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Ca2+ waves are known to trigger delayed after‐depolarizations that can cause malignant cardiac arrhythmias. However, modelling Ca2+ waves using physiologically realistic models has remained a major challenge. Existing models with low Ca2+ sensitivity of ryanodine receptors (RyRs) necessitate large release currents, leading to an unrealistically large Ca2+ transient amplitude incompatible with the experimental observations. Consequently, current physiologically detailed models of delayed after‐depolarizations resort to unrealistic cell architectures to produce Ca2+ waves with a normal Ca2+ transient amplitude. Here, we address these challenges by incorporating RyR cooperativity into a physiologically detailed model with a realistic cell architecture. We represent RyR cooperativity phenomenologically through a Hill coefficient within the sigmoid function of RyR open probability. Simulations in permeabilized myocytes with high Ca2+ sensitivity reveal that a sufficiently large Hill coefficient is required for Ca2+ wave propagation via the fire–diffuse–fire mechanism. In intact myocytes, propagating Ca2+ waves can occur only within an intermediate Hill coefficient range. Within this range, the spark rate is neither too low, enabling Ca2+ wave propagation, nor too high, allowing for the maintenance of a high sarcoplasmic reticulum load during diastole of the action potential. Moreover, this model successfully replicates other experimentally observed manifestations of Ca2+‐wave‐mediated triggered activity, including phase 2 and phase 3 early after‐depolarizations and high‐frequency voltage–Ca2+ oscillations. These oscillations feature an elevated take‐off potential with depolarization mediated by the L‐type Ca2+ current. The model also sheds light on the roles of luminal gating of RyRs and the mobile buffer ATP in the genesis of these arrhythmogenic phenomena. Key points: Existing mathematical models of Ca2+ waves use an excessively large Ca2+‐release current or unrealistic diffusive coupling between release units.Our physiologically realistic model, using a Hill coefficient in the ryanodine receptor (RyR) gating function to represent RyR cooperativity, addresses these limitations and generates organized Ca2+ waves at Hill coefficients ranging from ∼5 to 10, as opposed to the traditional value of 2.This range of Hill coefficients gives a spark rate neither too low, thereby enabling Ca2+ wave propagation, nor too high, allowing for the maintenance of a high sarcoplasmic reticulum load during the plateau phase of the action potential.Additionally, the model generates Ca2+‐wave‐mediated phase 2 and phase 3 early after‐depolarizations, and coupled membrane voltage with Ca2+ oscillations mediated by the L‐type Ca2+ current.This study suggests that pharmacologically targeting RyR cooperativity could be a promising strategy for treating cardiac arrhythmias linked to Ca2+‐wave‐mediated triggered activity. [ABSTRACT FROM AUTHOR]

Autonomic nerve activity is important in the mechanisms of paroxysmal atrial fibrillation (PAF). The purpose of this study was to test the hypothesis that a single burst of skin sympathetic nerve activity (SKNA) can toggle on and off PAF or premature atrial contraction (PAC) clusters. Simultaneous recording of SKNA and electrocardiogram (neuECG) recording was performed over 7 days in patients with PAF. In study 1, 8 patients (7 men and 1 woman; age 62 ± 8 years) had 124 episodes of PAF. An SKNA burst toggled both on and off PAF in 8 episodes (6.5%) (type 1), toggled on but not off in 12 episodes (9.7%) (type 2), and toggled on a PAC cluster followed by PAF in 4 episodes (3.2%) (type 3). The duration of these PAF episodes was <10 minutes. The remaining 100 episodes (80.6%) were associated with active SKNA bursts throughout PAF (type 4) and lasted longer than type 1 (P =.0185) and type 2 (P =.0027) PAF. There were 47 PAC clusters. Among them, 24 (51.1%) were toggled on and off, and 23 (48.9%) were toggled on but not off by an SKNA burst. In study 2, 17 patients (9 men and 8 women; age 58 ± 12 years) had <10 minutes of PAF (4, 8, 0, and 31 of types 1, 2, 3, and 4, respectively). There were significant circadian variations of all types of PAF. A single SKNA burst can toggle short-duration PAF and PAC cluster episodes on and off. The absence of continued SKNA after the onset might have affected the maintenance of these arrhythmias. [ABSTRACT FROM AUTHOR]

The article describes the case of a 54-year old male athlete who was diagnosed with adenosine-sensitive ventricular tachycardia. Results of the patient's intracardiac electrograms revealed A-V dissociation on the coronary sinus electrograms during trachycardia. A pace-mapping was performed on the patient with an ablation catheter and radiofrequency energy was applied to destroy the site of the trachycardia origin. The differential diagnosis of trarchycardia is also discussed.

Background Sustained forms of tachycardia especially from the left ventricular summit are rare. Adenosine sensitive outflow tachycardias, especially from the left ventricular summit, are rarer still. These arrhythmias may be exercise or stress induced as they are facilitated by catecholamines and characteristically terminate with adenosine, vagal manoeuvres, and beta-blockers. The surface 12-lead electrocardiogram can be used to localize the anatomic site of origin before catheter ablation; however, prediction of the precise origin may still be challenging due to the intimate and complex anatomy of the outflow tracts. Case summary A 12-year-old female presented to an emergency room with frequent runs of wide complex tachycardia that terminated with adenosine but would spontaneously reinitiate. After three additional temporary terminations with adenosine and because of an inability to completely eliminate tachycardia, she was started on an esmolol infusion that resulted in an abrupt termination of tachycardia. At follow-up, she reported breakthrough episodes of tachycardia with exercise, especially associated with beta-blocker non-compliance. The rest of her cardiac testing was normal apart from an anomalous right coronary artery origin from the left coronary sinus. Given the increased frequency of symptomatic palpitations and medication non-compliance, she underwent an electrophysiology study. During the study, a ventricular tachycardia was successfully mapped to an epicardial focus at the left ventricle summit and was successfully ablated. Discussion The response of this patient's ventricular tachycardia to adenosine suggests a triggered mechanism. To our knowledge, this is the first unambiguous example of left ventricular tachycardia due to cAMP-mediated triggered activity in this age group. [ABSTRACT FROM AUTHOR]

Termination of focal atrial tachycardia with adenosine is considered a defining feature for triggered activity. Recent evidence, however, suggests that the perinodal adenosine-sensitive AT has reentry as the mechanism of tachycardia. In this report, we were able to confirm the mechanism of AT as reentry by observing the response to programmed electrical stimulation and demonstrating the fallacy of traditional teaching that the adenosine responsiveness of AT is a criterion for labeling the mechanism as triggered activity.

We aimed to investigate the effect of apocynin (APO) on delayed afterdepolarizations (DADs) in rat atrial myocytes and the underlying mechanisms. Rat atrial myocytes were isolated by a Langendorff perfusion apparatus. DADs were induced by isoproterenol (ISO). Action potentials (APs) and ion currents were recorded by the whole-cell clamp technique. The fluorescent indicator fluo-4 was used to visualize intracellular Ca2+ transients, and western blotting was used to measure the expression of related proteins. The incidence of DADs in rat atrial myocytes increased significantly after ISO treatment, leading to an increased incidence of triggered activity (TA). The incidence of ISO-induced DADs and TA were reduced by 100.0 μM APO from 48.89% to 25.56% and 17.78% to 5.56%, respectively. In the range of 3.0 μM–300.0 μM, the effect of APO was concentration dependent, with a half maximal inhibitory concentration (IC 50) of 120.1 μM and a Hill coefficient of 1.063. APO reversed the increase in transient inward current (I ti) and Na+/Ca2+-exchange current (I NCX) densities induced by ISO in atrial myocytes. The frequency of spontaneous Ca2+ transients in atrial myocytes was reduced by 100.0 μM APO. Compared with ISO, APO downregulated the expression of NOX2 and increased the phosphorylation of PLNSer16 and the sarcoplasmic reticulum Ca2+-ATPase-2a (SERCA2a) level; however, it had little effect on ryanodine-receptor channel type-2 (RyR2). These findings showed that APO may block I ti and I NCX and reduce intracellular Ca2+ levels in rat atrial myocytes, thus reducing the incidence of ISO-induced DADs and TA. • APO decreased the incidence of ISO-induced DADs and TA. • APO reduced the ISO-induced increase in the inward I NCX density and the I ti density. • APO reduced the frequency of ISO-induced spontaneous calcium transients. • APO downregulated NOX2 level and increased the phosphorylation of PLNSer16 and SERCA2a level. [ABSTRACT FROM AUTHOR]

Background: The role of sympathetic nerve activity to maintain sinus rate acceleration remains unclear.Objective: The purpose of this study was to test the hypothesis that sustained (>30 seconds) sinus rate acceleration can be associated with either a sympathetic driven or a sympathetic toggled mechanism.Methods: We used a patch monitor to record skin sympathetic nerve activity (SKNA) and electrocardiogram over 24 hours. Study 1 included chronic orthostatic intolerance (OI) (n = 18), atrial fibrillation (n = 7), and asymptomatic normal control (n = 19) groups. Study 2 included 17 participants with chronic OI not treated with ivabradine, pyridostigmine, or β-blockers.Results: While a majority of sinus rate acceleration was driven by persistent SKNA in study 1, some episodes were toggled on and off by SKNA bursts without persistent SKNA elevation. The sympathetic toggled sinus rate acceleration episodes were found in 7 of 18 participants with chronic OI (39%), 2 of 7 participants with atrial fibrillation (29%), and 6 of 19 normal control participants (32%) (P = .847) and were faster and longer in the chronic OI group than in other groups. In study 2, there were a total of 11 episodes of sinus rate acceleration that persisted for >200 seconds. Among these episodes, 6 (35%) were toggled on and off by SKNA bursts.Conclusion: Sustained sinus rate acceleration (may be toggled on or off) is associated with SKNA bursts in participants with chronic OI, participants with atrial fibrillation, and normal controls. Patients with OI had more frequent and longer episodes than did other groups. [ABSTRACT FROM AUTHOR]

Background: Ca 2+ mishandling in cardiac Purkinje cells is a well-known cause of cardiac arrhythmias. The Purkinje cell resident inositol 1,4,5-trisphosphate receptor 1 (ITPR1) is believed to play an important role in Ca 2+ handling, and ITPR1 gain-of-function (GOF) has been implicated in cardiac arrhythmias. However, nearly all known disease-associated ITPR1 variants are loss-of-function and are primarily linked to neurological disorders. Whether ITPR1 GOF has pathological consequences, such as cardiac arrhythmias, is unclear. This study aimed to identify human ITPR1 GOF variants and determine the impact of ITPR1 GOF on Ca 2+ handling and arrhythmia susceptibility., Methods: There are a large number of rare ITPR1 missense variants reported in open data repositories. Based on their locations in the ITPR1 channel structure, we selected and characterized 33 human ITPR1 missense variants from open databases and identified 21 human ITPR1 GOF variants. We generated a mouse model carrying a human ITPR1 GOF variant, ITPR1-W1457G (W1447G in mice)., Results: We showed that the ITPR1-W1447G ± and recently reported ITPR1-D2594K ± GOF mutant mice were susceptible to stress-induced ventricular arrhythmias. Confocal Ca 2+ and voltage imaging in situ in heart slices and Ca 2+ imaging and patch-clamp recordings of isolated Purkinje cells showed that ITPR1-W1447G ± and ITPR1-D2594K ± variants increased the occurrence of stress-induced spontaneous Ca 2+ release, delayed afterdepolarization, and triggered activity in Purkinje cells. To assess the potential role of ITPR1 variants in arrhythmia susceptibility in humans, we looked up a gene-based association study in the UK Biobank data set and identified 7 rare ITPR1 missense variants showing potential association with cardiac arrhythmias. Remarkably, in vitro functional characterization revealed that all these 7 ITPR1 variants resulted in GOF., Conclusions: Our studies in mice and humans reveal that enhanced function of ITPR1, a well-known movement disorder gene, increases the risk for cardiac arrhythmias.

Aims Cardiac arrhythmias are a major factor in the occurrence of morbidity and sudden death in patients with cardiovascular disease. Disturbances of Ca2+ homeostasis in the heart contribute to the initiation and maintenance of cardiac arrhythmias. Extrasystolic increases in intracellular Ca2+ lead to delayed afterdepolarizations and triggered activity, which can result in heart rhythm abnormalities. It is being suggested that the Ca2+-activated nonselective cation channel TRPM4 is involved in the aetiology of triggered activity, but the exact contribution and in vivo significance are still unclear. Methods and results In vitro electrophysiological and calcium imaging technique as well as in vivo intracardiac and telemetric electrocardiogram measurements in physiological and pathophysiological conditions were performed. In two distinct Ca2+-dependent proarrhythmic models, freely moving Trpm4−/− mice displayed a reduced burden of cardiac arrhythmias. Looking further into the specific contribution of TRPM4 to the cellular mechanism of arrhythmias, TRPM4 was found to contribute to a long-lasting Ca2+ overload-induced background current, thereby regulating cell excitability in Ca2+ overload conditions. To expand these results, a compound screening revealed meclofenamate as a potent antagonist of TRPM4. In line with the findings from Trpm4−/− mice, 10 µM meclofenamate inhibited the Ca2+ overload-induced background current in ventricular cardiomyocytes and 15 mg/kg meclofenamate suppressed catecholaminergic polymorphic ventricular tachycardia-associated arrhythmias in a TRPM4-dependent manner. Conclusion The presented data establish that TRPM4 represents a novel target in the prevention and treatment of Ca2+-dependent triggered arrhythmias. [ABSTRACT FROM AUTHOR]

RATIONALE:The major role of a transverse-tubular (TT) network in a cardiac cell is to facilitate effective excitation-contraction coupling and signaling. The TT network structures are heterogeneous within a single cell, and vary between different types of cells and species. They are also remodeled in cardiac diseases. However, how different TT network structures predispose cardiac cells to arrhythmogenesis remains to be revealed. OBJECTIVE:To systematically investigate the roles of TT network structure and the underlying mechanisms in the genesis of intracellular calcium (Ca2+) alternans and triggered activity (TA). METHODS AND RESULTS:Based on recent experimental observations, different TT network structures, including uniformly and non-uniformly random TT distributions, were modeled in a cardiac cell model consisting of a three-dimensional network of Ca2+ release units (CRUs). Our simulations showed that both Ca2+ alternans and Ca2+ wave-mediated TA were promoted when the fraction of orphaned CRUs was in an intermediate range, but suppressed in cells exhibiting either well-organized TT networks or low TT densities. Ca2+ alternans and TA could be promoted by low TT densities when the cells were small or the CRU coupling was strong. Both alternans and TA occurred more easily in uniformly random TT networks than in non-uniformly random TT networks. Subcellular spatially discordant Ca2+ alternans was promoted by non-uniformly random TT networks but suppressed by increasing CRU coupling strength. These mechanistic insights provide a holistic understanding of the effects of TT network structure on the susceptibility to arrhythmogenesis. CONCLUSIONS:The TT network plays important roles in promoting Ca2+ alternans and TA, and different TT network structures may predispose cardiac cells differently to arrhythmogenesis.

Cardiac myocytes normally initiate action potentials in response to a current stimulus that depolarizes the membrane above an excitation threshold. Aberrant excitation can also occur due to spontaneous calcium (Ca2+) release (SCR) from intracellular stores after the end of a preceding action potential. SCR drives the Na+/Ca2+ exchange current inducing a "delayed afterdepolarization" that can in turn trigger an action potential if the excitation threshold is reached. This "triggered activity" is known to cause arrhythmias, but how it is initiated and terminated is not understood. Using computer simulations of a ventricular myocyte model, we show that initiation and termination are inherently random events. We determine the probability of those events from statistical measurements of the number of beats before initiation and before termination, respectively, which follow geometric distributions. Moreover, we elucidate the origin of randomness by a statistical analysis of SCR events, which do not follow a Poisson process observed in other eukaryotic cells. Due to synchronization of Ca2+ releases during the action potential upstroke, waiting times of SCR events after the upstroke are narrowly distributed, whereas SCR amplitudes follow a broad normal distribution with a width determined by fluctuations in the number of independent Ca2+ wave foci. This distribution enables us to compute the probabilities of initiation and termination of bursts of triggered activity that are maintained by a positive feedback between the action potential upstroke and SCR. Our results establish a theoretical framework for interpreting complex and varied manifestations of triggered activity relevant to cardiac arrhythmias.

While recent advances clarified the molecular and cellular modes of action of antiarrhythmic drugs (AADs), their link to suppression of dynamical arrhythmia mechanisms remains only partially understood. The current classifications of AADs (Classes I, III, and IV) rely on blocking peak Na, K and L-type calcium currents (ICa,L), with Class II with dominant beta receptor blocking activity and Class V including drugs with diverse classes of actions. The discovery that the calcium and redox sensor, cardiac Ca/calmodulin-dependent protein kinase II (CaMKII) enhances both the late Na (INa-L) and the late ICa,L in patients at high risk of VT/VF provided a new and a rational AAD target. Pathological rise of either or both of INa-L and late ICa,L are demonstrated to promote cellular early afterdepolarizations (EADs) and EAD-mediated triggered activity that can initiate VT/VF in remodeled hearts. Selective inhibition of the INa-L without affecting their peak transients with the highly specific prototype drug, GS-967 suppresses these EAD-mediated VT/VFs. As in the case of INa-L, selective inhibition of the late ICa,L without affecting its peak with the prototype drug, roscovitine suppressed oxidative EAD-mediated VT/VF. These findings indicate that specific blockers of the late inward currents without affecting their peaks (gating modifiers), offer a new and effective AAD class action i.e., "Class VI." The development of safe drugs with selective Class VI actions provides a rational and effective approach to treat VT/VF particularly in cardiac conditions associated with enhanced CaMKII activity such as heart failure.

Although considerable progress has been made in developing ablation approaches to cure atrial fibrillation (AF), outcomes are still suboptimal, especially for persistent and long-lasting persistent AF. In this topical review, we review the arrhythmia mechanisms, both reentrant and nonreentrant, that are potentially relevant to human AF at various stages/settings. We describe arrhythmia mapping techniques used to distinguish between the different mechanisms, with a particular focus on the detection of rotors. We discuss which arrhythmia mechanisms are likely to respond to ablation, and the challenges and prospects for improving upon current ablation strategies to achieve better outcomes.

Abstract Background Carvedilol is one of the most effective beta-blockers in reducing ventricular tachyarrhythmias and mortality in patients with heart failure. One of the possible antiarrhythmic mechanisms of carvedilol is the suppression of store overload-induced Ca2+ release, especially for the triggered activity. Objectives Premature ventricular complex (PVC) originating from the ventricular outflow tract (OT) is the most common form of idiopathic PVC, and its main mechanism is related to triggered activity. We evaluate the efficacy of carvedilol to suppress the OT PVC. Methods The electronic medical records at our hospital were screened to identify OT PVC patients treated with carvedilol. Clinical, electrocardiographic, and Holter monitoring studies were reviewed. Results A total of 25 patients who underwent Holter monitoring before and after carvedilol administration were found and enrolled. The mean age of the patients was 54.9 ± 13.9 years, and the mean dose of carvedilol was 18.2 ± 10.2 mg (sustained release formulation, 8/16/32 mg). The 24-h burden of PVC in 18 (72%) of 25 patients was significantly reduced from 12.2 ± 9.7% to 4.4 ± 6.7% (P = 0.006). In seven patients, the burden of PVC was changed from 7.1 ± 6.1% to 9.8 ± 8.4% (P = 0.061). There was no difference in age, carvedilol dose, duration of treatment, ventricular function, and left atrial size between responding and non-responding groups. Conclusion In this retrospective pilot study, treatment with carvedilol showed PVC suppression in 72% of patients. Now, we are conducting a prospective, randomized, multicenter study to evaluate the effect of carvedilol on OT PVC (Clinical trial registration: FOREVER trial, Clinical-Trials.gov: NCT03587558).

Dysfunctional sarcoplasmic reticulum Ca2+ handling is commonly observed in heart failure, and thought to contribute to arrhythmogenesis through several mechanisms. Some time ago we developed a cardiomyocyte-specific inducible SERCA2 knockout mouse, which is remarkable in the degree to which major adaptations to sarcolemmal Ca2+ entry and efflux overcome the deficit in SR reuptake to permit relatively normal contractile function. Conventionally, those adaptations would also be expected to dramatically increase arrhythmia susceptibility. However, that susceptibility has never been tested, and it is possible that the very rapid repolarization of the murine action potential (AP) allows for large changes in sarcolemmal Ca2+ transport without substantially disrupting electrophysiologic stability. We investigated this hypothesis through telemetric ECG recording in the SERCA2-KO mouse, and patch-clamp electrophysiology, Ca2+ imaging, and mathematical modeling of isolated SERCA2-KO myocytes. While the SERCA2-KO animals exhibit major (and unique) electrophysiologic adaptations at both the organ and cell levels, they remain resistant to arrhythmia. A marked increase in peak L-type calcium (ICaL) current and slowed ICaL decay elicited pronounced prolongation of initial repolarization, but faster late repolarization normalizes overall AP duration. Early afterdepolarizations were seldom observed in KO animals, and those that were observed exhibited a mechanism intermediate between murine and large mammal dynamical properties. As expected, spontaneous SR Ca2+ sparks and waves were virtually absent. Together these findings suggest that intact SR Ca2+ handling is an absolute requirement for triggered arrhythmia in the mouse, and that in its absence, dramatic changes to the major inward currents can be resisted by the substantial K+ current reserve, even at end-stage disease.

Dysfunctional sarcoplasmic reticulum Ca2+ handling is commonly observed in heart failure, and thought to contribute to arrhythmogenesis through several mechanisms. Some time ago we developed a cardiomyocyte-specific inducible SERCA2 knockout mouse, which is remarkable in the degree to which major adaptations to sarcolemmal Ca2+ entry and efflux overcome the deficit in SR reuptake to permit relatively normal contractile function. Conventionally, those adaptations would also be expected to dramatically increase arrhythmia susceptibility. However, that susceptibility has never been tested, and it is possible that the very rapid repolarization of the murine action potential (AP) allows for large changes in sarcolemmal Ca2+ transport without substantially disrupting electrophysiologic stability. We investigated this hypothesis through telemetric ECG recording in the SERCA2-KO mouse, and patch-clamp electrophysiology, Ca2+ imaging, and mathematical modeling of isolated SERCA2-KO myocytes. While the SERCA2-KO animals exhibit major (and unique) electrophysiologic adaptations at both the organ and cell levels, they remain resistant to arrhythmia. A marked increase in peak L-type calcium (I CaL) current and slowed I CaL decay elicited pronounced prolongation of initial repolarization, but faster late repolarization normalizes overall AP duration. Early afterdepolarizations were seldom observed in KO animals, and those that were observed exhibited a mechanism intermediate between murine and large mammal dynamical properties. As expected, spontaneous SR Ca2+ sparks and waves were virtually absent. Together these findings suggest that intact SR Ca2+ handling is an absolute requirement for triggered arrhythmia in the mouse, and that in its absence, dramatic changes to the major inward currents can be resisted by the substantial K+ current reserve, even at end-stage disease. [ABSTRACT FROM AUTHOR]

• PEG- l -ASNase was immobilized on UCNPs modified with PEI coating by adsorption. • UCNPs had rod-shaped particles and particle size between 200 and 400 nm. • Immobilized PEG- l -ASNases exhibited enhanced thermal and storage stability. • Immobilized PEG- l -ASNases maintained their activity to ≥55 % after 20 cycles. • PEG- l -ASNase activity was triggered ≥2-fold under NIR irradiation. Immobilized enzymes have encountered two main challenges: Reduced enzyme activity compared to free enzymes and exhausted immobilized enzymes due to reusability. Herein, we suggested a promising activity enhancement strategy to overcome these challenges. The emission from upconversion nanoparticles (UCNPs) under near-infrared (NIR) excitation can increase the activity of PEG- l -ASNase due to Förster Resonance Energy Transfer. For this purpose, UCNPs were initially synthesized using the hydrothermal method. Subsequently, these UCNPs were functionalized with a polycationic polymer, branched polyethyleneimine (PEI), and the immobilization of PEG- l -ASNase was achieved through adsorption. We preliminarily explored the parameters such as enzyme concentration, incubation time, pH, temperature, reusability, storage stability, and kinetic study, etc. Further, the in vitro biocompatibility, hemolytic behavior, and anticancer activity of the produced UCNPs were also analyzed as crucial parameters. The results showed the pH durance, thermal and storage stability of the immobilized PEG- l -ASNases were enhanced. The immobilized PEG- l -ASNases maintained their activity to ≥55 % after 20 cycles. Enzyme immobilization led to a decrease in Km and Vmax compared to PEG- l -ASNase. In vitro assays revealed that immobilized enzyme further reduced the proliferation of human leukemia cell line (HL-60) upon NIR irradiation exposure but did not cause toxicity. This research may provide a new strategy to promote the catalytic activity of l -ASNase and demonstrates its potential application on human leukemia cells. Finally, these outcomes are valuable for the use of NIR induction in enzymatic reactions. [Display omitted] [ABSTRACT FROM AUTHOR]

Background: Asymptomatic premature ventricular complex (PVC) in childhood often disappears over time. However, predictive factors for persistent PVC are unknown. We examined predictive factors for persistent PVCs on initial Holter electrocardiogram (ECG) in pediatric patients with asymptomatic PVC.Methods and Results: The initial Holter ECG findings of untreated PVC patients (n=216) between 2010 and 2021 were examined. Multivariable analysis was performed to clarify predictive factors for subsequent persistent PVC burden for each index (age, sex, PVC burden, PVC origin, minimum and maximum mean RR intervals [RRmin and RRmax, respectively]) of the 3 heartbeats of baseline sinus rhythm immediately before the PVC. The median age at initial Holter ECG was 11.6 years (range 5.8-18.8 years), the PVC burden was 5.22% (range 0.01-44.21%), RRmin was 660 ms, RRmax was 936 ms, RRrange (=RRmax-RRmin) was 273 ms, and 15 (7%) PVC runs were identified. The median follow-up period was 5.1 years (range 0.8-9.4 years), and the final Holter PVC burden was 3.99% (range 0-36.38%). In multivariate analysis, RRrange was the only independent risk factor for predicting a final Holter PVC burden >10%, with an area under the curve of 0.920 using an RRrange of 600 ms as the cut-off value., Conclusions: A wide RRrange at the initial Holter ECG may be a predictive indicator for persistent PVC in childhood.

It has long been known that the electrophysiological effects of many cardioactive drugs strongly depend on the rate dependent frequency. This was recognized first for class I antiarrhythmic agents: their V max suppressive effect was attenuated at long cycle lengths. Later many Ca2+ channel blockers were also found to follow such kinetics. The explanation was provided by the modulated and the guarded receptor theories. Regarding the duration of cardiac action potentials (APD) an opposite frequency-dependence was observed, i.e. the drug-induced changes in APD were proportional with the cycle length of stimulation, therefore it was referred as "reverse rate-dependency". The beat-to-beat, or short term variability of APD (SV) has been recognized as an important proarrhythmic mechanism, its magnitude can be used as an arrhythmia predictor. SV is modulated by several cardioactive agents, however, these drugs modify also APD itself. In order to clear the drug-specific effects on SV from the concomitant unspecific APD-change related ones, the term of "relative variability" was introduced. Relative variability is increased by ion channel blockers that decrease the negative feedback control of APD (i.e. blockers of I Ca , I Kr and I Ks) and also by elevation of cytosolic Ca2+. Cardiac arrhythmias are also often categorized according to the characteristic heart rate (tachy- and bradyarrhythmias). Tachycardia is proarrhythmic primarily due to the concomitant Ca2+ overload causing delayed afterdepolarizations. Early afterdepolarizations (EADs) are complications of the bradycardic heart. What is common in the reverse rate-dependent nature of drug action on APD, increased SV and EAD incidence associated with bradycardia. [ABSTRACT FROM AUTHOR]

The electrophysiological properties of pulmonary vein (PV)‐cardiomyocytes, and their responses to the sympathetic neurotransmitter, noradrenaline (NA), are thought to differ from those of the left atrium (LA) and contribute to atrial ectopy. The aim of this study was to examine rat PV cardiomyocyte electrophysiology and responses to NA in comparison with LA cells. LA and PV cardiomyocytes were isolated from adult male Wistar rat hearts, and membrane potentials and ion currents recorded at 36°C using whole‐cell patch‐clamp techniques. PV and LA cardiomyocytes did not differ in size. In control, there were no differences between the two cell‐types in zero‐current potential or action potential duration (APD) at 1 Hz, although the incidence of early afterdepolarizations (EADs) was greater in PV than LA cardiomyocytes. The L‐type Ca2+ current (ICaL) was ~×1.5 smaller (p =.0029, Student's t test) and the steady‐state K+ current (IKss) was ~×1.4 larger (p =.0028, Student's t test) in PV than in LA cardiomyocytes. PV cardiomyocyte inward‐rectifier current (IK1) was slightly smaller than LA cardiomyocyte IK1. In LA cardiomyocytes, NA significantly prolonged APD30. In PV cells, APD30 responses to 1 μM NA were heterogeneous: while the mean percentage change in APD30 was not different from 0 (16.5 ± 9.7%, n cells/N animals = 12/10, p =.1177, one‐sample t test), three cells showed shortening (‐18.8 ± 6.0%) whereas nine showed prolongation (28.3 ± 10.1%, p =.008, Student's t test). NA had no effect on IK1 in either cell‐type but inhibited PV IKss by 41.9 ± 4.1% (n/N = 23/11 p <.0001), similar to LA cells. NA increased ICaL in most PV cardiomyocytes (median × 2.2‐increase, p <.0001, n/N = 32/14, Wilcoxon‐signed‐rank test), although in 7/32 PV cells ICaL was decreased following NA. PV cardiomyocytes differ from LA cells and respond heterogeneously to NA. [ABSTRACT FROM AUTHOR]

Different factors may trigger arrhythmias in diseased hearts, including fibrosis, cardiomyocyte hypertrophy, hypoxia, and inflammation. This makes it difficult to establish the relative contribution of each of them to the occurrence of arrhythmias. Accordingly, in this study, we used an in vitro model of pathological cardiac hypertrophy (PCH) to investigate its proarrhythmic features and the underlying mechanisms independent of fibrosis or other PCH-related processes. Neonatal rat ventricular cardiomyocyte (nr-vCMC) monolayers were treated with phorbol 12-myristate 13-acetate (PMA) to create an in vitro model of PCH. The electrophysiological properties of PMA-treated and control monolayers were analyzed by optical mapping at day 9 of culture. PMA treatment led to a significant increase in cell size and total protein content. It also caused a reduction in sarcoplasmic/endoplasmic reticulum Ca2+ ATPase 2 level (32%) and an increase in natriuretic peptide A (42%) and -1-skeletal muscle actin (34%) levels, indicating that the hypertrophic response induced by PMA was, indeed, pathological in nature. PMA-treated monolayers showed increases in action potential duration (APD) and APD dispersion, and a decrease in conduction velocity (CV; APD30 of 306 ± 39 vs. 148 ± 18 ms, APD30 dispersion of 85 ± 19 vs. 22 ± 7 and CV of 10 ± 4 vs. 21 ± 2 cm/s in controls). Upon local 1-Hz stimulation, 53.6% of the PMA-treated cultures showed focal tachyarrhythmias based on triggered activity (n 82), while the control group showed 4.3% tachyarrhythmias (n 70). PMA-treated nr-vCMC cultures may, thus, represent a well-controllable in vitro model for testing new therapeutic interventions targeting specific aspects of hypertrophyassociated arrhythmias. [ABSTRACT FROM AUTHOR]

If the appropriate immobilization method and carrier support are not selected, partial decreases in the activity of enzymes may occur after immobilization. Herein, to overcome this challenge, an excitation mechanism that enables energy transfer was proposed. Modified upconverting nanoparticles (UCNPs) were constructed and the important role of near-infrared (NIR) excitation in enhancing the catalytic activity of the enzyme was demonstrated. For this purpose, UCNPs were first synthesized via the hydrothermal method, functionalized with isocyanate groups, and then, PEG-L-ASNase was immobilized via covalent binding. UCNPs with and without PEG-L-ASNase were extensively characterized by different methods. These supports had immobilization yield and activity efficiency of >96 % and 78 %, respectively. Moreover, immobilized enzymes exhibited improved pH, thermal, and storage stability. In addition, they retained >65 % of their initial activity even after 20 catalytic cycles. Biochemical and histological findings did not indicate a trend of toxicity in rats due to UCNPs. Most importantly, PEG-L-ASNase activity was triggered approximately 5- and 2-fold under in vitro and in vivo conditions, respectively. Overall, it is anticipated that this pioneering work will shed new light on the realistic and promising usage of NIR-excited UCNPs for the immobilization of enzymes in expensive and extensive applications. [Display omitted] • Upconverting nanoparticles (UCNPs) were designed for PEG-L-ASNase immobilization. • UCNPs had rod-shaped particles and particle size from 80.8 ± 27.2 nm. • The immobilized enzyme showed high stability and reusability. • No biochemical or histological toxicity due to UCNPs was observed in rats. • PEG-L-ASNase activity was triggered 2-fold as in vivo under NIR irradiation. [ABSTRACT FROM AUTHOR]

Abstract Cardiac Purkinje cells (PCs) are more susceptible to action potential abnormalities as compared to ventricular myocytes (VMs), which could be associated with their distinct intracellular calcium handling. We developed a detailed biophysical model of a mouse cardiac PC, which importantly reproduces the experimentally observed biphasic cytosolic calcium waves. The model includes a stochastic gating formulation for the opening and closing of ryanodine receptor (RyR) channels, simulated with a Monte Carlo method, to accurately reproduce cytosolic calcium wave propagation and the effects of spontaneous calcium release events. Simulations predict that during an action potential, smaller cytosolic calcium wavelets propagated from the sarcolemma towards the center of the cell and initiated larger magnitude cell‐wide calcium waves via a calcium‐induced‐calcium release mechanism. In the presence of RyR mutations, frequent spontaneous calcium leaks from sarcoplasmic reticulum (SR) initiated calcium waves, which upon reaching the cell periphery produced delayed afterdepolarizations (DADs) via sodium‐calcium exchanger (NCX) and T‐type calcium (ICaT) channel activation. In the presence of isoproterenol‐mediated effects, DADs induced triggered activity by reactivation of fast sodium channels. Based on our model, we found that the activation of either L‐type calcium channels (ICaL), ICaT, sodium‐potassium exchanger (INaK) or NCX is sufficient for occurrence of triggered activity; however, a partial blockade of ICaT or INaK is essential for its successful termination. Our modeling study highlights valuable insights into the mechanisms of DAD‐induced triggered activity mediated via cytosolic calcium waves in cardiac PCs and may elucidate the increased arrhythmogeneity in PCs.

Ca2+ activated non-selective (CAN) cation channels have been described in cardiomyocytes since the advent of the patch clamp technique. It has been hypothesized that this type of ion channel contributes to the triggering of cardiac arrhythmias. TRPM4 is to date the only molecular candidate for a CAN cation channel in cardiomyocytes. Its significance for arrhythmogenesis in living animals remains, however, unclear. In this study, we have tested whether increased expression of wild-type (WT) TRPM4 augments the risk of arrhythmias in living mice. Overexpression of WT TRPM4 was achieved via tail vein injection of adeno-associated viral vector serotype 9 (AAV9) particles, which have been described to be relatively cardiac specific in mice. Subsequently, we performed ECG-measurements in freely moving mice to determine their in vivo cardiac phenotype. Though cardiac muscle was transduced with TRPM4 viral particles, the majority of viral particles accumulated in the liver. We did not observe any difference in arrhythmic incidents during baseline conditions. Instead, WT mice that overexpress TRPM4 were more vulnerable to develop premature ventricular ectopic beats during exercise-induced β-adrenergic stress. Conduction abnormalities were rare and not more frequent in transduced mice compare to WT mice. Taken together, we provide evidence that overexpression of TRPM4 increases the susceptibility of living mice to stress-induced arrhythmias.

Risk for Atrial Fibrillation (AF), the most common human arrhythmia, has a major genetic component. The T-box transcription factor TBX5 influences human AF risk, and adult-specific Tbx5-mutant mice demonstrate spontaneous AF. We report that TBX5 is critical for cellular Ca2+ homeostasis, providing a molecular mechanism underlying the genetic implication of TBX5 in AF. We show that cardiomyocyte action potential (AP) abnormalities in Tbx5-deficient atrial cardiomyocytes are caused by a decreased sarcoplasmic reticulum (SR) Ca2+ ATPase (SERCA2)-mediated SR calcium uptake which was balanced by enhanced trans-sarcolemmal calcium fluxes (calcium current and sodium/calcium exchanger), providing mechanisms for triggered activity. The AP defects, cardiomyocyte ectopy, and AF caused by TBX5 deficiency were rescued by phospholamban removal, which normalized SERCA function. These results directly link transcriptional control of SERCA2 activity, depressed SR Ca2+ sequestration, enhanced trans-sarcolemmal calcium fluxes, and AF, establishing a mechanism underlying the genetic basis for a Ca2+-dependent pathway for AF risk.

Most forms of sustained ventricular tachycardia (VT) are caused by re-entry, resulting from altered myocardial conduction and refractoriness secondary to underlying structural heart disease. In contrast, VT caused by triggered activity (TA) is unrelated to an abnormal structural substrate and is often caused by molecular defects affecting ion channel function or regulation of intracellular calcium cycling. This review summarizes the cellular and molecular bases underlying TA and exemplifies their clinical relevance with selective representative scenarios. The underlying basis of TA caused by delayed afterdepolarizations is related to sarcoplasmic reticulum calcium overload, calcium waves, and diastolic sarcoplasmic reticulum calcium leak. Clinical examples of TA caused by delayed afterdepolarizations include sustained right and left ventricular outflow tract tachycardia and catecholaminergic polymorphic VT. The other form of afterpotentials, early afterdepolarizations, are systolic events and inscribe early afterdepolarizations during phase 2 or phase 3 of the action potential. The fundamental defect is a decrease in repolarization reserve with associated increases in late plateau inward currents. Malignant ventricular arrhythmias in the long QT syndromes are initiated by early afterdepolarization-mediated TA. An understanding of the molecular and cellular bases of these arrhythmias has resulted in generally effective pharmacologic-based therapies, but these are nonspecific agents that have off-target effects. Therapeutic efficacy may need to be augmented with an implantable defibrillator. Next-generation therapies will include novel agents that rescue arrhythmogenic abnormalities in cellular signaling pathways and gene therapy approaches that transfer or edit pathogenic gene variants or silence mutant messenger ribonucleic acid., Competing Interests: Funding Support and Author Disclosures The authors have reported that they have no relationships relevant to the contents of this paper to disclose., (Copyright © 2024 American College of Cardiology Foundation. Published by Elsevier Inc. All rights reserved.)

Ca2+ activated non-selective (CAN) cation channels have been described in cardiomyocytes since the advent of the patch clamp technique. It has been hypothesized that this type of ion channel contributes to the triggering of cardiac arrhythmias. TRPM4 is to date the only molecular candidate for a CAN cation channel in cardiomyocytes. Its significance for arrhythmogenesis in living animals remains, however, unclear. In this study, we have tested whether increased expression of wild-type (WT) TRPM4 augments the risk of arrhythmias in living mice. Overexpression of WT TRPM4 was achieved via tail vein injection of adeno-associated viral vector serotype 9 (AAV9) particles, which have been described to be relatively cardiac specific in mice. Subsequently, we performed ECG-measurements in freely moving mice to determine their in vivo cardiac phenotype. Though cardiac muscle was transduced with TRPM4 viral particles, the majority of viral particles accumulated in the liver. We did not observe any difference in arrhythmic incidents during baseline conditions. Instead, WT mice that overexpress TRPM4 were more vulnerable to develop premature ventricular ectopic beats during exercise-induced β-adrenergic stress. Conduction abnormalities were rare and not more frequent in transduced mice compare to WT mice. Taken together, we provide evidence that overexpression of TRPM4 increases the susceptibility of living mice to stress-induced arrhythmias. [ABSTRACT FROM AUTHOR]

Ventricular tachycardia (VT) secondary to myocardial infarction (MI) remain a major cause of sudden death in adults. Premature ventricular complexes (PVCs), the first initiating beats of a portion of these arrhythmias, arise from triggered activity in the infarct border zone (BZ). At the cellular scale, spontaneous calcium release (SCR) events are a known cause of triggered activity and have been reported in cells that survive MI. At the tissue scale, fibrosis has been shown to play an important role in creating the substrate for VT. However, the interplay between SCR-mediated triggered activity and fibrosis upon VT formation in infarcted hearts has not been fully investigated. Here, we conduct in-silico experiments to assess how macroscopic and microscopic anatomical properties of the BZ can create a substrate for SCR-mediated VT formation. To study this question, we employ a stochastic subcellular-scale model of SCR events and action potential to simulate different cardiac preparations. Within 2D sheet models with idealized infarct scars and BZ we show that the probability of PVCs is higher, 55%, in preparations with thin conducting isthmuses (0.2 mm) transcending the scar. In an anatomically-detailed model of the rabbit ventricles with a realistic representation of intramural scars, we show that the heart's protective source-sink mismatch prevents ectopy. Furthermore, we demonstrate that fibrosis disrupts this antiarrhythmic mechanism making PVCs more likely. PVC probability is highest (≥25%) when fibrosis accounts for 60 and 90% of the BZ in the 2D sheet and the 3D anatomical model, respectively. Above these thresholds, PVC occurrence decreases because of: (1) the reduced number of myocytes in the BZ; (2) conduction block. Block is caused either by disconnection of BZ cells from the myocardium or due to source-sink mismatches at regions of rapid tissue expansion. Moreover, while outward propagation to healthy tissue may fail, PVCs traveling inward through the scar might encounter more favorable loading conditions. These PVCs may exit to the myocardium and reenter back at the region of block. Overall, our findings indicate that thin isthmuses and strands of myocytes interspersed with fibrosis can be arrhythmogenic. Ablation of these microscopic structures may prevent VT formation.

We describe a case wherein the presence of premature ventricular contractions was related to an increased heart rate that occurred due to supra‐ventricular tachycardia: atrial tachycardia or atrioventricular nodal reentry tachycardia. [ABSTRACT FROM AUTHOR]

Background: Atrial tachyarrhythmia (AT) is a common complication in patients who have undergone a Fontan operation. In this study, we investigated whether abnormal Ca2+ handling contributes to the Fontan operation-related atrial arrhythmogenic substrate.Methods: Mongrel dogs were randomly assigned to sham and Fontan groups. The Fontan operation model was developed by performing an atriopulmonary anastomosis. After 14 days, an electrophysiological study was performed to evaluate the AT vulnerability. Ca2+ handling properties were measured by loading atrial cardiomyocytes (CMs) with fura-2 AM. The L-type Ca2+ (ICa−L) and Na+-Ca2+ exchanger (INCX) currents of the CMs were recorded by the whole-cell patch-clamp technique. The key Ca2+ handling proteins expression was assessed by Western blotting.Results: The AT inducibility was higher in the Fontan group than in the sham group (85.71 vs. 14.29%, P < 0.05). The Fontan operation resulted in decreased Ca2+ transient (CaT) amplitude and sarcoplasmic reticulum (SR) Ca2+ content, but in enhanced diastolic intracellular Ca2+ concentration and SR Ca2+ leak in the atrial CMs. The spontaneous CaT events, triggered ectopic activity and INCX density were increased, but ICa−L density was reduced in CMs from the Fontan atria (all P < 0.05). Additionally, the Fontan operation resulted in decreased SR Ca2+ ATPase expression and Cav1.2 expression, but in increased NCX1 and Ser2814-phosphorylated ryanodine receptor 2. The calmodulin-dependent protein kinase II expression and function were markedly enhanced in the Fontan atria.Conclusion: The Fontan operation caused atrial CM Ca2+ handling abnormalities that produced arrhythmogenic-triggered activity and increased vulnerability to AT in experimental Fontan dogs. [ABSTRACT FROM AUTHOR]

Hibernating animals show resistance to hypothermia-induced cardiac arrhythmias. However, it is not clear whether and how mammalian hibernators are resistant to ischemia-induced arrhythmias. The goal of this investigation was to determine the susceptibility of woodchucks (Marmota monax) to arrhythmias and their mechanisms after coronary artery occlusion at the same room temperature in both winter, the time for hibernation, and summer, when they do not hibernate. By monitoring telemetric electrocardiograms, we found significantly higher arrhythmia scores, calculated as the severity of arrhythmias, with incidence of ventricular tachycardia, ventricular fibrillation, and thus sudden cardiac death (SCD) in woodchucks in summer than they had in winter. The level of catalase expression in woodchuck hearts was significantly higher, whereas the level of oxidized Ca2+/calmodulin-dependent protein kinase II (CaMKII) was lower in winter than it was in summer. Ventricular myocytes isolated from woodchucks in winter were more resistant to H2O2-induced early afterdepolarizations (EADs) compared with myocytes isolated from woodchucks in summer. The EADs were eliminated by inhibiting CaMKII (with KN-93), l-type Ca current (with nifedipine), or late Na+ current (with ranolazine). In woodchucks, in the summer, the arrhythmia score was significantly reduced by overexpression of catalase (via adenoviral vectors) or the inhibition of CaMKII (with KN-93) in the heart. This study suggests that the heart of the mammalian hibernator is more resistant to ischemia-induced arrhythmias and SCD in winter. Increased antioxidative capacity and reduced CaMKII activity may confer resistance in woodchuck hearts against EADs and arrhythmias during winter. The profound protection conferred by catalase overexpression or CaMKII inhibition in this novel natural animal model may provide insights into clinical directions for therapy of arrhythmias. [ABSTRACT FROM AUTHOR]

The electrophysiologic mechanisms responsible for the initiation and maintenance of ventricular tachycardia (VT) include enhanced automaticity, triggered activity and reentry. Differentiating between these three mechanisms can be challenging for the clinician and usually requires an invasive electrophysiology study. Establishing the underlying VT mechanism in a particular patient is helpful to define the optimal therapeutic approach, including the selection of pharmacologic agents or delineation of an ablation strategy. The purpose of this review is to provide insight into the possible VT mechanisms based on noninvasive clues from the clinical history, 12-lead electrocardiogram, tachycardia onset and termination and the response to pharmacologic manipulation. [ABSTRACT FROM AUTHOR]

• Early afterdepolarization (EAD) is repolarization abnormalities in ventricular cardiomyocytes. • Depolarizing waves by EADs accentuate repolarization heterogeneity within the ventricle. • Synchronous EAD developments form repolarization potential difference boundaries within the tissue. • Repolarization potential difference boundary shape is a key to triggered activity formation. • Repolarization heterogeneity due to EAD developments could trigger lethal ventricular tachyarrhythmias. Excessive prolongation of QT interval on ECGs in patients with congenital/acquired long QT syndrome and heart failure is a sign suggesting the development of early afterdepolarization (EAD), an abnormal repolarization in the action potential of ventricular cardiomyocytes. The development of EAD has been believed to be a trigger for fatal tachyarrhythmia, which can be a risk for sudden cardiac death. The role of EAD in triggering ventricular tachycardia (VT) remains unclear. The aim of this study was to elucidate the mechanism of EAD-induced triggered activity formation that leads to the VT such as Torsades de Pointes. We investigated the relationship between EAD and tachyarrhythmia initiation by constructing homogeneous myocardial sheet models consisting of the mid-myocardial cell version of a human ventricular myocyte model and performing simulations of excitation propagation. A solitary island-like (clustering) occurrence of EADs in the homogeneous myocardial sheet could induce a focal excitation wave. However, reentrant excitation, an entity of tachyarrhythmia, was not able to be triggered regardless of the EAD cluster size when the focal excitation wave formed a repolarization potential difference boundary consisting of only a convex surface. The discontinuous distribution of multiple EAD clusters in the ventricular tissue formed a specific repolarization heterogeneity due to the repolarization potential difference, the shape of which depended on EAD cluster size and placed intervals. We found that the triggered activity was formed in such a manner that the repolarization potential difference boundary included a concave surface. The formation of triggered activity that led to tachyarrhythmia required not only the occurrence of EAD onset-mediated focal excitation wave but also a repolarization heterogeneity-based specific repolarization potential difference boundary shape formed within the tissue. [ABSTRACT FROM AUTHOR]

Although atrial fibrillation (AF) is the most commonly encountered cardiac arrhythmia, the basic mechanisms underlying this disorder remain incompletely understood. During the past decade or so, it has become clear that alterations in intracellular Ca2+ handling may play a role in the pathogenesis of AF. Studies in small and large animal models, as well as atrial samples from patients with different forms of AF, have implicated ryanodine receptor type 2 (RyR2) dysfunction and enhanced spontaneous Ca2+ release events from the sarcoplasmic reticulum (SR) as a potential cause of proarrhythmic cellular ectopic (triggered) activity in AF. The molecular mechanisms leading to RyR2 dysfunction and SR Ca2+ leak depend on the clinical stage of AF or specific animal model studied. This review focuses on the mechanisms and role of calcium-mediated cellular triggered activity in AF, and addresses some of the current controversies in the field. [ABSTRACT FROM AUTHOR]

Aims: The chronic complete atrioventricular block (CAVB) dog is highly sensitive for drug-induced torsade de pointes (TdP) arrhythmias. Focal mechanisms have been suggested as trigger for TdP onset; however, its exact mechanism remains unclear. In this study, detailed mapping of the ventricles was performed to assess intraventricular heterogeneity of repolarization in relation to the initiation of TdP.Methods and Results: In 8 CAVB animals, 56 needles, each containing 4 electrodes, were inserted in the ventricles. During right ventricular apex pacing (cycle length: 1000-1500 ms), local unipolar electrograms were recorded before and after administration of dofetilide to determine activation and repolarization times (RTs). Maximal RT differences were calculated in the left ventricle (LV) within adjacent electrodes in different orientations (transmural, vertical, and horizontal) and within a square of four needles (cubic dispersion). Dofetilide induced TdP in five out of eight animals. Right ventricle-LV was similar between inducible and non-inducible dogs at baseline (327 ± 30 vs. 345 ± 17 ms) and after dofetilide administration (525 ± 95 vs. 508 ± 15 ms). All measurements of intraventricular dispersion were not different at baseline, but this changed for horizontal (206 ± 20 vs. 142 ± 34 ms) and cubic dispersion (272 ± 29 vs. 176 ± 48 ms) after dofetilide: significantly higher values in inducible animals. Single ectopic beats and the first TdP beat arose consistently from a subendocardially located electrode terminal with the shortest RT in the region with largest RT differences.Conclusion: Chronic complete atrioventricular block dogs susceptible for TdP demonstrate higher RT differences. Torsade de pointes arises from a region with maximal heterogeneity of repolarization suggesting that a minimal gradient is required in order to initiate TdP. [ABSTRACT FROM AUTHOR]