Mechanism of right precordial ST-segment elevation in structural heart disease: excitation failure by current-to-load mismatch

Electrophysiological patterns and structural substrates of Brugada syndrome: Critical appraisal and computational analyses

Brugada syndrome (BrS) is a cardiac electrophysiological disease with unknown etiology, associated with sudden cardiac death. Symptomatic patients are treated with implanted cardiac defibrillator, but no risk stratification strategy is effective in patients that are at low to medium arrhythmic risk. Cardiac computational modeling is an emerging tool that can be used to verify the hypotheses of pathogenesis and inspire new risk stratification strategies. However, to obtain reliable results computational models must be validated with consistent experimental data. We reviewed the main electrophysiological and structural variables from BrS clinical studies to assess which data could be used to validate a computational approach. Activation delay in the epicardial right ventricular outflow tract is a consistent finding, as well as increased fibrosis and subclinical alterations of right ventricular functional and morphological parameters. The comparison between other electrophysiological variables is hindered by methodological differences between studies, which we commented. We conclude by presenting a recent theory unifying electrophysiological and structural substrate in BrS and illustrate how computational modeling could help translation to risk stratification.

Distinct Electrogram Features and Ventricular Arrhythmia Induction Modes Between Repolarization and Conduction Heterogeneities

BACKGROUND

Recent clinical studies have indicated the presence of localized electrical abnormalities in idiopathic ventricular fibrillation and J-wave syndrome patients.

OBJECTIVES

This study aims to characterize the specific electrical signatures of localized repolarization and conduction heterogeneities and their respective role in vulnerability to arrhythmias.

METHODS

Optical mapping was performed in porcine right ventricles with local: 1) repolarization shortening; 2) conduction slowing; or 3) structural heterogeneity induced by locally perfusing: 1) pinacidil (20 μmol/L, n = 13); or 2) flecainide (2 μmol/L, n = 13) via an epicardial catheter; or 3) by local epicardial tissue destruction (9 radiofrequency lesions n = 12). Electrograms were recorded (n = 5 in each group) and spontaneous and induced arrhythmias were quantified and optically mapped.

RESULTS

Electrograms were normal in (1) but showed local fragmentation in 40% of preparations in (2) with greater effects observed at high pacing frequencies dependent on the wavefront direction. In (3), the structural substrate alone increased the width and number of peaks in the electrograms, and addition of flecainide induced pronounced fragmentation (≥3 peaks and ≥70 ms) in all cases. Occurrence of spontaneous arrhythmias was significantly increased in (1) and (2) (P < 0.0001 and 0.05, respectively, vs baseline) and were triggered by ectopies. Vulnerability to arrhythmias at high pacing frequencies (≥2 Hz) was the lowest in (1) and greatest in (2).

CONCLUSIONS

Microstructural substrates have the most pronounced impact on electrograms, especially when combined with sodium channel blockers, whereas local action potential duration shortening does not lead to electrogram fragmentation even though it is associated with the highest prevalence of spontaneous arrhythmias.

A Review of Personalised Cardiac Computational Modelling Using Electroanatomical Mapping Data

Computational models of cardiac electrophysiology have gradually matured during the past few decades and are now being personalised to provide patient-specific therapy guidance for improving suboptimal treatment outcomes. The predictive features of these personalised electrophysiology models hold the promise of providing optimal treatment planning, which is currently limited in the clinic owing to reliance on a population-based or average patient approach. The generation of a personalised electrophysiology model entails a sequence of steps for which a range of activation mapping, calibration methods and therapy simulation pipelines have been suggested. However, the optimal methods that can potentially constitute a clinically relevant in silico treatment are still being investigated and face limitations, such as uncertainty of electroanatomical data recordings, generation and calibration of models within clinical timelines and requirements to validate or benchmark the recovered tissue parameters. This paper is aimed at reporting techniques on the personalisation of cardiac computational models, with a focus on calibrating cardiac tissue conductivity based on electroanatomical mapping data.

Biophysical mechanisms of myocardium sodium channelopathies

Genetic variants of gene SCN5A encoding the alpha-subunit of cardiac voltage-gated sodium channel Nav1.5 are associated with various diseases, including long QT syndrome (LQT3), Brugada syndrome (BrS1), and progressive cardiac conduction disease (PCCD). In the last decades, the great progress in understanding molecular and biophysical mechanisms of these diseases has been achieved. The LQT3 syndrome is associated with gain-of-function of sodium channels Nav1.5 due to impaired inactivation, enhanced activation, accelerated recovery from inactivation or the late current appearance. In contrast, BrS1 and PCCD are associated with the Nav1.5 loss-of-function, which in electrophysiological experiments can be manifested as reduced current density, enhanced fast or slow inactivation, impaired activation, or decelerated recovery from inactivation. Genetic variants associated with congenital arrhythmias can also disturb interactions of the Nav1.5 channel with different proteins or drugs and cause unexpected reactions to drug administration. Furthermore, mutations can affect post-translational modifications of the channels and their sensitivity to pH and temperature. Here we briefly review the current knowledge on biophysical mechanisms of LQT3, BrS1 and PCCD. We focus on limitations of studies that use heterologous expression systems and induced pluripotent stem cells (iPSC) derived cardiac myocytes and summarize our understanding of genotype-phenotype relations of SCN5A mutations.

Left Ventricular Abnormal Substrate in Brugada Syndrome

BACKGROUND

Slow-conductive structural abnormalities located in the epicardium of the right ventricle (RV) underlie Brugada syndrome (BrS). The extent of such substrate in the left ventricle (LV) has not been investigated.

OBJECTIVES

This study sought to characterize the extent of epicardial substrate abnormalities in BrS.

METHODS

We evaluated 22 consecutive patients (mean age 46 ± 11 years, 21 male) referred for recurrent ventricular arrhythmias (mean 10 ± 13 episodes) in the setting of BrS. The patients underwent clinical investigations and wide genetic screening to identify SCN5A mutations and common risk variants. High-density biventricular epicardial mapping was performed to detect prolonged (>70 ms) fragmented electrograms, indicating abnormal substrate area.

RESULTS

All patients presented with abnormal substrate in the epicardial anterior RV (27 ± 11 cm2). Abnormal substrate was also identified on the LV epicardium in 10 patients (45%), 9 at baseline and 1 after ajmaline infusion, covering 15 ± 11 cm2. Of these, 4 had severe LV fascicular blocks. Patients with LV substrate had a longer history of arrhythmia (11.4 ± 6.7 years vs 4.3 ± 4.3 years; P = 0.003), longer PR (217 ± 24 ms vs 171 ± 14 ms; P < 0.001) and HV (60 ± 12 ms vs 46 ± 5 ms; P = 0.005) intervals, and abnormal substrate also extending into the inferior RV (100% vs 33%; P = 0.001). SCN5A mutation was present in 70% of patients with LV substrate (vs 25%; P = 0.035). SCN5A BrS patients with recurrent ventricular arrhythmias present a higher polygenic risk score compared with a nonselected BrS population (median of differences: -0.86; 95% CI: -1.48 to -0.27; P = 0.02).

CONCLUSIONS

A subset of patients with BrS present an abnormal substrate extending onto the LV epicardium and inferior RV that is associated with SCN5A mutations and multigenic variants.

Genetic and Molecular Mechanisms in Brugada Syndrome

Brugada syndrome is a rare hereditary arrhythmia disorder characterized by a distinctive electrocardiogram pattern and an elevated risk of ventricular arrhythmias and sudden cardiac death in young adults. Despite recent advances, it remains a complex condition, encompassing mechanisms, genetics, diagnosis, arrhythmia risk stratification, and management. The underlying electrophysiological mechanism of Brugada syndrome requires further investigation, with current theories focusing on abnormalities in repolarization, depolarization, and current-load match. The genetic basis of the syndrome is strong, with mutations found in genes encoding subunits of cardiac sodium, potassium, and calcium channels, as well as genes involved in channel trafficking and regulation. While the initial discovery of mutations in the SCN5A gene provided valuable insights, Brugada syndrome is now recognized as a multifactorial disease influenced by several loci and environmental factors, challenging the traditional autosomal dominant inheritance model. This comprehensive review aims to provide a current understanding of Brugada syndrome, focusing on its pathophysiology, genetic mechanisms, and novel models of risk stratification. Advancements in these areas hold the potential to facilitate earlier diagnosis, improve risk assessments, and enable more targeted therapeutic interventions.

Subepicardial Cardiomyopathy: A Disease Underlying J-Wave Syndromes and Idiopathic Ventricular Fibrillation

Brugada syndrome (BrS), early repolarization syndrome (ERS), and idiopathic ventricular fibrillation (iVF) have long been considered primary electrical disorders associated with malignant ventricular arrhythmia and sudden cardiac death. However, recent studies have revealed the presence of subtle microstructural abnormalities of the extracellular matrix in some cases of BrS, ERS, and iVF, particularly within right ventricular subepicardial myocardium. Substrate-based ablation within this region has been shown to ameliorate the electrocardiographic phenotype and to reduce arrhythmia frequency in BrS. Patients with ERS and iVF may also exhibit low-voltage and fractionated electrograms in the ventricular subepicardial myocardium, which can be treated with ablation. A significant proportion of patients with BrS and ERS, as well as some iVF survivors, harbor pathogenic variants in the voltage-gated sodium channel gene, SCN5A, but the majority of genetic susceptibility of these disorders is likely to be polygenic. Here, we postulate that BrS, ERS, and iVF may form part of a spectrum of subtle subepicardial cardiomyopathy. We propose that impaired sodium current, along with genetic and environmental susceptibility, precipitates a reduction in epicardial conduction reserve, facilitating current-to-load mismatch at sites of structural discontinuity, giving rise to electrocardiographic changes and the arrhythmogenic substrate.

CineECG analysis provides new insights into Familial ST-segment Depression Syndrome

AIMS

Familial ST-segment Depression Syndrome (Fam-STD) is a novel inherited cardiac disease associated with arrhythmias and sudden cardiac death. This study aimed at investigating the cardiac activation pathway in patients with Fam-STD, modelling the electrocardiogram (ECG) phenotype, and performing in-depth ST-segment analyses.

METHODS AND RESULTS

CineECG analysis of patients with Fam-STD and age- and sex-matched controls. The groups were compared using the CineECG software which included the trans-cardiac ratio and the electrical activation pathway. We simulated the Fam-STD ECG phenotype by adjusting action potential duration (APD) and action potential amplitude (APA) in specific cardiac regions. High-resolution ST-segment analyses were performed per lead by dividing the ST-segment into nine 10 ms subintervals. Twenty-seven Fam-STD patients (74% females, mean age 51.6 ± 6.2 years) and 83 matched controls were included. Among Fam-STD patients, electrical activation pathway analysis in the anterior-basal orientation showed significantly abnormal direction toward the basal areas of the heart starting from QRS 60-89 ms until Tpeak-Tend (all P < 0.001). Simulations with shortened APD and reduced APA in the left ventricle basal regions recapitulated the Fam-STD ECG phenotype. Detailed ST-segment analyses showed significant differences in all nine 10 ms subintervals (all P < 0.01), with the most prominent findings during the 70-79/80-89 ms intervals.

CONCLUSION

CineECG analyses indicated abnormal repolarization with basal directions, and the Fam-STD ECG phenotype was simulated by reducing APD and APA in the left ventricle basal regions. Detailed ST-analysis showed amplitudes consistent with the proposed diagnostic criteria for Fam-STD patients. Our findings provide new insight into the electrophysiological abnormalities of Fam-STD.

Brugada Syndrome: From Molecular Mechanisms and Genetics to Risk Stratification

Brugada syndrome (BrS) is a rare hereditary arrhythmia disorder, with a distinctive ECG pattern, correlated with an increased risk of ventricular arrhythmias and sudden cardiac death (SCD) in young adults. BrS is a complex entity in terms of mechanisms, genetics, diagnosis, arrhythmia risk stratification, and management. The main electrophysiological mechanism of BrS requires further research, with prevailing theories centered on aberrant repolarization, depolarization, and current-load match. Computational modelling, pre-clinical, and clinical research show that BrS molecular anomalies result in excitation wavelength (k) modifications, which eventually increase the risk of arrhythmia. Although a mutation in the SCN5A (Sodium Voltage-Gated Channel Alpha Subunit 5) gene was first reported almost two decades ago, BrS is still currently regarded as a Mendelian condition inherited in an autosomal dominant manner with incomplete penetrance, despite the recent developments in the field of genetics and the latest hypothesis of additional inheritance pathways proposing a more complex mode of inheritance. In spite of the extensive use of the next-generation sequencing (NGS) technique with high coverage, genetics remains unexplained in a number of clinically confirmed cases. Except for the SCN5A which encodes the cardiac sodium channel NaV1.5, susceptibility genes remain mostly unidentified. The predominance of cardiac transcription factor loci suggests that transcriptional regulation is essential to the Brugada syndrome's pathogenesis. It appears that BrS is a multifactorial disease, which is influenced by several loci, each of which is affected by the environment. The primary challenge in individuals with a BrS type 1 ECG is to identify those who are at risk for sudden death, researchers propose the use of a multiparametric clinical and instrumental strategy for risk stratification. The aim of this review is to summarize the latest findings addressing the genetic architecture of BrS and to provide novel perspectives into its molecular underpinnings and novel models of risk stratification.

Cardiac Morphofunctional Characteristics of Individuals with Early Repolarization Pattern: A Literature Review

The early repolarization pattern (ERP) is an electrocardiographic phenomenon characterized by the appearance of a distinct J-wave or J-point elevation at the terminal part of the QRS complex. ERP is associated with an increased risk of ventricular arrhythmias in susceptible individuals. The cardiac morphofunctional parameters in subjects with ERP have been characterized mainly by imaging techniques, which suggests that certain changes could be identified in the background of the electrical pathomechanism: however, in this regard, current data are often contradictory or insufficiently detailed. For clarification, a more comprehensive cardiac imaging evaluation of a large patient population is necessary. This review summarizes and analyses the data from the literature related to cardiac morphofunctional characteristics in individuals with ERP.

Diffuse fibrosis and repolarization disorders explain ventricular arrhythmias in Brugada syndrome: a computational study

In this work, we reported a computational study to quantitatively determine the individual contributions of three candidate arrhythmic factors associated with Brugada Syndrome. In particular, we focused our analysis on the role of structural abnormalities, dispersion of repolarization, and size of the diseased region. We developed a human phenomenological model capable of replicating the action potential characteristics both in Brugada Syndrome and in healthy conditions. Inspired by physiological observations, we employed the phenomenological model in a 2D geometry resembling the pathological RVOT coupled with healthy epicardial tissue. We assessed the insurgence of sustained reentry as a function of electrophysiological and structural abnormalities. Our computational study indicates that both structural and repolarization abnormalities are essential to induce sustained reentry. Furthermore, our results suggest that neither dispersion of repolarization nor structural abnormalities are sufficient on their own to induce sustained reentry. It should be noted how our study seems to explain an arrhythmic mechanism that unifies the classic repolarization and depolarization hypotheses of the pathophysiology of the Brugada Syndrome. Finally, we believe that this work may offer a new perspective on the computational and clinical investigation of Brugada Syndrome and its arrhythmic behaviour.

The Genetics of Brugada Syndrome

Brugada syndrome is a heritable channelopathy characterized by a peculiar electrocardiogram (ECG) pattern and increased risk of cardiac arrhythmias and sudden death. The arrhythmias originate because of an imbalance between the repolarizing and depolarizing currents that modulate the cardiac action potential. Even if an overt structural cardiomyopathy is not typical of Brugada syndrome, fibrosis and structural changes in the right ventricle contribute to a conduction slowing, which ultimately facilitates ventricular arrhythmias. Currently, Mendelian autosomal dominant transmission is detected in less than 25% of all clinical confirmed cases. Although 23 genes have been associated with the condition, only SCN5A, encoding the cardiac sodium channel, is considered clinically actionable and disease causing. The limited monogenic inheritance has pointed toward new perspectives on the possible complex genetic architecture of the disease, involving polygenic inheritance and a polygenic risk score that can influence penetrance and risk stratification. Expected final online publication date for the Annual Review of Genomics and Human Genetics, Volume 23 is October 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Using QRS loop descriptors to characterize the risk of sudden cardiac death in patients with structurally normal hearts

The predictive value of non-invasive electrocardiographic examination findings for the risk of sudden cardiac death (SCD) in populations with structurally normal hearts remains unclear. This study aimed to investigate the characteristics of the QRS vectorcardiography of surface electrocardiography in patients with structurally normal hearts who experienced SCD. We consecutively enrolled patients who underwent vectorcardiography between March 2017 and December 2018 in a tertiary referral medical center. These patients didn’t have structural heart diseases, histories of congestive heart failure, or reduced ejection fraction, and they were classified into SCD (with aborted SCD history and cerebral performance category score of 1) and control groups (with an intervention for atrioventricular node reentrant tachycardia and without SCD history). A total of 162 patients (mean age, 54.3±18.1 years; men, 75.9%), including 59 in the SCD group and 103 in the control group, underwent propensity analysis. The baseline demographic variables, underlying diseases, QRS loop descriptors (the percentage of the loop area, loop dispersion, and inter-lead QRS dispersion), and other electrocardiographic parameters were compared between the two groups. In the univariate and multivariate analyses, a smaller percentage of the loop area (odds ratio, 0.0003; 95% confidence interval, 0.00–0.02; p<0.001), more significant V4-5 dispersion (odds ratio, 1.04; 95% confidence interval, 1.02–1.07; p = 0.002), and longer QRS duration (odds ratio, 1.05; 95% confidence interval, 1.00–1.10; p = 0.04) were associated with SCD. In conclusion, the QRS loop descriptors of surface electrocardiography could be used as non-invasive markers to identify patients experiencing aborted SCD from a healthy population. A decreased percentage of loop area and elevated V4-5 QRS dispersion values assessed using vectorcardiography were associated with an increased risk of SCD in patients with structurally normal hearts.

Mechanism of the Effects of Sodium Channel Blockade on the Arrhythmogenic Substrate of Brugada Syndrome

BACKGROUND

The mechanisms by which sodium channel blockade and high-rate pacing modify electrogram substrates of Brugada syndrome (BrS) have not been elucidated.

OBJECTIVES

To determine the effect of ajmaline and high pacing rate on the BrS substrates.

METHODS

Thirty-two BrS patients (age 40 ± 12 years) with frequent ventricular fibrillation (VF) episodes underwent right ventricular outflow tract (RVOT) substrate electroanatomical and electrocardiogram imaging (ECGI) mapping before and after ajmaline administration and during high-rate atrial pacing. In 4 patients, epicardial mapping was performed using open thoracotomy with targeted biopsies.

RESULTS

Ajmaline increased the activation time delay in the substrate (33%; p = 0.002), ST elevation in the right precordial leads (74%; p < 0.0001), and the area of delayed activation (170%; p < 0.0001), coinciding with increased substrate size (75%; p < 0.0001). High atrial pacing rate increased the abnormal electrogram (EGM) duration at the RVOT areas from 112 ± 48 to 143 ± 66 ms (p = 0.003) and produced intermittent conduction block and/or excitation failure at the substrate sites, especially after ajmaline. Biopsies from the 4 patients with thoracotomy showed epicardial fibrosis where EGMs were normal at baseline but became fractionated after ajmaline. In some areas, local activation was absent and unipolar EGMs had a monophasic morphology resembling the shape of the action potential.

CONCLUSIONS

I_Na reduction with ajmaline severely compromises impulse conduction at the BrS fibrotic substrates by producing fractionated EGMs, conduction block, or excitation failure, leading to the Brugada ECG pattern and favoring VF genesis.

Multisite conduction block in the epicardial substrate of Brugada syndrome

BACKGROUND

The Brugada pattern manifests as a spontaneous variability of the electrocardiographic marker, suggesting a variability of the underlying electrical substrate.

OBJECTIVE

The purpose of this study was to investigate the response of the epicardial substrate of Brugada syndrome (BrS) to programmed ventricular stimulation and to Na blocker infusion.

METHODS

We investigated 6 patients (all male; mean age 54 ± 14 years) with BrS and recurrent ventricular fibrillation. Five had no type 1 BrS electrocardiogram pattern at admission. They underwent combined epicardial-endocardial mapping using multielectrode catheters. Changes in epicardial electrograms were evaluated during single endocardial extrastimulation and after low-dose ajmaline infusion (0.5 mg/kg in 5 minutes).

RESULTS

All patients had a region in the anterior epicardial right ventricle with prolonged multicomponent electrograms. Single extrastimulation prolonged late epicardial components by 59 ± 31 ms and in 4 patients abolished epicardial components at some sites, without reactivation by surrounding activated sites. These localized blocks occurred at an initial coupling interval of 335 ± 58 ms and then expanded to other sites, being observed in up to 40% of epicardial sites. Ajmaline infusion prolonged electrogram duration in all and produced localized blocks in 62% of sites in the same patients as during extrastimulation. Epicardial conduction recovery after ajmaline occurred intermittently and at discontinuous sites and produced beat-to-beat changes in local repolarization, resulting in an area of marked electrical disparity. These changes were consistent with models based on microstructural alterations under critical propagation conditions.

CONCLUSION

In BrS, localized functional conduction blocks occur at multiple epicardial sites and with variable patterns, without being reactivated from the surrounding sites.

Prevalence and characteristics of the Brugada electrocardiogram pattern in patients with arrhythmogenic right ventricular cardiomyopathy

BACKGROUND

Despite distinct pathophysiology, arrhythmogenic right ventricular cardiomyopathy (ARVC) and Brugada syndrome (BrS) exhibit overlapping phenotypes. We investigated the prevalence and characteristics of the Brugada electrocardiogram (ECG) pattern in ARVC patients.

METHODS

A total of 114 ARVC patients fulfilling the revised Task Force Criteria were enrolled. The Brugada ECG pattern was evaluated according to the consensus report on right precordial leads, and 1141 ECGs (median, 1; interquartile range, 1-16 ECGs/patient) were analyzed.

RESULTS

Five patients (4%) showed a Brugada ECG pattern, which disappeared in four patients with ECGs recorded more than 2 years afterward. ARVC patients with the Brugada ECG pattern had a longer PQ interval (220 ± 62 ms vs 180 ± 35 ms, P = .02) and longer QRS duration (138 ± 25 ms vs 102 ± 23 ms, P < .001) than patients without the pattern. During follow-up (median, 11.4; interquartile range, 5.5-17.1 years), 19 ARVC patients experienced cardiac death and 29 experienced heart failure (HF) hospitalization. Kaplan-Meier analysis determined that the Brugada ECG pattern increased the risk of cardiac death and HF hospitalization (log-rank; P < .001, P < .001 respectively). The mean J-point and S-wave amplitudes of the Brugada ECG pattern were 0.29 ± 0.05 mV and 0.34 ± 0.21 mV, respectively, which were significantly lower than those of 26 age-matched BrS patients with a previous ventricular fibrillation episode (0.66 ± 0.33 mV, P < .001 and 0.67 ± 0.39 mV, P = .02 respectively).

CONCLUSION

The Brugada ECG pattern was infrequently encountered, was transient in ARVC patients, and was associated with a longer PQ interval, longer QRS duration, and cardiac events.

Heritable arrhythmia syndromes associated with abnormal cardiac sodium channel function: ionic and non-ionic mechanisms

The cardiac sodium channel NaV1.5, encoded by the SCN5A gene, is responsible for the fast upstroke of the action potential. Mutations in SCN5A may cause sodium channel dysfunction by decreasing peak sodium current, which slows conduction and facilitates reentry-based arrhythmias, and by enhancing late sodium current, which prolongs the action potential and sets the stage for early afterdepolarization and arrhythmias. Yet, some NaV1.5-related disorders, in particular structural abnormalities, cannot be directly or solely explained on the basis of defective NaV1.5 expression or biophysics. An emerging concept that may explain the large disease spectrum associated with SCN5A mutations centres around the multifunctionality of the NaV1.5 complex. In this alternative view, alterations in NaV1.5 affect processes that are independent of its canonical ion-conducting role. We here propose a novel classification of NaV1.5 (dys)function, categorized into (i) direct ionic effects of sodium influx through NaV1.5 on membrane potential and consequent action potential generation, (ii) indirect ionic effects of sodium influx on intracellular homeostasis and signalling, and (iii) non-ionic effects of NaV1.5, independent of sodium influx, through interactions with macromolecular complexes within the different microdomains of the cardiomyocyte. These indirect ionic and non-ionic processes may, acting alone or in concert, contribute significantly to arrhythmogenesis. Hence, further exploration of these multifunctional effects of NaV1.5 is essential for the development of novel preventive and therapeutic strategies.

How Cardiac Embryology Translates into Clinical Arrhythmias

The electrophysiological signatures of the myocardium in cardiac structures, such as the atrioventricular node, pulmonary veins or the right ventricular outflow tract, are established during development by the spatial and temporal expression of transcription factors that guide expression of specific ion channels. Genome-wide association studies have shown that small variations in genetic regions are key to the expression of these transcription factors and thereby modulate the electrical function of the heart. Moreover, mutations in these factors are found in arrhythmogenic pathologies such as congenital atrioventricular block, as well as in specific forms of atrial fibrillation and ventricular tachycardia. In this review, we discuss the developmental origin of distinct electrophysiological structures in the heart and their involvement in cardiac arrhythmias.

Fibrosis and Conduction Abnormalities as Basis for Overlap of Brugada Syndrome and Early Repolarization Syndrome

Brugada syndrome and early repolarization syndrome are both classified as J-wave syndromes, with a similar mechanism of arrhythmogenesis and with the same basis for genesis of the characteristic electrocardiographic features. The Brugada syndrome is now considered a conduction disorder based on subtle structural abnormalities in the right ventricular outflow tract. Recent evidence suggests structural substrate in patients with the early repolarization syndrome as well. We propose a unifying mechanism based on these structural abnormalities explaining both arrhythmogenesis and the electrocardiographic changes. In addition, we speculate that, with increasing technical advances in imaging techniques and their spatial resolution, these syndromes will be reclassified as structural heart diseases or cardiomyopathies.

Mechanisms of Arrhythmias in the Brugada Syndrome

Arrhythmias in Brugada syndrome patients originate in the right ventricular outflow tract (RVOT). Over the past few decades, the characterization of the unique anatomy and electrophysiology of the RVOT has revealed the arrhythmogenic nature of this region. However, the mechanisms that drive arrhythmias in Brugada syndrome patients remain debated as well as the exact site of their occurrence in the RVOT. Identifying the site of origin and mechanism of Brugada syndrome would greatly benefit the development of mechanism-driven treatment strategies.

Structurally Abnormal Myocardium Underlies Ventricular Fibrillation Storms in a Patient Diagnosed With the Early Repolarization Pattern

OBJECTIVES

The aim of this study was to investigate the mechanism underlying QRS-slurring in a patient with the early repolarization pattern in the electrocardiogram (ECG) and ventricular fibrillation (VF) storms.

BACKGROUND

The early repolarization pattern refers to abnormal ending of the QRS complex in subjects with structurally normal hearts and has been associated with VF.

METHODS

We studied a patient with slurring of the QRS complex in leads II, III, and aVF of the ECG and recurrent episodes of VF. Echocardiographic and imaging studies did not reveal any abnormalities. Endocardial mapping was normal but subxyphoidal epicardial access was not possible. Open chest epicardial mapping was performed.

RESULTS

Mapping showed that the inferior right ventricular free wall activated the latest with local J-waves in unipolar electrograms. The last moment of epicardial activation concurred with QRS-slurring in the ECG whereas the J-waves in the local unipolar electrograms occurred in the ST-segment of the ECG. Myocardial biopsies obtained from the late activated tissue showed severe fibrofatty alterations in the inferior right ventricular wall where fractionation and local J-waves were present. After ablation, the early repolarization pattern in the ECG disappeared and arrhythmias have been absent since (follow-up 18 months).

CONCLUSIONS

In this patient, the electrocardiographic early repolarization pattern was caused by late activation due to structurally abnormal myocardium. The late activated areas were marked by J-waves in local electrograms. Ablation of these regions prevented arrhythmia recurrence and normalized the ECG.

The AMP-activated protein kinase modulates hypothermia-induced J wave

BACKGROUND

The mechanism underlying the occurrence of the J wave in low temperature remains unclear. However, low temperature is associated with metabolic disorder and 5' AMP-activated protein kinase (AMPK), which modulates ionic currents and cardiac metabolism. This study investigated whether AMPK regulation can modulate the occurrence of the J wave at low temperature.

METHODS

Unipolar and bipolar leads were used to record monophasic action potential (the endocardium and epicardium) and pseudo-electrocardiograms (inferior leads) to study the cardiac electrical activity. Measurements were taken in isolated Langendorff rabbit hearts at both 30℃ and 37℃ before and after administration of 4-aminopyridine (an ultrarapid delayed rectifier potassium current inhibitor, I_Kur , 50 µmol L^-1 ), PF06409577 (an AMPK activator, 1 µmol L^-1 ), compound C (an AMPK inhibitor, 10 µmol L^-1 ) and glibenclamide (an ATP-sensitive inward rectifier potassium channel inhibitor, I_KATP , 20 µmol L^-1 ).

RESULTS

The amplitude of the J wave (2.46 ± 0.34 mV vs. 1.11 ± 0.23 mV, P < .01) at 30℃ (n = 15) was larger than that at 37℃ (n = 15). PF06409577 (1 µmol L^-1 ) increased the J waves at both 30℃ and 37℃. In contrast, compound C (10 µmol L^-1 ) reduced J wave at both 37℃ and 30℃. Low-temperature-induced J waves were individually suppressed by 4-AP (50 µmol L^-1 ) and glibenclamide (20 µmol L^-1 ).

CONCLUSIONS

AMPK inhibition reduces low-temperature-induced J waves and possible ventricular arrhythmogenesis by modulating I_KATP and I_Kur channels.

From Genome-Wide Association Studies to Cardiac Electrophysiology: Through the Maze of Biological Complexity

Genome Wide Association Studies (GWAS) have provided an enormous amount of data on genomic loci associated with cardiac electrophysiology and arrhythmias. Clinical relevance, however, remains unclear since GWAS do not provide a mechanistic explanation for this association. Determining the electrophysiological relevance of variants for arrhythmias would aid development of risk stratification models for patients with arrhythmias. In this review, we give an overview of genetic variants related to ECG intervals and arrhythmogenic pathologies and discuss how these variants may influence cardiac electrophysiology and the occurrence of arrhythmias.

Brugada syndrome: A comprehensive review of pathophysiological mechanisms and risk stratification strategies

Brugada syndrome (BrS) is an inherited ion channel channelopathy predisposing to ventricular arrhythmias and sudden cardiac death. Originally believed to be predominantly associated with mutations in SCN5A encoding for the cardiac sodium channel, mutations of 18 genes other than SCN5A have been implicated in the pathogenesis of BrS to date. Diagnosis is based on the presence of a spontaneous or drug-induced coved-type ST segment elevation. The predominant electrophysiological mechanism underlying BrS remains disputed, commonly revolving around the three main hypotheses based on abnormal repolarization, depolarization or current-load match. Evidence from computational modelling, pre-clinical and clinical studies illustrates that molecular abnormalities found in BrS lead to alterations in excitation wavelength (λ), which ultimately elevates arrhythmic risk. A major challenge for clinicians in managing this condition is the difficulty in predicting the subset of patients who will suffer from life-threatening ventricular arrhythmic events. Several repolarization risk markers have been used thus far, but these neglect the contributions of conduction abnormalities in the form of slowing and dispersion. Indices incorporating both repolarization and conduction based on the concept of λ have recently been proposed. These may have better predictive values than the existing markers. Current treatment options include pharmacological therapy to reduce the occurrence of arrhythmic events or to abort these episodes, and interventions such as implantable cardioverter-defibrillator insertion or radiofrequency ablation of abnormal arrhythmic substrate.

NS5806 Induces Electromechanically Discordant Alternans and Arrhythmogenic Voltage-Calcium Dynamics in the Isolated Intact Rabbit Heart

Background: NS5806 activates the transient outward potassium current I _to, and has been claimed to reproduce Brugada Syndrome (BrS) in ventricular wedge preparations. I _to modulates excitation-contraction coupling, which is critical in alternans dynamics. We explored NS5806-arrhythmogenic effects in the intact whole heart and its impact on alternans. Methods: Langendorff-perfused rabbit hearts (n = 20) underwent optical AP and Ca mapping during pacing at decremental cycle lengths (CL). Spontaneous arrhythmias and pacing-induced alternans was characterized at baseline (BL), after perfusing with NS5806, before and after adding verapamil (VP), and SEA0400 (SEA, n = 5 each), to modulate Ca-current and Na-Ca exchange, the main AP-Ca coupling mechanisms. Results: NS5806 induced BrS-like ECG features in 6 out of 20 hearts. NS5806 prolonged steady-state (3 Hz) action potential duration (APD) by 16.8%, Ca decay constant by 34%, and decreased conduction velocity (CV) by 52.6%. After NS5806 infusion, spontaneous ventricular ectopy (VE) and AP/Ca alternans occurred. Pacing-induced alternans during NS5806 infusion occurred at longer CL and were AP/Ca discordant from its onset. Spatially discordant alternans after NS5806 infusion had non-propagation-driven nodal line distribution. No spontaneous phase-2 reentry occurred. Under NS5806 + VP, alternans became AP/Ca concordant and only induced in two out of five; NS5806 + SEA did not affect alternans but suppressed spontaneous ectopy. Conclusions: NS5806 disrupts AP-Ca coupling and leads to Ca-driven, AP/Ca-discordant alternans and VE. Despite BrS-like ECG features, no spontaneous sustained arrhythmias or phase-2 reentry occurred. NS5806 does not fully reproduce BrS in the intact rabbit heart.

The Spectrum of Idiopathic Ventricular Fibrillation and J-Wave Syndromes: Novel Mapping Insights

Idiopathic ventricular fibrillation and J-wave syndromes are causes of sudden cardiac death (SCD) without any identified structural cardiac disease after extensive investigations. Recent data show that high-density electrophysiological mapping may ultimately offer diagnoses of subclinical diseases in most patients including those termed "unexplained" SCD. Three major conditions can underlie the occurrence of SCD: (1) localized depolarization abnormalities (due to microstructural myocardial alteration), (2) Purkinje abnormalities manifesting as triggering ectopy and inducible reentry; or (3) repolarization heterogeneities. Each condition may result from a spectrum of pathophysiologic processes with implications for individual therapy.

New electromechanical substrate abnormalities in high-risk patients with Brugada syndrome

BACKGROUND

The relationship between the typical electrocardiographic pattern and electromechanical abnormalities has never been systematically explored in Brugada syndrome (BrS).

OBJECTIVES

The aims of this study were to characterize the electromechanical substrate in patients with BrS and to evaluate the relationship between electrical and mechanical abnormalities.

METHODS

We enrolled 50 consecutive high-risk patients with BrS (mean age 42 ± 7.2 years), with implantable cardioverter-defibrillator implantation for primary or secondary prevention of ventricular tachyarrhythmias (ventricular tachycardia/ventricular fibrillation [VT/VF]), undergoing substrate mapping and ablation. Patients underwent 3-dimensional (3D) echocardiography with 3D wall motion/deformation quantification and electroanatomic mapping before and after ajmaline administration (1 mg/kg in 5 minutes); 3D mechanical changes were compared with 50 age- and sex-matched controls. The effect of substrate ablation on electromechanical abnormalities was also assessed.

RESULTS

In all patients, ajmaline administration induced Brugada type 1 pattern, with a significant increase in the electrical substrate (P < .001), particularly in patients with previous spontaneous VT/VF (P = .007). Induction of Brugada pattern was associated with lowering of right ventricular (RV) ejection fraction (P < .001) and worsening of 3D RV mechanical function (P < .001), particularly in the anterior free wall of the RV outflow tract, without changes in controls. RV electrical and mechanical abnormalities were highly correlated (r = 0.728, P < .001). By multivariate analysis, only the area of RV dysfunction was an independent predictor of spontaneous VT/VF (odds ratio 1.480; 95% confidence interval 1.159-1.889; P = .002). Substrate ablation abolished both BrS-electrocardiographic pattern and mechanical abnormalities, despite ajmaline rechallenge.

CONCLUSION

BrS is an electromechanical disease affecting the RV. The typical BrS pattern reflects an extensive RV arrhythmic substrate, driving consistent RV mechanical abnormalities. Substrate ablation abolished both Brugada pattern and mechanical abnormalities.

Pharmacological Modulation of Right Ventricular Endocardial-Epicardial Gradients in Brugada Syndrome

Background We explored the hypothesis that increased cholinergic tone exerts its proarrhythmic effects in Brugada syndrome (BrS) through increasing dispersion of transmural repolarization in patients with spontaneous and drug-induced BrS. Methods BrS and supraventricular tachycardia patients were studied after deploying an Ensite Array in the right ventricular outflow tract and a Cardima catheter in the great cardiac vein to record endo and epicardial signals, respectively. S₁-S₂ restitution curves from the right ventricular apex were conducted at baseline and after edrophonium challenge to promote increased cholinergic tone. The local unipolar electrograms were then analyzed to study transmural conduction and repolarization dynamics. Results The study included 8 BrS patients (5 men:3 women; mean age, 56 years) and 8 controls patients with supraventricular tachycardia (5 men:3 women; mean age, 48 years). Electrophysiological studies in controls demonstrated shorter endocardial than epicardial right ventricular activation times (mean difference: 26 ms; P<0.001). In contrast, patients with BrS showed longer endocardial than epicardial activation time (mean difference: -15 ms; P=0.001). BrS hearts, compared with controls, showed significantly larger transmural gradients in their activation recovery intervals (mean intervals, 20.5 versus 3.5 ms; P<0.01), with longer endocardial than epicardial activation recovery intervals. Edrophonium challenge increased such gradients in both controls (to a mean of 16 ms [ P<0.001]) and BrS (to 29.7 ms; P<0.001). However, these were attributable to epicardial and endocardial activation recovery interval prolongations in control and BrS hearts, respectively. Dynamic changes in repolarization gradients were also observed across the BrS right ventricular wall in BrS. Conclusions Differential contributions of conduction and repolarization were identified in BrS which critically modulated transmural dispersion of repolarization with significant cholinergic effects only identified in the patients with BrS. This has important implications for explaining the proarrhythmic effects of increased vagal tone in BrS, as well as evaluating autonomic modulation and epicardial ablation as therapeutic strategies.

Early repolarization in the inferior leads after accessory pathway ablation is highly correlated with atrial fibrillation in Wolff-Parkinson-White syndrome

BACKGROUND

Both atrial fibrillation (AF) and early repolarization (ER) are highly prevalent in patients with Wolff-Parkinson-White (WPW) syndrome.

METHODS

We retrospectively identified 123 WPW patients with manifest accessory pathway (mAP) and 105 patients with concealed accessory pathways (cAP) who underwent successful ablation of the APs. AF history prior to the ablation was investigated. The presence of ER was evaluated from the 12-lead electrocardiograms performed both before and immediately after the ablation. Patients were further followed up for AF occurrence after the AP ablation.

RESULTS

Compared with cAP patients, WPW patients presented with higher incidence of previous AF, as well as ER after the AP ablation. Compared with those without AF history, WPW patients with comorbid AF were more prone to presenting ER, especially in the inferior leads, after the mAP ablation. Regression analysis suggested that the post-ablation ER in the inferior leads was strongly associated with AF in WPW patients [OR = 5.85; 95% confidence interval (CI): 2.29-14.96], even after adjusting for age and left atrial diameter (OR = 5.14; 95% CI: 1.80-14.74). Moreover, post-ablation inferior-lead ER was predictive of AF recurrence after mAP ablation during the follow-up of 22.6 ± 11.1 months. In comparison, ER was correlated with neither AF history nor AF recurrence in patients undergoing cAP ablation.

CONCLUSIONS

ER in the inferior leads after the mAP ablation is highly correlated with AF history and recurrence in WPW patients.

Depolarization versus repolarization abnormality underlying inferolateral J-wave syndromes: New concepts in sudden cardiac death with apparently normal hearts

Early repolarization indicates a distinct electrocardiographic phenotype affecting the junction between the QRS complex and the ST segment in inferolateral leads (inferolateral J-wave syndromes). It has been considered a benign electrocardiographic variant for decades, but recent clinical studies have demonstrated its arrhythmogenicity in a small subset, supported by experimental studies showing transmural dispersion of repolarization. Here we review the current knowledge and the issues of risk stratification that limit clinical management. In addition, we report on new mapping data of patients refractory to pharmacologic treatment using high-density electrogram mapping at the time of inscription of J wave. These data demonstrate that distinct substrates, delayed depolarization, and abnormal early repolarization underlie inferolateral J-wave syndromes, with significant implications. Finally, based on these data, we propose a new simplified mechanistic classification of sudden cardiac deaths without apparent structural heart disease.

QRS alternans due to localized intraventricular block during ventricular tachycardia in Uhl's anomaly: a case report

Background

Uhl's anomaly is a rare congenital heart disease characterized by 'Paper-like thinning of the right ventricular (RV) wall'. Since most patients with Uhl's anomaly die in infancy or childhood, the adult cases of this disorder have been limitedly reported and there were no past report describing this anomaly with ventricular tachycardia (VT), in which catheter ablation was successfully performed.

Case summary

We report the case of a 43-year-old man with a suggested Uhl's anomaly and VT. He underwent a catheter ablation of the recurrent VT. An electrophysiological study showed the VT (cycle length = 460 ms) with a right bundle branch block pattern and inferior axis represented QRS alternans. Concealed entrainment by pacing from the anterior right ventricle (RV) suggested the mechanism of this VT was re-entry and the QRS alternans was simultaneously reproduced during the pacing. Furthermore, the intracardiac electrograms exhibiting local conduction blocks were documented on the RV free wall near the apex. The electrograms seen in a 2:1 ratio were coincident with slurs in the initial QRS on the 12-lead electrocardiogram. Since the anterior RV wall was considered to be a part of the VT circuit, a linear ablation in that area was performed resulting in VT termination.

Discussion

The local conduction blocks near the RV apex were related to the mechanism of the QRS alternans during the VT in this case. That might be based on the variations in the myocardium lying in the RV, which is the specific feature of Uhl's anomaly.

Clinical Characteristics and Electrophysiological Mechanisms Underlying Brugada ECG in Patients With Severe Hyperkalemia

Background Several metabolic conditions can cause the Brugada ECG pattern, also called Brugada phenotype (BrPh). We aimed to define the clinical characteristics and outcome of BrPh patients and elucidate the mechanisms underlying BrPh attributed to hyperkalemia. Methods and Results We prospectively identified patients hospitalized with severe hyperkalemia and ECG diagnosis of BrPh and compared their clinical characteristics and outcome with patients with hyperkalemia but no BrPh ECG. Computer simulations investigated the roles of extracellular potassium increase, fibrosis at the right ventricular outflow tract, and epicardial/endocardial gradients in transient outward current. Over a 6-year period, 15 patients presented severe hyperkalemia with BrPh ECG that was transient and disappeared after normalization of their serum potassium. Most patients were admitted because of various severe medical conditions causing hyperkalemia. Six (40%) patients presented malignant arrhythmias and 6 died during admission. Multiple logistic regression analysis revealed that higher serum potassium levels (odds ratio, 15.8; 95% CI, 3.1-79; P=0.001) and male sex (odds ratio, 17; 95% CI, 1.05-286; P=0.045) were risk factors for developing BrPh ECG in patients with severe hyperkalemia. In simulations, hyperkalemia yielded BrPh by promoting delayed and heterogeneous right ventricular outflow tract activation attributed to elevation of resting potential, reduced availability of inward sodium channel conductance, and increased right ventricular outflow tract fibrosis. An elevated transient outward current gradient contributed to, but was not essential for, the BrPh phenotype. Conclusions In patients with severe hyperkalemia, a BrPh ECG is associated with malignant arrhythmias and all-cause mortality secondary to resting potential depolarization, reduced sodium current availability, and fibrosis at the right ventricular outflow tract.

Clinical significance of J wave in prediction of ventricular arrhythmia in patients with acute myocardial infarction

BACKGROUND

J wave syndrome and myocardial ischemia are related with malignant ventricular arrhythmia (VA). The characteristics of dynamic J wave in patients with early phase of acute myocardial infarction (AMI) and subsequent VA or electrical storm (ES) have not been well evaluated.

OBJECTIVE

We investigated the utility of J wave in the prediction of VA and ES in patients within the early phase of AMI.

METHODS

This study retrospectively enrolled 208 patients (mean age 69±15 years, 171 males) with AMI. Of them, 50 patients had experienced VA during hospitalization and 24 had ES. The clinical and electrocardiographic characteristics of these patients with and without VA were compared.

RESULTS

Patients with VA had a higher incidence of chronic kidney disease (CKD) and J wave compared with those without VA. The hazard ratio (HR) of J wave for VA was 4.31 (p<0.01) and CKD was 2.64 (p<0.01). In the VA group, ES patients had a higher incidence of diabetes mellitus (DM) (HR 2.73, p=0.02) and J wave (HR 4.21, p<0.01). If the AMI patients had J wave, the OR for mortality was 2.14 (p=0.03), VA events was 6.23 (p<0.01), and ES events was 12.15 (p<0.01). If VA patients had J wave, the mortality rate will significantly increase (OR 68.62, p=0.01).

CONCLUSION

The AMI patients who develop VA in the early phase of AMI had a higher incidence of J wave and CKD, and those who develop ES had a higher incidence of J wave and DM. It seems that J wave in AMI patients is a poor prognostic factor, and we found that J wave will increase mortality, VA events, and ES events. The majority locations of J wave were inferior leads although there was no relationship between the locations and VA incidence. If the VA patients had inferior or lateral J wave, it would further increase the risk of mortality.

Scalable and Accurate ECG Simulation for Reaction-Diffusion Models of the Human Heart

Realistic electrocardiogram (ECG) simulation with numerical models is important for research linking cellular and molecular physiology to clinically observable signals, and crucial for patient tailoring of numerical heart models. However, ECG simulation with a realistic torso model is computationally much harder than simulation of cardiac activity itself, so that many studies with sophisticated heart models have resorted to crude approximations of the ECG. This paper shows how the classical concept of electrocardiographic lead fields can be used for an ECG simulation method that matches the realism of modern heart models. The accuracy and resource requirements were compared to those of a full-torso solution for the potential and scaling was tested up to 14,336 cores with a heart model consisting of 11 million nodes. Reference ECGs were computed on a 3.3 billion-node heart-torso mesh at 0.2 mm resolution. The results show that the lead-field method is more efficient than a full-torso solution when the number of simulated samples is larger than the number of computed ECG leads. While the initial computation of the lead fields remains a hard and poorly scalable problem, the ECG computation itself scales almost perfectly and, even for several hundreds of ECG leads, takes much less time than the underlying simulation of cardiac activity.

KV4.3 Expression Modulates NaV1.5 Sodium Current

In cardiomyocytes, the voltage-gated transient outward potassium current (I_to) is responsible for the phase-1 repolarization of the action potential (AP). Gain-of-function mutations in KCND3, the gene encoding the I_to carrying K_V4.3 channel, have been associated with Brugada syndrome (BrS). While the role of I_to in the pro-arrhythmic mechanism of BrS has been debated, recent studies have suggested that an increased I_to may directly affect cardiac conduction. However, the effects of an increased I_to on AP upstroke velocity or sodium current at the cellular level remain unknown. We here investigated the consequences of K_V4.3 overexpression on Na_V1.5 current and consequent sodium channel availability. We found that overexpression of K_V4.3 protein in HEK293 cells stably expressing Na_V1.5 (HEK293-Na_V1.5 cells) significantly reduced Na_V1.5 current density without affecting its kinetic properties. In addition, K_V4.3 overexpression decreased AP upstroke velocity in HEK293-Na_V1.5 cells, as measured with the alternating voltage/current clamp technique. These effects of K_V4.3 could not be explained by alterations in total Na_V1.5 protein expression. Using computer simulations employing a multicellular in silico model, we furthermore demonstrate that the experimentally observed increase in K_V4.3 current and concurrent decrease in Na_V1.5 current may result in a loss of conduction, underlining the potential functional relevance of our findings. This study gives the first proof of concept that K_V4.3 directly impacts on Na_V1.5 current. Future studies employing appropriate disease models should explore the potential electrophysiological implications in (patho)physiological conditions, including BrS associated with KCND3 gain-of-function mutations.

Brugada syndrome in children - Stepping into unchartered territory

Brugada syndrome (BrS) is an autosomal dominant inherited channelopathy. It is associated with a typical pattern of ST-segment elevation in the precordial leads V1-V3 and potentially lethal ventricular arrhythmias in otherwise healthy patients. It is frequently seen in young Asian males, in whom it has previously been described as sudden unexplained nocturnal death syndrome. Although it typically presents in young adults, it is also known to present in children and infants, especially in the presence of fever. Our understanding of the genetic pathogenesis and management of BrS has grown substantially considering that it has only been 24 years since its first description as a unique clinical entity. However, there remains much to be learned, especially in the pediatric population. This review aims to discuss the epidemiology, genetics, and pathogenesis of BrS. We will also discuss established standards and new innovations in the diagnosis, prognostication, risk stratification, and management of BrS. Literature search was run on the National Center for Biotechnology Information's website, using the Medical Subject Headings (MeSH) database with the search term "Brugada Syndrome" (MeSH), and was run on the PubMed database using the age filter (birth-18 years), yielding 334 results. The abstracts of all these articles were studied, and the articles were categorized and organized. Articles of relevance were read in full. As and where applicable, relevant references and citations from the primary articles were further explored and read in full.

Readthrough-Promoting Drugs Gentamicin and PTC124 Fail to Rescue Nav1.5 Function of Human-Induced Pluripotent Stem Cell-Derived Cardiomyocytes Carrying Nonsense Mutations in the Sodium Channel Gene SCN5A

BACKGROUND

Several compounds have been reported to induce translational readthrough of premature stop codons resulting in the production of full-length protein by interfering with ribosomal proofreading. Here we examined the effect of 2 of these compounds, gentamicin and PTC124, in human-induced pluripotent stem cell (hiPSC)-derived cardiomyocytes bearing nonsense mutations in the sodium channel gene SCN5A, which are associated with conduction disease and potential lethal arrhythmias.

METHODS AND RESULTS

We generated hiPSC from 2 patients carrying the mutations R1638X and W156X. hiPSC-derived cardiomyocytes from both patients recapitulated the expected electrophysiological phenotype, as evidenced by reduced Na⁺ currents and action potential upstroke velocities compared with hiPSC-derived cardiomyocytes from 2 unrelated control individuals. While we were able to confirm the readthrough efficacy of the 2 drugs in Human Embryonic Kidney 293 cells, we did not observe rescue of the electrophysiological phenotype in hiPSC-derived cardiomyocytes from the patients.

CONCLUSIONS

We conclude that these drugs are unlikely to present an effective treatment for patients carrying the loss-of-function SCN5A gene mutations examined in this study.

Hypothetical "anatomy" of Brugada phenomenon: "Long QT sine Long QT" syndrome implicating morphologically undefined specific "Brugada's myocells"

The Brugada syndrome (BrS) is associated with increased risk of ventricular arrhythmias and sudden cardiac death. It generates genetically mediated arrhythmias posing a true pathophysiological challenge. In search of the similarities between BrS and long QT syndrome some novel insights are suggested. In patients with BrS the duration of QT interval is usually normal. Some investigators have found prolonged QT interval in the syndrome's natural course or the duration of QT segment have been extended by provocative tests unmasking BrS. Thus, BrS might be characterized as "long QT sine long QT" syndrome. The existence of two functional types of myocites is suspected. Regarding structure and function the majority of ventricular myocardium is probably mostly healthy. The rest of myocardium (preferably the subepicardium of right ventricular outflow tract) due to its genotypic peculiarities demonstrates no negative influence on ventricular performance until early adulthood is reached and/or other unstable preconditions are fulfilled (nocturnal time, fever, specific drugs, etc.). Based on published findings of positive outcomes, following the epicardial ablation of the right ventricular outflow tract region, a new hypothetical concept suggesting the presence of specific, genetically affected "Brugada's myocells" is proposed. These cells as a suitable arrhythmogenic substrate reside intramurally within the subepicardial region of the outflow tract of right ventricle. In the daytime these cells likely are dormant but at rest their nocturnal proarrhythmic behavior is activated occasionally. Presumptions regarding the pathophysiology of BrS might be the focus of further discussion.

Local Left Ventricular Epicardial J Waves and Late Potentials in Brugada Syndrome Patients with Inferolateral Early Repolarization Pattern

Background: Brugada syndrome (BrS) is characterized by J-point or ST-segment elevation on electrocardiograms (ECGs) and increased risk of ventricular fibrillation (VF). In BrS, epicardial depolarization abnormality with delayed potential on the right ventricular outflow tract is reportedly the predominant mechanism underlying VF. Yet VF occurrence is also associated with early repolarization (ER) pattern in the inferolateral ECG leads, which may represent the inferior and/or left lateral ventricular myocardium. The aim of this study was to examine epicardial electrograms recorded directly at the left ventricle (LV) in BrS patients after VF episodes.

Methods: In 12 BrS patients who had experienced VF episodes and 17 control subjects, a multipolar catheter was introduced into the left lateral coronary vein for unipolar and bipolar electrogram recordings at the LV epicardium. Both inferior and lateral ER patterns on ECG were observed in three BrS patients and six control subjects.

Results: In the epicardium, prominent J waves were detected using unipolar recording, and potentials after the QRS complex were detected using bipolar recording in three of the 12 BrS patients. These three patients also showed both inferior and lateral ER patterns on ECG. Neither prominent J waves nor potentials after the QRS complex were recorded at the endocardium of the LV in any of these three patients; nor were they seen at the epicardium in any of the control subjects. These features were accentuated on pilsicainide administration (n = 2) but diminished on constant atrial pacing (n = 3) and isoproterenol administration (n = 1). The J waves observed through unipolar recording coincided with the potentials after QRS complex observed through bipolar recording and with the inferolateral ER patterns on ECG.

Conclusions: We recorded prominent J waves in unipolar electrogram and potentials after QRS complex in bipolar electrogram at the LV epicardium in BrS patients with global ER pattern. The prominent J waves coincided with the potentials after QRS complex and the inferolateral ER pattern on ECG. The characteristics of the inferolateral ER pattern on ECG in these patients primarily represent depolarization feature.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.

Electrophysiological Mechanisms of Brugada Syndrome: Insights from Pre-clinical and Clinical Studies

Brugada syndrome (BrS), is a primary electrical disorder predisposing affected individuals to sudden cardiac death via the development of ventricular tachycardia and fibrillation (VT/VF). Originally, BrS was linked to mutations in the SCN5A, which encodes for the cardiac Na+ channel. To date, variants in 19 genes have been implicated in this condition, with 11, 5, 3, and 1 genes affecting the Na+, K+, Ca2+, and funny currents, respectively. Diagnosis of BrS is based on ECG criteria of coved- or saddle-shaped ST segment elevation and/or T-wave inversion with or without drug challenge. Three hypotheses based on abnormal depolarization, abnormal repolarization, and current-load-mismatch have been put forward to explain the electrophysiological mechanisms responsible for BrS. Evidence from computational modeling, pre-clinical, and clinical studies illustrates that molecular abnormalities found in BrS lead to alterations in excitation wavelength (λ), which ultimately elevates arrhythmic risk. A major challenge for clinicians in managing this condition is the difficulty in predicting the subset of patients who will suffer from life-threatening VT/VF. Several repolarization risk markers have been used thus far, but these neglect the contributions of conduction abnormalities in the form of slowing and dispersion. Indices incorporating both repolarization and conduction and based on the concept of λ have recently been proposed. These may have better predictive values than the existing markers.