In this study, we demonstrate the existence and contribution of SOCE in normal adult mouse cardiac myocytes. For an apparent SOCE activation, complete depletion of sarcoplasmic reticulum (SR) Ca²⁺ by employing both caffeine (10 mM) and thapsigargin (1 μM) or cyclopiazonic acid (10 μM) was required. Consistent with the notion that SOCE may be mediated by heteromultimeric TRPC channels, SOCEs observed from those myocytes were significantly reduced by the pretreatment with anti-TRPC1, 3, and 6 antibodies as well as by gadolinium, a non-selective TRPC channel blocker. In addition, we showed that SOCE may regulate spontaneous SR Ca²⁺ release, Ca²⁺ waves, and triggered activities which may manifest cardiac arrhythmias. Since the spontaneous depolarization in membrane potential preceded the elevation of intracellular Ca²⁺, an inward membrane current presumably via TRPC channels was considered as the predominant cause of cellular arrhythmias. The selective TRPC6 activator hyperforin (0.1–10 μM) significantly facilitated the SOCE, SOCE-mediated inward current, and calcium load in the ventricular myocytes. ECG recording further demonstrated the proarrhythmic effects of hyperforin in ex vivo mouse hearts.

Conclusion and Implications: We suggest that SOCE, which is at least partially mediated by TRPC channels, exists in adult mouse ventricular myocytes. TRPC channels and SOCE mechanism may be involved in cardiac arrhythmogenesis via promotion of spontaneous Ca²⁺ waves and triggered activities under hyperactivated conditions.

Mitochondrial Dysfunction-Associated Arrhythmogenic Substrates in Diabetes Mellitus

There is increasing evidence that diabetic cardiomyopathy increases the risk of cardiac arrhythmia and sudden cardiac death. While the detailed mechanisms remain incompletely understood, the loss of mitochondrial function, which is often observed in the heart of patients with diabetes, has emerged as a key contributor to the arrhythmogenic substrates. In this mini review, the pathophysiology of mitochondrial dysfunction in diabetes mellitus is explored in detail, followed by descriptions of several mechanisms potentially linking mitochondria to arrhythmogenesis in the context of diabetic cardiomyopathy.

Epac Proteins and Calmodulin as Possible Arrhythmogenesis Trigger in Alcoholic Cardiomyopathy

The expression of Epac proteins (exchange protein directly activated by cAMP) and calmodulin (CaM) was assessed by the content of the corresponding mRNA in biopsy specimens of cardiac atrium, left ventricle, and thoracic aorta of rats with alcoholic cardiomyopathy. In the myocardium, overexpression of Еpac1, Ерас2, and СаМ mRNA was found. The content of Epac2 mRNA in the left ventricle was elevated by 2.9 times (p=0.000001), in the left atrium by 3.2 times (p=0.00001), in the right atrium by 3 times (p=0.00001). In contrast to the myocardial tissue, the content of CaM mRNA in the thoracic aorta was not increased, but showed a tendency to decrease, when compared to the control values, while the level of Epac1 and Epac2 mRNA was increased. The assumption is made that regulatory proteins Epac and CaM can play a key role in arrhythmogenesis development under conditions of alcoholic cardiomyopathy.

Atrial GIRK Channels Mediate the Effects of Vagus Nerve Stimulation on Heart Rate Dynamics and Arrhythmogenesis

Diminished parasympathetic influence is central to the pathogenesis of cardiovascular diseases, including heart failure and hypertension. Stimulation of the vagus nerve has shown promise in treating cardiovascular disease, prompting renewed interest in understanding the signaling pathway(s) that mediate the vagal influence on cardiac physiology. Here, we evaluated the contribution of G protein-gated inwardly rectifying K+ (GIRK/Kir3) channels to the effect of vagus nerve stimulation (VNS) on heart rate (HR), HR variability (HRV), and arrhythmogenesis in anesthetized mice. As parasympathetic fibers innervate both atria and ventricle, and GIRK channels contribute to the cholinergic impact on atrial and ventricular myocytes, we collected in vivo electrocardiogram recordings from mice lacking either atrial or ventricular GIRK channels, during VNS. VNS decreased HR and increased HRV in control mice, in a muscarinic receptor-dependent manner. This effect was preserved in mice lacking ventricular GIRK channels, but was nearly completely absent in mice lacking GIRK channels in the atria. In addition, atrial-specific ablation of GIRK channels conferred resistance to arrhythmic episodes induced by VNS. These data indicate that atrial GIRK channels are the primary mediators of the impact of VNS on HR, HRV, and arrhythmogenesis in the anesthetized mouse.

Risk Stratification in Arrhythmic Right Ventricular Cardiomyopathy Without Implantable Cardioverter-Defibrillators

OBJECTIVES

The primary objective of this study is risk stratification of patients with arrhythmic right ventricular cardiomyopathy (ARVC).

BACKGROUND

There is a need to identify those who need an automatic implantable defibrillator (ICD) to prevent sudden death.

METHODS

This is an analysis of 88 patients with ARVC from three centers who were not treated with an ICD.

RESULTS

Risk factors for subsequent arrhythmic deaths were pre-enrollment sustained or nonsustained ventricular tachycardia (VT) and decreased left ventricular function.

CONCLUSION

These factors serve as proposed guidelines for implantation of an ICD in patients with ARVC to prevent sudden death.

Local Gradients in Electrotonic Loading Modulate the Local Effective Refractory Period: Implications for Arrhythmogenesis in the Infarct Border Zone

Ectopic electrical activity that originates in the peri-infarct region can give rise to potentially lethal re-entrant arrhythmias. The spatial variation in electrotonic loading that results from structural remodelling in the infarct border zone may increase the probability that focal activity will trigger electrical capture, but this has not previously been investigated systematically. This study uses in-silico experiments to examine the structural modulation of effective refractory period on ectopic beat capture. Informed by 3-D reconstructions of myocyte organization in the infarct border zone, a region of rapid tissue expansion is abstracted to an idealized representation. A novel metric is introduced that defines the local electrotonic loading as a function of passive tissue properties and boundary conditions. The effective refractory period correlates closely with local electrotonic loading, while the action potential duration, conduction, and upstroke velocity reduce in regions of increasing electrotonic load. In the presence of focal ectopic stimuli, spatial variation in effective refractory period can cause unidirectional conduction block providing a substrate for reentrant arrhythmias. Consequently, based on the observed results, a possible novel mechanism for arrhythmogenesis in the infarct border zone is proposed.

SR calcium handling dysfunction, stress-response signaling pathways, and atrial fibrillation

Atrial fibrillation (AF) is the most common sustained arrhythmia. It is associated with a markedly increased risk of premature death due to embolic stroke and also complicates co-existing cardiovascular diseases such as heart failure. The prevalence of AF increases dramatically with age, and aging has been shown to be an independent risk of AF. Due to an aging population in the world, a growing body of AF patients are suffering a diminished quality of life and causing an associated economic burden. However, effective pharmacologic treatments and prevention strategies are lacking due to a poor understanding of the molecular and electrophysiologic mechanisms of AF in the failing and/or aged heart. Recent studies suggest that altered atrial calcium handling contributes to the onset and maintenance of AF. Here we review the role of stress-response kinases and calcium handling dysfunction in AF genesis in the aged and failing heart.

Cytosolic calcium changes affect the incidence of early afterdepolarizations in canine ventricular myocytes

This study was designed to investigate the influence of cytosolic Ca(2+) levels ([Ca(2+)]i) on action potential duration (APD) and on the incidence of early afterdepolarizations (EADs) in canine ventricular cardiomyocytes. Action potentials (AP) of isolated cells were recorded using conventional sharp microelectrodes, and the concomitant [Ca(2+)]i was monitored with the fluorescent dye Fura-2. EADs were evoked at a 0.2 Hz pacing rate by inhibiting the rapid delayed rectifier K(+) current with dofetilide, by activating the late sodium current with veratridine, or by activating the L-type calcium current with BAY K8644. These interventions progressively prolonged the AP and resulted in initiation of EADs. Reducing [Ca(2+)]i by application of the cell-permeant Ca(2+) chelator BAPTA-AM lengthened the AP at 1.0 Hz if it was applied alone, in the presence of veratridine, or in the presence of BAY K8644. However, BAPTA-AM shortened the AP if the cells were pretreated with dofetilide. The incidence of the evoked EADs was strongly reduced by BAPTA-AM in dofetilide, moderately reduced in veratridine, whereas EAD incidence was increased by BAPTA-AM in the presence of BAY K8644. Based on these experimental data, changes in [Ca(2+)]i have marked effects on APD as well as on the incidence of EADs; however, the underlying mechanisms may be different, depending on the mechanism of EAD generation. As a consequence, reduction of [Ca(2+)]i may eliminate EADs under some, but not all, experimental conditions.

Thyroid hormone replacement therapy attenuates atrial remodeling and reduces atrial fibrillation inducibility in a rat myocardial infarction-heart failure model

BACKGROUND

Heart failure (HF) is associated with increased atrial fibrillation (AF) risk. Accumulating evidence suggests the presence of myocardial tissue hypothyroidism in HF, which may contribute to HF development. In a recent report we demonstrated that hypothyroidism, like hyperthyroidism, leads to increased AF inducibility. The present study was designed to investigate the effect of thyroid hormone (TH) replacement therapy on AF arrhythmogenesis in HF.

METHODS AND RESULTS

Myocardial infarction (MI) was produced in rats by means of coronary artery ligation. Rats with large MIs (>40%) were randomized into L-thyroxine (T4; n = 14) and placebo (n = 15) groups 2 weeks after MI. Rats received 3.3 mg T4 (in 60-day release form) or placebo pellets for 2 months. Compared with the placebo, T4 treatment improved cardiac function and decreased left ventricular internal diameters as well as left atrial diameter. T4 treatment attenuated atrial effective refractory period prolongation (45 ± 1.5 ms in placebo group vs 37 ± 1.6 ms in T4 group; P < .01) and reduced AF inducibility (AF/atrial flutter/tachycardia were inducible in 11/15 rats [73%] in the placebo- vs 4/14 rats [29%] in the T4-treated group; P < .05). Arrhythmia reduction was associated with decreased atrial fibrosis but was not associated with connexin 43 changes.

CONCLUSIONS

To our knowledge this is the first study demonstrating that TH replacement therapy in HF attenuates atrial remodeling and reduces AF inducibility after MI-HF. Clinical studies are needed to confirm such benefits in human patients.

Assessment of ventricular repolarization inhomogeneity in patients with mitral valve prolapse: value of T wave peak to end interval

Mitral valve prolapse (MVP) has been long known for causing susceptibility for ventricular arrhythmogenesis, and this risk was evaluated by various methods, mostly by using QT interval related measurements on surface electrocardiogram. T wave peak to end (Tp-e) interval is a relatively new marker for ventricular arrhythmogenesis and repolarization heterogeneity. Prolongation of this interval represents a period of potential vulnerability to re-entrant ventricular arrhythmias. However, there is no information available assessing the Tp-e interval and related calculations in patients with MVP. The aim of this study was to assess ventricular repolarization in patients with MVP by using QT, corrected QT (QTc) and Tp-e interval, Tp-e/QT ratio, and Tp-e/QTc ratio. Electrocardiogram of consecutive 72 patients, who were followed by outpatient clinic because of mitral valve prolapse, were obtained and scanned. Electrocardiograms of age and sex matched 60 healthy control individuals were also gained for comparison. QT, QTc, Tp-e/QT and Tp-e/QTc were calculated. Baseline characteristics were similar in both groups. QT (405.1±64.3 vs. 362.1±39.1; p<0.001), QTc (457.6±44.4 vs. 428.3±44.7; p<0.001), Tp-e (100.2±22.1 vs. 74.6±10.2; p<0.001) and Tp-e/QT (0.24 vs. 0.20; p<0.001) and Tp-e/QTc (0.21 vs. 0.17; p<0.001) were significantly worse in MVP group. Our study revealed that Tp-e interval and Tp-e/QT ratio were increased in MVP patients. Tp-e interval and Tp-e/QT ratio might be a useful marker of cardiovascular morbidity and mortality due to ventricular arrhythmias in patients with MVP.

Ageing-related cardiomyocyte functional decline is sex and angiotensin II dependent

Clinically, heart failure is an age-dependent pathological phenomenon and displays sex-specific characteristics. The renin-angiotensin system mediates cardiac pathology in heart failure. This study investigated the sexually dimorphic functional effects of ageing combined with angiotensin II (AngII) on cardiac muscle cell function, twitch and Ca(2+)-handling characteristics of isolated cardiomyocytes from young (~13 weeks) and aged (~87 weeks) adult wild type (WT) and AngII-transgenic (TG) mice. We hypothesised that AngII-induced contractile impairment would be exacerbated in aged female cardiomyocytes and linked to Ca(2+)-handling disturbances. AngII-induced cardiomyocyte hypertrophy was evident in young adult mice of both sexes and accentuated by age (aged adult ~21-23 % increases in cell length relative to WT). In female AngII-TG mice, ageing was associated with suppressed cardiomyocyte contractility (% shortening, maximum rate of shortening, maximum rate of relaxation). This was associated with delayed cytosolic Ca(2+) removal during twitch relaxation (Tau ~20 % increase relative to young adult female WT), and myofilament responsiveness to Ca(2+) was maintained. In contrast, aged AngII-TG male cardiomyocytes exhibited peak shortening equivalent to young TG; yet, myofilament Ca(2+) responsiveness was profoundly reduced with ageing. Increased pro-arrhythmogenic spontaneous activity was evident with age and cardiac AngII overexpression in male mice (42-55 % of myocytes) but relatively suppressed in female aged transgenic mice. Female myocytes with elevated AngII appear more susceptible to an age-related contractile deficit, whereas male AngII-TG myocytes preserve contractile function with age but exhibit desensitisation of myofilaments to Ca(2+) and a heightened vulnerability to arrhythmic activity. These findings support the contention that sex-specific therapies are required for the treatment of age-progressive heart failure.

Functional, anatomical, and molecular investigation of the cardiac conduction system and arrhythmogenic atrioventricular ring tissue in the rat heart

Background

The cardiac conduction system consists of the sinus node, nodal extensions, atrioventricular (AV) node, penetrating bundle, bundle branches, and Purkinje fibers. Node‐like AV ring tissue also exists at the AV junctions, and the right and left rings unite at the retroaortic node. The study aims were to (1) construct a 3‐dimensional anatomical model of the AV rings and retroaortic node, (2) map electrical activation in the right ring and study its action potential characteristics, and (3) examine gene expression in the right ring and retroaortic node.

Methods and Results

Three‐dimensional reconstruction (based on magnetic resonance imaging, histology, and immunohistochemistry) showed the extent and organization of the specialized tissues (eg, how the AV rings form the right and left nodal extensions into the AV node). Multiextracellular electrode array and microelectrode mapping of isolated right ring preparations revealed robust spontaneous activity with characteristic diastolic depolarization. Using laser microdissection gene expression measured at the mRNA level (using quantitative PCR) and protein level (using immunohistochemistry and Western blotting) showed that the right ring and retroaortic node, like the sinus node and AV node but, unlike ventricular muscle, had statistically significant higher expression of key transcription factors (including Tbx3, Msx2, and Id2) and ion channels (including HCN4, Ca_v3.1, Ca_v3.2, K_v1.5, SK1, K_ir3.1, and K_ir3.4) and lower expression of other key ion channels (Na_v1.5 and K_ir2.1).

Conclusions

The AV rings and retroaortic node possess gene expression profiles similar to that of the AV node. Ion channel expression and electrophysiological recordings show the AV rings could act as ectopic pacemakers and a source of atrial tachycardia.

Independent contribution of catecholamines to arrhythmogenesis during evolving infarction in the isolated rat heart

Ventricular fibrillation (VF) in conscious rats with coronary artery ligation occurs in two phases, before (phase 1) and after (phase 2) 90 min of ischaemia respectively. The mechanisms of phase 2 VF are not established. Interestingly, phase 2 VF is absent in isolated (denervated) buffer-perfused rat hearts. We investigated whether catecholamine supplementation (to mimic sympathetic drive) was sufficient to restore phase 2 VF in such hearts. Isolated rat hearts (n=10 per group) underwent coronary ligation for 240 min. At 90 min, during a period of relative electrical stability, the perfusion solution was switched from standard (Krebs) to identical solution or Krebs containing catecholamines (313 nM noradrenaline and 75 nM adrenaline) with or without 10 microM trimazosin (an alpha(1)-adrenoceptor antagonist) or 10 microM atenolol (a beta(1)-adrenoceptor antagonist). Although in all groups the incidence of phase 1 VF was high (80 - 100%), the temporal distribution of VF was monophasic, i.e. only one heart in one group developed phase 2 VF (P=NS). Other ventricular arrhythmias (e.g., tachycardia; VT) exhibited a similar temporal distribution. Nevertheless, haemodynamic changes confirmed sympathomimetic effects of catecholamines, e.g., heart rate was increased from 278+/-7 beats min(-1) in controls to 335+/-8 beats min(-1) (P<0.05) by catecholamines, an effect that could be blocked by atenolol (285+/-7 beats min(-1)) but not by trimazosin (342+/-12 beats min(-1)). Coronary flow was correspondingly increased from 7.7+/-0.7 ml min(-1) g(-1) to 16.5+/-1.3 ml min(-1) g(-1) (P<0.05); this effect could be blocked by atenolol (8.1+/-0.6 ml min(-1) g(-1)) and was enhanced by trimazosin (20.7+/-2.4 ml min(-1) g(-1)). In conclusion, despite evidence of adequate alpha- and beta-adrenoceptor activation, catecholamine supplementation to isolated buffer-perfused rat hearts was insufficient to restore phase 2 VF. It therefore appears unlikely that catecholamines alone mediate phase 2 VF.