The M cell: its contribution to the ECG and to normal and abnormal electrical function of the heart

Theoretical prediction of early afterdepolarization-evoked triggered activity formation initiating ventricular reentrant arrhythmias

BACKGROUND AND OBJECTIVE

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Predicting Long-Term Ventricular Arrhythmia Risk in Children with Acute Lymphoblastic Leukemia Using Normal Values of Ventricular Repolarization Markers Established from Japanese Cohort Study

BACKGROUND

Cardiac complications due to anthracycline treatment may become evident several years after chemotherapy and are recognized as a serious cause of morbidity and mortality in cancer patients or childhood cancer survivors.

OBJECTIVES

We analyzed ventricular repolarization parameters in electrocardiography for pediatric acute lymphoblastic leukemia patients during chemotherapy and in long-term follow-up. To establish the reference values of ventricular repolarization parameters in children, we retrospectively summarized the Tpe interval, QT interval, QTc interval, and Tpe/QT ratio in healthy Japanese children.

METHODS

Electrocardiography data recorded from students in 1st and 7th grades were randomly selected from a database maintained by the school-based screening system in the Oita city cohort, Japan. Subsequently, chronological data of the Tpe/QT ratio in 17 pediatric patients with acute lymphoblastic leukemia were analyzed over time.

RESULTS

The mean ± standard deviation of the Tpe interval in 1st and 7th graders was 70 ± 7 and 78 ± 17 ms, respectively, while the mean ± standard deviation of the Tpe/QT ratio was 0.21 ± 0.02 and 0.22 ± 0.02 ms, respectively. During the intensive phase of treatment, the Tpe/QT ratios of 3 high-risk patients among the 17 patients with acute lymphoblastic leukemia exceeded the upper limit.

CONCLUSION

The Tpe/QT ratio has a potential clinical application in predicting the risk of long-term ventricular arrhythmia of cancer patients or childhood cancer survivors from childhood to adulthood.

T-Wave Analysis on the 24 h Holter ECG Monitoring as a Predictive Assessment of Major Adverse Cardiovascular Events in Patients with Myocardial Infarction: A Literature Review and Future Perspectives

Myocardial ischemia is a pathophysiological state characterized by inadequate perfusion of the myocardium, resulting in an imbalance between myocardial oxygen demand and supply. It is most commonly caused by coronary artery disease, in which atherosclerotic plaques lead to luminal narrowing and reduced blood flow to the heart. Myocardial ischemia can manifest as angina pectoris or silent myocardial ischemia and can progress to myocardial infarction or heart failure if left untreated. Diagnosis of myocardial ischemia typically involves a combination of clinical evaluation, electrocardiography and imaging studies. Electrocardiographic parameters, as assessed by 24 h Holter ECG monitoring, can predict the occurrence of major adverse cardiovascular events in patients with myocardial ischemia, independent of other risk factors. The T-waves in patients with myocardial ischemia have prognostic value for predicting major adverse cardiovascular events, and their electrophysiological heterogeneity can be visualized using various techniques. Combining the electrocardiographic findings with the assessment of myocardial substrate may offer a better picture of the factors that can contribute to cardiovascular death.

The U-wave: A remaining enigma of the electrocardiogram

The U-wave's electrophysiological origin remains unknown and is subject to debate. It is rarely used for diagnosis in clinical practice. The aim of this study was to review new information regarding the U-wave. Further to present the proposed theories behind the U-wave's origin along with potential pathophysiologic and prognostic implications related to its presence, polarity and morphology.

METHOD

Literature searches were conducted to retrieve publications related to the electrocardiogram U-wave in the literature database Embase.

RESULTS

The review of the literature revealed the following major theories that will be discussed; late depolarisation, delayed or prolonged repolarisation, electro-mechanical stretch and IK1 dependent intrinsic potential differences in the terminal part of the action potential. Various pathologic conditions were found to correlate with the presence and properties of the U-wave, such as its amplitude and polarity. Abnormal U-waves can, for example, be observed in coronary artery disease with ongoing myocardial ischemia or infarction, ventricular hypertrophy, congenital heart disease, primary cardiomyopathy and valvular defects. Negative U-waves are highly specific for the presence of heart diseases. Concordantly negative T- and U-waves are especially associated with cardiac disease. Patients with negative U-waves tend to have higher blood pressure and history of hypertension, higher heart rate, cardiac disease and left ventricular hypertrophy compared to subjects with normal U-waves. Negative U-waves have been found to be associated with increased risk of all-cause mortality, cardiac death and cardiac hospitalisation in men.

CONCLUSIONS

The origin of the U-wave is still not established. U-wave diagnostics may reveal cardiac disorders and the cardiovascular prognosis. Including the U-wave characteristics in the clinical ECG assessment may be useful.

The predictive value of electrocardiographic polarization parameters on appropriate ICD shock in primary prevention heart failure patients

OBJECT

The effect of frontal QRS-T angle, Tp-e and Tp-e/QT ratio on cardiac events have been shown in many studies. In this study, we aimed to determine the prognostic value of frontal QRS-T angle, TPe and Tp-e/QT ratio on ICD shock in patients who had ICD (Implantable Cardioverter Defibrillator) implanted due to heart failure with reduced ejection fraction (HFrEF).

MATERIAL AND METHOD

158 patients with HFrEF who had previous ICD implantation were retrospectively analyzed. 27 patients were found to have an appropriate shock. Frontal QRS-T angle, Tp-e interval, Tp-e/QT ratio were calculated by evaluating the basal ECG records of the patients. Comparisons of these arrhythmogenic predictors were made in patients with and without ICD shock at follow-up.

RESULT

When 158 patients with previous ICD implantation were analyzed in two groups with and without ICD shock, the number of patients with frontal QRS-T angle >120°, Tp-e interval > 105 ms, Tp-e/QT > 0.2 in the shock group (n: 27) was found to be high with a different significance (p:<0.01, p:<0.01, p:<0.01). There was no significant difference between the two groups regarding other ECG parameters such as QRS duration, QT interval, PR interval, fragmented QRS and positive T wave. In addition, more amiodarone use was observed in the shock group, and more hyperlipidemia cases were observed in the non-shocked group (p:0.01; p:<0.01).

CONCLUSION

Increased frontal QRS-T angle, Tp-e interval, and Tp-e/QT ratio are arrhythmogenic parameters and predict appropriate ICD shock.

Anti-malarial drugs: Mechanisms underlying their proarrhythmic effects

Malaria remains the leading cause of parasitic death in the world. Artemisinin resistance is an emerging threat indicating an imminent need for novel combination therapy. Given the key role of mass drug administration, it is pivotal that the safety of anti-malarial drugs is investigated thoroughly prior to widespread use. Cardiotoxicity, most prominently arrhythmic risk, has been a concern for anti-malarial drugs. We clarify the likely underlying mechanisms by which anti-malarial drugs predispose to arrhythmias. These relate to disruption of (1) action potential upstroke due to effects on the sodium currents, (2) action potential repolarisation due to effects on the potassium currents, (3) cellular calcium homeostasis, (4) mitochondrial function and reactive oxygen species production and (5) cardiac fibrosis. Together, these alterations promote arrhythmic triggers and substrates. Understanding these mechanisms is essential to assess the safety of these drugs, stratify patients based on arrhythmic risk and guide future anti-malarial drug development.

Ventricular arrhythmias in acute myocardial ischaemia-Focus on the ageing and sex

Annually, approximately 17 million people die from cardiovascular diseases worldwide, half of them suddenly. The most common direct cause of sudden cardiac death is ventricular arrhythmia triggered by an acute coronary syndrome (ACS). The study summarizes the knowledge of the mechanisms of arrhythmia onset during ACS in humans and in animal models and factors that may influence the susceptibility to life-threatening arrhythmias during ACS with particular focus on the age and sex. The real impact of age and sex on the arrhythmic susceptibility within the setting of acute ischaemia is masked by the fact that ACSs result from coronary artery disease appearing with age much earlier among men than among women. However, results of researches show that in ageing process changes with potential pro-arrhythmic significance, such as increased fibrosis, cardiomyocyte hypertrophy, decrease number of gap junction channels, disturbances of the intracellular Ca²⁺ signalling or changes in electrophysiological parameters, occur independently of the development of cardiovascular diseases and are more severe in male individuals. A review of the literature also indicates a marked paucity of research in this area in female and elderly individuals. Greater awareness of sex differences in the aging process could help in the development of personalized prevention methods targeting potential pro-arrhythmic factors in patients of both sexes to reduce mortality during the acute phase of myocardial infarction. This is especially important in an era of aging populations in which women will predominate due to their longer lifespan.

Tpeak-Tend Interval during Pregnancy and Postpartum

BACKGROUND

Pregnancy is a condition in which new cardiac arrhythmias can occur or prior undiagnosed arrhythmias may provide symptoms. The occurrence of severe ventricular arrhythmias and polymorphic ventricular tachycardia that may lead to fainting or sudden cardiac death is promoted by the prolongation of the QTc interval. The post-partum adaptation period is the most arrhythmogenic. TpTe (Tpeak-Tend interval) is a novel marker of arrhythmogenesis by many considered a more sensitive marker than QTc.

OBJECTIVE

The aim of our work was to determine the TpTe interval (Tpeak-Tend) in women in the first, second and third trimester of pregnancy and the post-partum period.

MATERIALS AND METHODS

The study group consisted of 128 women in pregnancy or postpartum and a control group of 32 non-pregnant women. A standard 12-lead ECG (electrocardiograph) recording with evaluation of the duration of TpTe and QTc was performed in all patients.

RESULTS

In comparison to the non-pregnant women, higher values of QTc and TpTe were observed starting in the first trimester with highest values observed in the postpartum period. Mean duration of TpTe interval during pregnancy (81.59 ± 5.92 ms) and in the whole study group (pregnancy + postpartum) (85.46 ± 6.45 ms) was significantly longer (p < 0.001) compared to the TpTe interval in the control group (74.06 ± 6.14 ms). During pregnancy and postpartum, the increase in the TpTe interval in comparison to the increase in the QTc parameter (31.10% vs. 4.18%) was significantly higher (p < 0.001).

CONCLUSIONS

The study showed an increase in the duration of the TpTe interval and QTc parameter during pregnancy and postpartum with the highest values in the postpartum period. TpTe interval increase was significantly higher compared to QTc increase during pregnancy and postpartum. Changes of TpTe interval were not associated with any clinical outcome or measure of arrythmia burden. Further studies are needed in order to see the clinical significance of these ECG findings, in particular for larger groups of patients with automatic measurement in correlation with echocardiography.

A Novel In Silico Electromechanical Model of Human Ventricular Cardiomyocyte

Contractility has become one of the main readouts in computational and experimental studies on cardiomyocytes. Following this trend, we propose a novel mathematical model of human ventricular cardiomyocytes electromechanics, BPSLand, by coupling a recent human contractile element to the BPS2020 model of electrophysiology. BPSLand is the result of a hybrid optimization process and it reproduces all the electrophysiology experimental indices captured by its predecessor BPS2020, simultaneously enabling the simulation of realistic human active tension and its potential abnormalities. The transmural heterogeneity in both electrophysiology and contractility departments was simulated consistent with previous computational and in vitro studies. Furthermore, our model could capture delayed afterdepolarizations (DADs), early afterdepolarizations (EADs), and contraction abnormalities in terms of aftercontractions triggered by either drug action or special pacing modes. Finally, we further validated the mechanical results of the model against previous experimental and in silico studies, e.g., the contractility dependence on pacing rate. Adding a new level of applicability to the normative models of human cardiomyocytes, BPSLand represents a robust, fully-human in silico model with promising capabilities for translational cardiology.

An Automata-Based Cardiac Electrophysiology Simulator to Assess Arrhythmia Inducibility

Personalized cardiac electrophysiology simulations have demonstrated great potential to study cardiac arrhythmias and help in therapy planning of radio-frequency ablation. Its application to analyze vulnerability to ventricular tachycardia and sudden cardiac death in infarcted patients has been recently explored. However, the detailed multi-scale biophysical simulations used in these studies are very demanding in terms of memory and computational resources, which prevents their clinical translation. In this work, we present a fast phenomenological system based on cellular automata (CA) to simulate personalized cardiac electrophysiology. The system is trained on biophysical simulations to reproduce cellular and tissue dynamics in healthy and pathological conditions, including action potential restitution, conduction velocity restitution and cell safety factor. We show that a full ventricular simulation can be performed in the order of seconds, emulate the results of a biophysical simulation and reproduce a patient’s ventricular tachycardia in a model that includes a heterogeneous scar region. The system could be used to study the risk of arrhythmia in infarcted patients for a large number of scenarios.

Conduction system pacing versus biventricular pacing: Reduced repolarization heterogeneity in addition to improved depolarization

INTRODUCTION

His-bundle pacing (HBP) and left-bundle-area pacing (LBAP) are conduction system pacing (CSP) modalities increasingly used as alternatives to conventional biventricular pacing (BiVP). While effects of CSP on ventricular depolarization have been reported, effects on ventricular repolarization have not.

METHODS

QRS duration (QRSd) and validated ECG parameters of ventricular repolarization associated with arrhythmic risk (T-peak-to-T-end_Transmural , T-peak-to-T-end_Total , T-peak dispersion, QTc, QTc dispersion) were analyzed post-implant in 107 patients: 60 with CSP (HBP: n = 35, LBAP: n = 25) and 47 with BiVP. T-wave memory resolution and QTc shortening were analyzed on ECGs obtained ≥25 days post-implant. Twenty blinded measurements were obtained by both authors to assess Interobserver variability.

RESULTS

Although QRSd was shorter with HBP versus LBAP (119 ± 7 ms vs. 132 ± 9 ms, p = .02), there were no significant differences in any repolarization parameters between these methods of CSP. However, when comparing CSP (HBP + LBAP) to BiVP, both QRSd (125 ± 5 ms vs. 147 ± 7 ms, p < .0001) and repolarization parameters (T-peak-to-T-end_Transmural : 83 ± 5 ms vs. 107 ± 8 ms; T-peak-to-T-end_Total : 110 ± 7 ms vs. 137 ± 10 ms; QTc: 470 ± 12 ms vs. 506 ± 12 ms; all p ≤ .0001) were significantly shorter with CSP. Improved T-peak-to-T-end values were unrelated to pre-implant QRSd or LV function. Interobserver variability was 4.6 ± 1.9 ms. Frontal QRS-T angle narrowing (132° to 104°, p = .001) and QTc shortening (483 ± 13 ms to 464 ± 12 ms, p = .008) were seen only with CSP.

CONCLUSIONS

In addition to improved depolarization, CSP reduced repolarization heterogeneity and provided greater T-wave memory resolution as compared to BiVP. Both modalities of CSP (HBP + LBAP) resulted in comparably reduced repolarization heterogeneity regardless of baseline QRSd and LV function. These observations may confer lower arrhythmogenic risk and warrant further study.

Long-Term Prognostic Significance of Ventricular Repolarization Dispersion in Patients with Cardiac Sarcoidosis

Cardiac sarcoidosis (CS) is frequently complicated by fatal ventricular arrhythmias. T-peak to T-end interval to QT interval ratio (TpTe/QT) on electrocardiograms (ECG) was proposed as a marker of ventricular repolarization dispersion. Although this ratio could be associated with the incidence of ventricular arrhythmias in cardiovascular diseases, its prognostic implication in patients with CS is unclear. We sought to investigate whether TpTe/QT was associated with long-term clinical outcomes in patients with CS. Ninety consecutive patients with CS in 2 tertiary hospitals who had ECG data before initiation of immunosuppressive therapy between November 1995 and March 2019 were examined. The primary outcome was a composite of advanced atrioventricular block, ventricular tachycardia or ventricular fibrillation (VT/VF), heart failure hospitalization, and all-cause death. During a median follow-up period of 4.70 (interquartile range 2.06-7.23) years, the primary outcome occurred in 21 patients (23.3%). Survival analyses revealed that the primary outcome (p < 0.001), especially VT/VF or sudden cardiac death (p = 0.002), occurred more frequently in patients with higher TpTe/QT (≥ 0.242, the median) than in those with lower TpTe/QT. Multivariable Cox regression analysis showed that a higher TpTe/QT was independently associated with increased subsequent risk of adverse events (hazard ratio1.11, 95% confidence interval 1.03-1.20, p = 0.008) even after adjustment for the significant covariates. In conclusion, a higher TpTe/QT was associated with worse long-term clinical outcomes, especially fatal ventricular arrhythmic events, in patients with cardiac sarcoidosis, suggesting the importance of assessing TpTe/QT as a surrogate for risk stratification in these patients.

Importance of electrocardiographic markers in predicting cardiac events in children

ECG is a common diagnostic tool in medical practice. Sudden cardiac death (SCD) is a rare but devastating event. The most common cause of SCD in the young is a primary arrhythmic event, which is often produced by malignant ventricular arrhythmia. Several electrocardiographic markers for ventricular repolarization and depolarization have been proposed to predict this arrhythmic risk and SCD in children. Although many of these parameters can easily be used in clinical practice, some of them need specific techniques for interpretation. In this review, we summarized the current knowledge regarding the clinical importance and the ability of these ECG parameters to predict adverse cardiac events in the pediatric population.

Tp-e and (Tp-e)/QT ratio as a non-invasive risk factors for malignant ventricular arrhythmia in patients with idiopathic ventricular premature complexes

Background

To evaluate the role of Tp‐e and (Tp‐e)/QT ratio in differentiating benign ventricular premature complex (VPC) and malignant polymorphic ventricular tachycardia (PVT).

Methods

From January 2017 to December 2017, patients with documented polymorphic ventricular tachycardia (PVT) or ventricular fibrillation (VF) were consecutive included and classified as PVT/VF group. Sixty age‐ and sex‐matched healthy individuals were recruited as comparative control and subdivided into non‐VPC and VPC group. Clinical characteristics and Tp‐e and Tp‐e/QT ratio between the three groups were compared.

Results

Tp‐e and (Tp‐e)/QT ratio were significantly higher in patients of PVT/VF group compared with the other two groups (P < .001). Episodes of syncope were more frequent in patients with PVT/VF (P < .05). The sensitivity and specificity of a Tp‐e interval ≥86 ms for malignant arrhythmias triggered by VPCs were 88% and 66%, respectively, while the sensitivity and specificity of the Tp‐e/QT ratio ≥0.24 were 82% and 70%, respectively. Five patients complained recurrence of syncope in the PVT/VF group and 1 patient died with mean follow‐up of 18 months.

Conclusion

Tp‐e interval and the Tp‐Te/QT ratio is significantly increased in patients with PVT/VF and may be used as a novel non‐invasive marker of differentiating malignant and benign VPC.

First episode of ventricular fibrillation in an 84-year-old man with long-QT type 2 syndrome: A case report

Congenital long QT syndrome (LQTS) is associated with ventricular arrhythmia and an increased risk of sudden cardiac death in young people. However, it is extremely rare for an elderly man to experience ventricular fibrillation (VF) due to congenital LQTS as a first episode. We describe the case of an 84-year-old man who experienced syncope after urination. He had a medical history of hypertension and asthma, but no history of syncope. Electrocardiographic findings in 2017 showed QT prolongation (corrected QT = 505 ms). No medication that could induce QT prolongation was administered. Blood test results on admission showed no electrolyte abnormalities, and there were no abnormal findings on echocardiography. The second episode of loss of consciousness occurred during hospitalization, and electrocardiography revealed incessant torsade de pointes, caused by R-on-T with short-long-short (SLS) sequences due to bradyarrhythmia. Coronary angiography did not detect myocardial ischemia, and an implantable cardioverter-defibrillator was implanted for secondary prevention. Genetic testing revealed a mutation of the KCNH2 gene, indicating LQTS type 2. In summary, we report a rare case of prolonged QT interval with SLS sequences due to sick sinus syndrome triggering VF as the first attack in an elderly patient with LQTS type 2. <Learning objective: Physicians should be aware of the prolongation of QT as a cause of syncope in elderly patients and should pay attention to QT duration. Furthermore, patients with elderly-onset QT prolongation may have a genetic background associated with congenital long QT syndrome (LQTS); therefore, we should not hesitate to perform genetic testing in cases where LQTS is suspected in elderly patients.>.

Body Surface Mapping of Ventricular Repolarization Heterogeneity: An Ex-vivo Multiparameter Study

Background

Increased heterogeneity of ventricular repolarization is associated with life-threatening arrhythmia and sudden cardiac death (SCD). T-wave analysis through body surface potential mapping (BSPM) is a promising tool for risk stratification, but the clinical effectiveness of current electrocardiographic indices is still unclear, with limited experimental validation. This study aims to investigate performance of non-invasive state-of-the-art and novel T-wave markers for repolarization dispersion in an ex vivo model.

Methods

Langendorff-perfused pig hearts (N = 7) were suspended in a human-shaped 256-electrode torso tank. Tank potentials were recorded during sinus rhythm before and after introducing repolarization inhomogeneities through local perfusion with dofetilide and/or pinacidil. Drug-induced repolarization gradients were investigated from BSPMs at different experiment phases. Dispersion of electrical recovery was quantified by duration parameters, i.e., the time interval between the peak and the offset of T-wave (T_PEAK-T_END) and QT interval, and variability over time and electrodes was also assessed. The degree of T-wave symmetry to the peak was quantified by the ratio between the terminal and initial portions of T-wave area (Asy). Morphological variability between left and right BSPM electrodes was measured by dynamic time warping (DTW). Finally, T-wave organization was assessed by the complexity of repolarization index (CR), i.e., the amount of energy non-preserved by the dominant eigenvector computed by principal component analysis (PCA), and the error between each multilead T-wave and its 3D PCA approximation (NMSE). Body surface indices were compared with global measures of epicardial dispersion of repolarization, and with local gradients between adjacent ventricular sites.

Results

After drug intervention, both regional and global repolarization heterogeneity were significantly enhanced. On the body surface, T_PEAK-T_END was significantly prolonged and less stable in time in all experiments, while QT interval showed higher variability across the interventions in terms of duration and spatial dispersion. The rising slope of the repolarization profile was steeper, and T-waves were more asymmetric than at baseline. Interventricular shape dissimilarity was enhanced by repolarization gradients according to DTW. Organized T-wave patterns were associated with abnormal repolarization, and they were properly described by the first principal components.

Conclusion

Repolarization heterogeneity significantly affects T-wave properties, and can be non-invasively captured by BSPM-based metrics.

Blinded In Silico Drug Trial Reveals the Minimum Set of Ion Channels for Torsades de Pointes Risk Assessment

Torsades de Pointes (TdP) is a type of ventricular arrhythmia which could be observed as an unwanted drug-induced cardiac side effect, and it is associated with repolarization abnormalities in single cells. The pharmacological evaluations of TdP risk in previous years mainly focused on the hERG channel due to its vital role in the repolarization of cardiomyocytes. However, only considering drug effects on hERG led to false positive predictions since the drug action on other ion channels can also have crucial regulatory effects on repolarization. To address the limitation of only evaluating hERG, the Comprehensive in Vitro Proarrhythmia Assay initiative has proposed to systematically integrate drug effects on multiple ion channels into in silico drug trial to improve TdP risk assessment. It is not clear how many ion channels are sufficient for reliable TdP risk predictions, and whether differences in IC₅₀ and Hill coefficient values from independent sources can lead to divergent in silico prediction outcomes. The rationale of this work is to investigate the above two questions using a computationally efficient population of human ventricular cells optimized to favor repolarization abnormality. Our blinded results based on two independent data sources confirm that simulations with the optimized population of human ventricular cell models enable efficient in silico drug screening, and also provide direct observation and mechanistic analysis of repolarization abnormality. Our results show that 1) the minimum set of ion channels required for reliable TdP risk predictions are Nav1.5 (peak), Cav1.2, and hERG; 2) for drugs with multiple ion channel blockage effects, moderate IC₅₀ variations combined with variable Hill coefficients can affect the accuracy of in silico predictions.

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.

Multiple Dynamical Mechanisms of Phase-2 Early Afterdepolarizations in a Human Ventricular Myocyte Model: Involvement of Spontaneous SR Ca 2+ Release

Early afterdepolarization (EAD) is known to cause lethal ventricular arrhythmias in long QT syndrome (LQTS). In this study, dynamical mechanisms of EAD formation in human ventricular myocytes (HVMs) were investigated using the mathematical model developed by ten Tusscher and Panfilov (Am J Physiol Heart Circ Physiol 291, 2006). We explored how the rapid (IKr) and slow (IKs) components of delayed-rectifier K+ channel currents, L-type Ca2+ channel current (ICa L), Na+/Ca2+ exchanger current (INCX), and intracellular Ca2+ handling via the sarcoplasmic reticulum (SR) contribute to initiation, termination and modulation of phase-2 EADs during pacing in relation to bifurcation phenomena in non-paced model cells. Parameter-dependent dynamical behaviors of the non-paced model cell were determined by calculating stabilities of equilibrium points (EPs) and limit cycles, and bifurcation points to construct bifurcation diagrams. Action potentials (APs) and EADs during pacing were reproduced by numerical simulations for constructing phase diagrams of the paced model cell dynamics. Results are summarized as follows: (1) A modified version of the ten Tusscher-Panfilov model with accelerated ICaL inactivation could reproduce bradycardia-related EADs in LQTS type 2 and β-adrenergic stimulation-induced EADs in LQTS type 1. (2) Two types of EADs with different initiation mechanisms, ICaL reactivation-dependent and spontaneous SR Ca2+ release-mediated EADs, were detected. (3) Termination of EADs (AP repolarization) during pacing depended on the slow activation of IKs. (4) Spontaneous SR Ca2+ releases occurred at higher Ca2+ uptake rates, attributable to the instability of steady-state intracellular Ca2+ concentrations. Dynamical mechanisms of EAD formation and termination in the paced model cell are closely related to stability changes (bifurcations) in dynamical behaviors of the non-paced model cell, but they are model-dependent. Nevertheless, the modified ten Tusscher-Panfilov model would be useful for systematically investigating possible dynamical mechanisms of EAD-related arrhythmias in LQTS.

Evaluation of Tpeak to end/QT and Tpeak to end/QTc ratios in patients with STEMI undergoing percutaneous intervention vs. thrombolytic therapy

INTRODUCTION

The patients with ST-segment Elevation Myocardial Infarction (STEMI) are significantly at increased risk of arrhythmia. Repolarization of myocardium has been evaluated by a series of electrical parameters including T wave peak to T wave end (Tp-Te) and Tp-Te/QT ratio. Which were compared with survival outcomes between two groups of STEMI patients treated with Primary Percutaneous Coronary Intervention (PPCI) and recombinant Tissue Plasminogen Activator (r-TPA).

METHODS

In this prospective study, 188 patients with STEMI were included in the study. 12‑Lead ECGs were obtained from all patients on time of admission and after 24 h after treatment. After dividing the patients into two groups based on their type of treatment (PPCI or r-TPA), The Tp-Te/QT and Tp-Te/QTc ratios were calculated using ECG records. The survival outcomes were compared between two groups.

RESULTS

95 patients (50.5%) underwent PPCI and 93 patients (49.5%) received r-TPA. Tp-Te/QT and Tp-Te/QTc ratios after administration of the treatments were significantly decreased in both groups (P-value = .001) with lower Tp-Te/QT and Tp-Te/QTc ratios in PPCI group (P-value = .001). 7 patients in PPCI group (7.3%) and 16 patients in r-TPA group (17.2%) were died during their hospitalization period (P-value = .04). The best combination of sensitivity and specificity of post treatment Tp-Te/QT ratio was at cutoff points of 29.4 with 82% sensitivity and 83% specificity.

CONCLUSION

Our study demonstrated that Tp-Te/QT and Tp-Te/QTc ratios decrease significantly after both PPCI and r-TPA therapies, but with PPCI these indexes decrease more than r-TPA, resulting a better survival outcome in patients with STEMI.

Changes in left ventricular repolarization after short-term testosterone replacement therapy in hypogonadal males

BACKGROUND AND AIM

Evidences suggest that androgen deficiency is associated with sudden cardiac death (SCD). Our purpose was to analyse some electrocardiographic (ECG) markers of repolarization phase in hypogonadal patients either at baseline or after testosterone replacement therapy (TRT).

PATIENTS AND METHODS

Baseline and after 6 months of testosterone replacement therapy, 14 hypogonadal patients and 10 age-matched controls underwent a short-term ECG recordings at rest and immediately after a maximal exercise test. The following ECG parameters have been collected: QTe (the interval between the q wave the end of T wave), QTp (the interval between the q wave and the peak of T wave), and Te (the interval between the peak and the end of T wave).

RESULTS

At baseline, in the hypogonadal patients, corrected QTe and QTp values were longer at rest than in the controls at rest (p < 0.05), whereas, during the recovery phase, only the QTp remained significantly longer (p < 0.05). After TRT, hypogonadal patients showed an improvement only in Te (p < 0.05). Conversely, any difference between hypogonadal patients and control subjects was found with respect to the markers of temporal dispersion of repolarization phases, except for a worse QTp → Te coherence (p = 0.001) obtained during the recovery phase.

CONCLUSIONS

In conclusion, at rest, hypogonadal patients suffer from a stable increase in the myocardial repolarization phase without an increase in its temporal dispersion and, hence, the SCD risk seems to be low.

Evaluation of the Tp-Te interval, Tp-Te/QTc ratio, and QT dispersion in patients with Turner syndrome

Objective:

To evaluate ventricular repolarization parameters using the interval from the peak to the end of the T wave (Tp–Te), together with QT and corrected QT (QTc) intervals, QT dispersion (QTd), and Tp-Te/QTc ratio in patients with Turner syndrome (pwTS) and to compare the results with those from healthy controls.

Methods:

In total, 38 patients previously diagnosed with Turner syndrome (TS) and 35 healthy girls (controls) were included in our cross-sectional study. Twelve-lead electrocardiography (ECG) and echocardiography after a 30-min rest were performed. The QT, QTc, QTd, Tp-Te interval, and Tp-Te/QTc ratio were determined.

Results:

No differences in age or sex were observed between the groups. QT intervals were similar in both groups [pwTS: 354.76±25.33 ms, controls (C): 353.29±17.51 ms, p=0.775]. pwTS had significantly longer QTc and QTd than controls (411.87±22.66 ms vs. 392.06±13.21 ms, p<0.001 and 40.31±2.02 ms vs. 37.54±1.83 ms, p<0.001, respectively). Similarly, the Tp-Te interval and Tp-Te/QTc ratio were significantly longer in pwTS than in controls (71.89±3.39 ms vs. 65.34±2.88 ms, p<0.001 and 0.17±0.01 vs. 0.16±0.01, p=0.01).

Conclusion:

As pwTS have longer QTc, QTd, Tp–Te interval, and Tp-Te/QTc ratio, an annual follow-up with ECG can provide awareness and even prevent sudden death in them. Also avoiding the use of drugs that makes repolarization anomaly and having knowledge about the side effects of these drugs are essential in pwTS.

Personalized Cardiac Computational Models: From Clinical Data to Simulation of Infarct-Related Ventricular Tachycardia

In the chronic stage of myocardial infarction, a significant number of patients develop life-threatening ventricular tachycardias (VT) due to the arrhythmogenic nature of the remodeled myocardium. Radiofrequency ablation (RFA) is a common procedure to isolate reentry pathways across the infarct scar that are responsible for VT. Unfortunately, this strategy show relatively low success rates; up to 50% of patients experience recurrent VT after the procedure. In the last decade, intensive research in the field of computational cardiac electrophysiology (EP) has demonstrated the ability of three-dimensional (3D) cardiac computational models to perform in-silico EP studies. However, the personalization and modeling of certain key components remain challenging, particularly in the case of the infarct border zone (BZ). In this study, we used a clinical dataset from a patient with a history of infarct-related VT to build an image-based 3D ventricular model aimed at computational simulation of cardiac EP, including detailed patient-specific cardiac anatomy and infarct scar geometry. We modeled the BZ in eight different ways by combining the presence or absence of electrical remodeling with four different levels of image-based patchy fibrosis (0, 10, 20, and 30%). A 3D torso model was also constructed to compute the ECG. Patient-specific sinus activation patterns were simulated and validated against the patient's ECG. Subsequently, the pacing protocol used to induce reentrant VTs in the EP laboratory was reproduced in-silico. The clinical VT was induced with different versions of the model and from different pacing points, thus identifying the slow conducting channel responsible for such VT. Finally, the real patient's ECG recorded during VT episodes was used to validate our simulation results and to assess different strategies to model the BZ. Our study showed that reduced conduction velocities and heterogeneity in action potential duration in the BZ are the main factors in promoting reentrant activity. Either electrical remodeling or fibrosis in a degree of at least 30% in the BZ were required to initiate VT. Moreover, this proof-of-concept study confirms the feasibility of developing 3D computational models for cardiac EP able to reproduce cardiac activation in sinus rhythm and during VT, using exclusively non-invasive clinical data.

Electrocardiographic conduction and repolarization markers associated with sudden cardiac death: moving along the electrocardiography waveform

The QT interval along with its heart rate corrected form (QTc) are well-established ECG markers that have been found to be associated with malignant ventricular arrhythmogenesis. However, extensive preclinical and clinical investigations over the years have allowed for novel clinical ECG markers to be generated as predictors of arrhythmogenesis and sudden cardiac death. Repolarization markers include the older QTc, QT dispersion and newer Tpeak - Tend intervals, (Tpeak - Tend) / QT ratios, T-wave alternans (TWA), microvolt TWA and T-wave area dispersion. Meanwhile, conduction markers dissecting the QRS complex, such as QRS dispersion (QRSD) and fragmented QRS, were also found to correlate conduction velocity and unidirectional block with re-entrant substrates in various cardiac conditions. Both repolarization and conduction parameters can be combined into the excitation wavelength (λ). A surrogate marker for λ is the index of Cardiac Electrophysiological Balance (iCEB: QT / QRSd). Other markers based on conduction-repolarization are [QRSD x (Tpeak-Tend) / QRSd] and [QRSD x (Tpeak-Tend) / (QRSd x QT)]. Advancement in technology permitted sophisticated electrophysiological analyses such as principal component analysis and periodic repolarization dynamics to further improve risk stratification. This was closely followed by other novel indices including ventricular ectopic QRS interval, the f99 index and EntropyXQT, which integrates mathematical and physical calculations for determining the risk markers. Though proven to be effective in limited patient cohorts, more clinical studies across different cardiac pathologies are required to confirm their validity. As such, this review seeks to encapsulate the development of old and new ECG markers along with their associated utility and shortcomings in clinical practice.

Integrative and theoretical research on the architecture of a biological system and its disorder

An organism stems from assemblies of a variety of cells and proteins. This complex system serves as a unit, and it exhibits highly sophisticated functions in response to exogenous stimuli that change over time. The complete sequencing of the entire human genome has allowed researchers to address the enigmas of life and disease at the gene- or molecular-based level. The consequence of such studies is the rapid accumulation of a multitude of data at multiple levels, ranging from molecules to the whole body, that has necessitated the development of entirely new concepts, tools, and methodologies to analyze and integrate these data. This necessity has given birth to systems biology, an advanced theoretical and practical research framework that has totally changed the directions of not only basic life science but also medicine. During the symposium of the 95th Annual Meeting of The Physiological Society of Japan 2018, five researchers reported on their respective studies on systems biology. The topics included reactions of drugs, ion-transport architecture in an epithelial system, multi-omics in renal disease, cardiac electrophysiological systems, and a software platform for computer simulation. In this review article these authors have summarized recent achievements in the field and discuss next-generation studies on health and disease.

Evaluation of myocardial dispersion of repolarization in patients with heart transplantation

Background/aim

The number of patients with heart transplantation has dramatically increased in the last decade. Considerable studies have suggested that the interval from the peak to the end of the electrocardiographic T wave (Tp-e) may correspond to the transmural dispersion of repolarization and increased Tp-e interval and Tp-e/QT ratio are associated with malignant ventricular arrhythmias. We analyzed the dispersion of myocardial repolarization using electrocardiographic Tp-e interval and Tp-e/QTc ratio in patients with heart transplantation.

Materials and methods

This observational study included 38 patients (12 female and 26 male) with heart transplantation and 38 well-matched controls. From electrocardiograms, Tp-e interval and Tp-e/QTc ratio were calculated and compared between the 2 groups.

Results

Noninvasive arrhythmia indicators including Tp-e interval (84.63 ± 14.17 ms vs 71.82 ± 7.47 ms, P < 0.001), Tp-e/QTc ratio (0.19 ± 0.04 vs 0.16 ± 0.02, P < 0.001) and QTc interval except QT interval were significantly higher in transplanted hearts compared to normal hearts.

Conclusion

Patients with heart transplantation have increased myocardial dispersion of repolarization.

Evaluation of ventricular repolarization parameters during migraine attacks

AIMS

Migraine is a chronic neurovascular disorder characterized by intermittent episodes of severe headache. Abnormalities in the autonomic nervous system (sympathetic and parasympathetic nervous systems) have been detected during migraine-free periods in patients with migraine. In these patients, disrupted autonomic innervations of the heart and coronary arteries may lead to electrocardiographic changes during a migraine attack. T-wave peak-to-end interval (Tp-e interval) and Tp-e/QT ratio are relatively new markers of ventricular arrhythmogenesis and repolarization heterogeneity. In the present observational study, we investigated the changes in ventricular repolarization during migraine attacks and attack-free periods by performing 12‑lead electrocardiography (ECG).

METHODS

This study included 63 patients (54 [86%] women; mean age: 33.3 ± 9.9 years) with migraine. The QT and corrected QT (QTc) intervals, Tp-e interval, and Tp-e/QT ratio of the patients during migraine attacks and attack-free periods were measured by performing 12‑lead ECG.

RESULTS

The QT and QTc intervals, Tp-e interval, and Tp-e/QT ratio were higher during migraine attacks than during attack-free periods (P < 0.001 for all).

CONCLUSION

These results indicate that migraine attacks are associated with an increase in ventricular repolarization parameters compared with attack-free periods possibly because of the dysregulation of the autonomic nervous system.

Fully coupled fluid-electro-mechanical model of the human heart for supercomputers

In this work, we present a fully coupled fluid-electro-mechanical model of a 50th percentile human heart. The model is implemented on Alya, the BSC multi-physics parallel code, capable of running efficiently in supercomputers. Blood in the cardiac cavities is modeled by the incompressible Navier-Stokes equations and an arbitrary Lagrangian-Eulerian (ALE) scheme. Electrophysiology is modeled with a monodomain scheme and the O'Hara-Rudy cell model. Solid mechanics is modeled with a total Lagrangian formulation for discrete strains using the Holzapfel-Ogden cardiac tissue material model. The three problems are simultaneously and bidirectionally coupled through an electromechanical feedback and a fluid-structure interaction scheme. In this paper, we present the scheme in detail and propose it as a computational cardiac workbench.

Possible predictive role of electrical risk score on transcatheter aortic valve replacement outcomes in older patients: preliminary data

Objectives

To evaluate the predicative power of the electrical risk score (ERS), a noninvasive and inexpensive test obtained by means of a standard 12-lead electrocardiogram (ECG), in a cohort of elderly patients who had undergone transcatheter aortic valve replacement (TAVR).

Methods

Survivors and non-survivors after TAVR at 1-year follow-up were compared in respect to the pre-procedural ERS as well as a number of other clinical and instrumental variables. ERS is composed of seven simple ECG markers: heart rate (>75 bpm); QRS duration (>110 ms); left ventricular hypertrophy (Sokolow–Lyon criteria); delayed QRS transition zone (≥ V4); frontal QRS-T angle (>90°); long QT_Bazett (>450 ms for men and >460 in women) or JT_Bazett (330 ms for men and >340 ms for women); and long Tpeak to Tend interval (T_p-e) (>89 ms). The trial was registered in ClinicalTrials.gov as NCT03145376.

Results

A total of 40 patients were evaluated. During the follow-up, the all-cause mortality rate was 25% (ten patients) with 15% of cardiovascular death (six patients). The ERS was the strongest predictor of all-cause (odds ratio 3.73, 95% CI: 1.44–9.66, P<0.05) or cardiovascular (odds ratio 3.95, 95% CI: 1.09–14.27, P<0.05) mortality. Receiver operating characteristic curves showed that ERS had the widest significant sensitivity-specificity area under the curve (AUC) predicting all-cause (AUC: 0.855, P<0.05) or cardiovascular mortality (AUC: 0.908, P<0.05).

Conclusion

ERS seems to be a useful noninvasive tool able to stratify the risk of mortality in 1-year follow-up of TAVR patients. These findings, however, require larger trials to be confirmed.

Mechanisms Underlying the Actions of Antidepressant and Antipsychotic Drugs That Cause Sudden Cardiac Arrest

A number of antipsychotic and antidepressant drugs are known to increase the risk of ventricular arrhythmias and sudden cardiac death. Based largely on a concern over the development of life-threatening arrhythmias, a number of antipsychotic drugs have been temporarily or permanently withdrawn from the market or their use restricted. While many antidepressants and antipsychotics have been linked to QT prolongation and the development of torsade de pointes arrhythmias, some have been associated with a Brugada syndrome phenotype and the development of polymorphic ventricular arrhythmias. This article examines the arrhythmic liability of antipsychotic and antidepressant drugs capable of inducing long QT and/or Brugada syndrome phenotypes. The goal of this article is to provide an update on the ionic and cellular mechanisms thought to be involved in, and the genetic and environmental factors that predispose to, the development of cardiac arrhythmias and sudden cardiac death among patients taking antidepressant and antipsychotic drugs that are in clinical use.

Paediatric breath-holding spells are associated with autonomic dysfunction and iron deficiency may play a role

AIM

This study assessed cardiac performance and iron in subjects aged 12-36 months with breath-holding spells (BHSs).

METHODS

We consecutively recruited 40 subjects (55% male) experiencing BHSs from the general paediatric outpatients department at the Children's Hospital, Ain Shams University, Egypt, from 2015 to 2016. The 20 matched comparisons were mainly healthy siblings. The workup included iron levels and electrocardiograms.

RESULTS

The age at the onset of BHSs was 5-24 months with a median monthly frequency of 13. Almost two-thirds of the patients had cyanotic spells, and one-third had pallid spells, lasting 25-90 seconds. Lower serum iron levels and higher QT dispersion and T-wave dispersion were recorded in patients than the comparison group, and 4.8% had dysrhythmia and bradycardia. We observed higher durations of bradycardia during attacks and higher occurrences of dysrhythmia during cyanotic spells, which were more frequent in patients with prolonged or frequent BHSs.

CONCLUSION

Our study of patients aged 12-13 months supported the theory of autonomic dysfunction in BHSs. The ECG findings, especially in patients with prolonged or frequent spells, need to be studied further to evaluate the risk of life-threatening events. Iron deficiency may play a role in autonomic dysfunction in patients with BHSs.

Meta-analysis of Tpeak-Tend and Tpeak-Tend/QT ratio for risk stratification in congenital long QT syndrome

BACKGROUND AND OBJECTIVES

Congenital long QT syndrome (LQTS) predisposes affected individuals to ventricular tachycardia/fibrillation (VF/VF), potentially resulting in sudden cardiac death. The T_peak-T_end interval and the T_peak-T_end/QT ratio, electrocardiographic markers of dispersion of ventricular repolarization, were proposed for risk stratification but their predictive values in LQTS have been controversial. A systematic review and meta-analysis was conducted to examine the value of T_peak-T_end intervals and T_peak-T_end/QT ratios in predicting arrhythmic and mortality outcomes in congenital LQTS.

METHOD

PubMed and Embase databases were searched until 9th May 2017, identifying 199 studies.

RESULTS

Five studies on long QT syndrome were included in the final meta-analysis. T_peak-T_end intervals were longer (mean difference [MD]: 13ms, standard error [SE]: 4ms, P=0.002; I²=34%) in congenital LQTS patients with adverse events [syncope, ventricular arrhythmias or sudden cardiac death] compared to LQTS patients without such events. By contrast, T_peak-T_end/QT ratios were not significantly different between the two groups (MD: 0.02, SE: 0.02, P=0.26; I²=0%).

CONCLUSION

This meta-analysis showed that T_peak-T_end interval is significant higher in individuals who are at elevated risk of adverse events in congenital LQTS, offering incremental value for risk stratification.

Evaluation of the Tp-Te Interval, QTc and P-Wave Dispersion in Patients With Coronary Artery Ectasia

Background

Coronary artery ectasia (CAE) is defined as a diffuse dilatation of the diameter of the ectatic segment of the coronary artery, 1.5 times greater than that of the adjacent segment. The Tp-Te interval, P-wave and QTc dispersions are relatively new electrocardiographic markers associated with an increased risk of developing arrhythmias. Despite CAE increasing in prevalence in recent years, there is a sparsity of data available about its arrhythmogenic effect. The aim of the study was to evaluate QTc, P-wave dispersion and Tp-Te and Tp-Te/QT ratio in patients with CAE.

Methods

A retrospective comparative study was designed for consecutive age- and sex-matched patients. Twenty patients with isolated CAE (group 1) and 20 control subjects (group 2), with normal coronary arteries, were included. All patients presented with chest pain and coronary angiogram was indicated. Outcome measures included Tp-Te interval, Tp-Te/QT ratio, QTc dispersion and P-wave dispersion. Measurement of electrocardiogram (ECG) parameters was conducted using standardized digital online software. Descriptive and inferential statistics were performed.

Results

Mean Tp-Te (95.5 ± 9.01 ms) and Tp-Te/QT ratio (0.22 ± 0.02) were significantly prolonged in CAE group (Tp-Te: 84 ± 5.62 ms, P = 0.00009; Tp-Te/QT ratio: 0.20 ± 0.01, P = 0.00004). In addition, QTc (31.2 ± 3.71 ms) and P-wave dispersion (31.9 ± 5.46 ms) were significantly increased in comparison to the control group (QTc: 27.6 ± 2.82 ms, P = 0.00532 and 20 ± 3.77 ms, P = 0.00003 respectively). However, there was no difference in ventricular activation time (VAT) between groups.

Conclusions

CAE ECGs were found to be associated with increased Tp-Te, Tp-Te/QT ratio, QTc intervals and P-wave dispersions. This may suggest that CAE existence has a pro-arrhythmogenic nature.

Long-term effects of cardiac resynchronization therapy on electrical remodeling in heart failure-A prospective study

INTRODUCTION

Effects of cardiac resynchronization therapy (CRT) on arrhythmogenicity and sudden death have not been fully ascertained. CRT has been shown to increase transmural dispersion of repolarization (TDR) immediately on implantation, which may favorably remodel on long-term follow-up. However, such a hypothesis has not been prospectively evaluated.

METHODS AND RESULTS

We included 35 consecutive patients who underwent CRT implantation between September 2013 and August 2014 (mean age 56.8 ± 11.09 years; 71.43% males). QT and Tpeak-Tend (Tp-e) intervals were measured during endocardial (RVendoP), epicardial (LVepiP), and biventricular pacing (BiVP) at CRT implantation and 1-year follow-up. Compared to RVendoP (130.41 ± 16.75 ms), Tp-e was significantly prolonged during BiVP (142.06 ± 21.98 ms; P < 0.001) and LVepiP (183.45 ± 27.87 ms; P < 0.001) at baseline. There was a significant decrease in Tp-e during BiVP on follow-up (117.93 ± 15.03 ms; P < 0.001). High responders had significantly lower Tp-e at 1 year compared to low responders (113.16 ± 14.3 ms vs 129.59 ± 9.75 ms, P  =  0.004). Tp-e at 1 year had strong negative correlation with reduction in LV end-systolic volumes (r  =  - 0.51; P  =  0.003). Seven patients with sustained ventricular arrhythmias during follow-up had significantly longer baseline Tp-e compared to those without arrhythmias (158.19 ± 17.59 ms vs 139.72 ± 20.94 ms, P  =  0.043). A baseline Tp-e value of ≥ 148 ms had a specificity of 75% and sensitivity of 71% to predict ventricular arrhythmias.

CONCLUSIONS

Baseline TDR is greater during BiVP and LV epiP compared with RVendoP in patients with heart failure. However, BiVP causes a significant reduction in TDR reflective of reverse electrical remodeling on long-term follow-up.

Hysteretic Dynamics of Multi-Stable Early Afterdepolarisations with Repolarisation Reserve Attenuation: A Potential Dynamical Mechanism for Cardiac Arrhythmias

Some cardiovascular and non-cardiovascular drugs frequently cause excessive prolongation of the cardiac action potential (AP) and lead to the development of early afterdepolarisations (EADs), which trigger lethal ventricular arrhythmias. Combining computer simulations in APs with numerical calculations based on dynamical system theory, we investigated stability changes of APs observed in a paced human ventricular myocyte model by decreasing and/or increasing the rapid (I_Kr) and slow (I_Ks) components of delayed rectifying K⁺ current. Upon reducing I_Kr, the APs without EADs (no-EAD response) showed gradual prolongation of AP duration (APD), and were annihilated without AP configuration changes due to the occurrence of saddle-node bifurcations. This annihilation caused a transition to an AP with EADs as a new stable steady state. Furthermore, reducing repolarisation currents (repolarisation reserve attenuation) evoked multi-stable states consisting of APs with different APDs, and caused multiple hysteretic dynamics. Depending on initial ion circumstances within ventricular myocytes, these multi-stable AP states might increase the local/global heterogeneity of AP repolarisations in the ventricle. Thus, the EAD-induced arrhythmias with repolarisation reserve attenuation might be attributed to the APD variability caused by multi-stability in cardiac AP dynamics.

Differences in Repolarization Heterogeneity Among Heart Failure With Preserved Ejection Fraction Phenotypic Subgroups

Heart failure with preserved ejection fraction (HFpEF) is a highly heterogeneous syndrome associated with multiple medical comorbidities and pathophysiologic pathways or phenotypes. We recently developed a phenomapping method combining deep phenotyping with machine learning analysis to classify HFpEF patients into 3 clinically distinct phenotypic subgroups (phenogroups) with different clinical outcomes. Phenogroup #1 was younger with lower B-type natriuretic peptide levels, phenogroup #2 had the highest prevalence of obesity and diabetes mellitus, and phenogroup #3 was the oldest with the most factors for chronic kidney disease, the most dysfunctional myocardial mechanics, and the highest adverse outcomes. The pathophysiological differences between these phenogroups, however, remain incompletely described. We sought to evaluate whether these 3 groups differ on the basis of repolarization heterogeneity, which has previously been linked to adverse outcomes in HFpEF. The T-peak to T-end (TpTe) interval, a well-validated index of repolarization heterogeneity, was measured by 2 readers blinded to each other and all other clinical data on the electrocardiograms of 201 HFpEF patients enrolled in a systematic observational study. TpTe duration was associated with higher B-type natriuretic peptide level (p = 0.006), increased QRS-T angle (p = 0.008), and lower septal e' velocity (p = 0.007). TpTe duration was greatest in phenogroup #3 (100.4 ± 24.5 ms) compared with phenogroups #1 (91.2 ± 17.3 ms) and #2 (90.2 ± 17.0 ms) (p = 0.0098). On multivariable analyses, increased TpTe was independently associated with the high-risk phenogroup #3 classification. In conclusion, repolarization heterogeneity is a marker of a specific subset of HFpEF patients identified using unsupervised machine learning analysis and therefore may be a key pathophysiologic marker in this subset of HFpEF patients.

Pro-arrhythmogenic Effects of the V141M KCNQ1 Mutation in Short QT Syndrome and Its Potential Therapeutic Targets: Insights from Modeling

Gain-of-function mutations in the pore-forming subunit of I_Ks channels, KCNQ1, lead to short QT syndrome (SQTS) and lethal arrhythmias. However, how mutant I_Ks channels cause SQTS and the possibility of I_Ks-specific pharmacological treatment remain unclear. V141M KCNQ1 is a SQTS associated mutation. We studied its effect on I_Ks gating properties and changes in the action potentials (AP) of human ventricular myocytes. Xenopus oocytes were used to study the gating mechanisms of expressed V141M KCNQ1/KCNE1 channels. Computational models were used to simulate human APs in endocardial, mid-myocardial, and epicardial ventricular myocytes with and without β-adrenergic stimulation. V141M KCNQ1 caused a gain-of-function in I_Ks characterized by increased current density, faster activation, and slower deactivation leading to I_Ks accumulation. V141M KCNQ1 also caused a leftward shift of the conductance-voltage curve compared to wild type (WT) I_Ks (V_1/2 = 33.6 ± 4.0 mV for WT, and 24.0 ± 1.3 mV for heterozygous V141M). A Markov model of heterozygous V141M mutant I_Ks was developed and incorporated into the O’Hara–Rudy model. Compared to the WT, AP simulations demonstrated marked rate-dependent shortening of AP duration (APD) for V141M, predicting a SQTS phenotype. Transmural electrical heterogeneity was enhanced in heterozygous V141M AP simulations, especially under β-adrenergic stimulation. Computational simulations identified specific I_K1 blockade as a beneficial pharmacologic target for reducing the transmural APD heterogeneity associated with V141M KCNQ1 mutation. V141M KCNQ1 mutation shortens ventricular APs and enhances transmural APD heterogeneity under β-adrenergic stimulation. Computational simulations identified I_K1 blockers as a potential antiarrhythmic drug of choice for SQTS.

Role of abnormal repolarization in the mechanism of cardiac arrhythmia

In cardiac patients, life-threatening tachyarrhythmia is often precipitated by abnormal changes in ventricular repolarization and refractoriness. Repolarization abnormalities typically evolve as a consequence of impaired function of outward K⁺ currents in cardiac myocytes, which may be caused by genetic defects or result from various acquired pathophysiological conditions, including electrical remodelling in cardiac disease, ion channel modulation by clinically used pharmacological agents, and systemic electrolyte disorders seen in heart failure, such as hypokalaemia. Cardiac electrical instability attributed to abnormal repolarization relies on the complex interplay between a provocative arrhythmic trigger and vulnerable arrhythmic substrate, with a central role played by the excessive prolongation of ventricular action potential duration, impaired intracellular Ca²⁺ handling, and slowed impulse conduction. This review outlines the electrical activity of ventricular myocytes in normal conditions and cardiac disease, describes classical electrophysiological mechanisms of cardiac arrhythmia, and provides an update on repolarization-related surrogates currently used to assess arrhythmic propensity, including spatial dispersion of repolarization, activation-repolarization coupling, electrical restitution, TRIaD (triangulation, reverse use dependence, instability, and dispersion), and the electromechanical window. This is followed by a discussion of the mechanisms that account for the dependence of arrhythmic vulnerability on the location of the ventricular pacing site. Finally, the review clarifies the electrophysiological basis for cardiac arrhythmia produced by hypokalaemia, and gives insight into the clinical importance and pathophysiology of drug-induced arrhythmia, with particular focus on class Ia (quinidine, procainamide) and Ic (flecainide) Na⁺ channel blockers, and class III antiarrhythmic agents that block the delayed rectifier K⁺ channel (dofetilide).

Ca2+-activated Cl- current is antiarrhythmic by reducing both spatial and temporal heterogeneity of cardiac repolarization

The role of Ca²⁺-activated Cl^- current (I_Cl(Ca)) in cardiac arrhythmias is still controversial. It can generate delayed afterdepolarizations in Ca²⁺-overloaded cells while in other studies incidence of early afterdepolarization (EAD) was reduced by I_Cl(Ca). Therefore our goal was to examine the role of I_Cl(Ca) in spatial and temporal heterogeneity of cardiac repolarization and EAD formation. Experiments were performed on isolated canine cardiomyocytes originating from various regions of the left ventricle; subepicardial, midmyocardial and subendocardial cells, as well as apical and basal cells of the midmyocardium. I_Cl(Ca) was blocked by 0.5mmol/L 9-anthracene carboxylic acid (9-AC). Action potential (AP) changes were tested with sharp microelectrode recording. Whole-cell 9-AC-sensitive current was measured with either square pulse voltage-clamp or AP voltage-clamp (APVC). Protein expression of TMEM16A and Bestrophin-3, ion channel proteins mediating I_Cl(Ca), was detected by Western blot. 9-AC reduced phase-1 repolarization in every tested cell. 9-AC also increased AP duration in a reverse rate-dependent manner in all cell types except for subepicardial cells. Neither I_Cl(Ca) density recorded with square pulses nor the normalized expressions of TMEM16A and Bestrophin-3 proteins differed significantly among the examined groups of cells. The early outward component of I_Cl(Ca) was significantly larger in subepicardial than in subendocardial cells in APVC setting. Applying a typical subepicardial AP as a command pulse resulted in a significantly larger early outward component in both subepicardial and subendocardial cells, compared to experiments when a typical subendocardial AP was applied. Inhibiting I_Cl(Ca) by 9-AC generated EADs at low stimulation rates and their incidence increased upon beta-adrenergic stimulation. 9-AC increased the short-term variability of repolarization also. We suggest a protective role for I_Cl(Ca) against risk of arrhythmias by reducing spatial and temporal heterogeneity of cardiac repolarization and EAD formation.

Cardiac voltage-gated ion channels in safety pharmacology: Review of the landscape leading to the CiPA initiative

Voltage gated ion channels are central in defining the fundamental properties of the ventricular cardiac action potential (AP), and are also involved in the development of drug-induced arrhythmias. Many drugs can inhibit cardiac ion currents, including the Na⁺ current (I_Na), L-type Ca²⁺ current (Ica-L), and K⁺ currents (I_to, I_K1, I_Ks, and I_Kr), and thereby affect AP properties in a manner that can trigger or sustain cardiac arrhythmias. Since publication of ICH E14 and S7B over a decade ago, there has been a focus on drug effects on QT prolongation clinically, and on the rapidly activating delayed rectifier current (I_Kr), nonclinically, for evaluation of proarrhythmic risk. This focus on QT interval prolongation and a single ionic current likely impacted negatively some drugs that lack proarrhythmic liability in humans. To rectify this issue, the Comprehensive in vitro proarrhythmia assay (CiPA) initiative has been proposed to integrate drug effects on multiple cardiac ionic currents with in silico modelling of human ventricular action potentials, and in vitro data obtained from human stem cell-derived ventricular cardiomyocytes to estimate proarrhythmic risk of new drugs with improved accuracy. In this review, we present the physiological functions and the molecular basis of major cardiac ion channels that contribute to the ventricle AP, and discuss the CiPA paradigm in drug development.

Potassium currents in the heart: functional roles in repolarization, arrhythmia and therapeutics

This is the second of the two White Papers from the fourth UC Davis Cardiovascular Symposium Systems Approach to Understanding Cardiac Excitation-Contraction Coupling and Arrhythmias (3-4 March 2016), a biennial event that brings together leading experts in different fields of cardiovascular research. The theme of the 2016 symposium was 'K⁺ channels and regulation', and the objectives of the conference were severalfold: (1) to identify current knowledge gaps; (2) to understand what may go wrong in the diseased heart and why; (3) to identify possible novel therapeutic targets; and (4) to further the development of systems biology approaches to decipher the molecular mechanisms and treatment of cardiac arrhythmias. The sessions of the Symposium focusing on the functional roles of the cardiac K⁺ channel in health and disease, as well as K⁺ channels as therapeutic targets, were contributed by Ye Chen-Izu, Gideon Koren, James Weiss, David Paterson, David Christini, Dobromir Dobrev, Jordi Heijman, Thomas O'Hara, Crystal Ripplinger, Zhilin Qu, Jamie Vandenberg, Colleen Clancy, Isabelle Deschenes, Leighton Izu, Tamas Banyasz, Andras Varro, Heike Wulff, Eleonora Grandi, Michael Sanguinetti, Donald Bers, Jeanne Nerbonne and Nipavan Chiamvimonvat as speakers and panel discussants. This article summarizes state-of-the-art knowledge and controversies on the functional roles of cardiac K⁺ channels in normal and diseased heart. We endeavour to integrate current knowledge at multiple scales, from the single cell to the whole organ levels, and from both experimental and computational studies.

QT Prolongation and Malignant Arrhythmia: How Serious a Problem?

QT prolongation constitutes one of the most frequently encountered electrical disorders of the myocardium. This is due not only to the presence of several associated congenital syndrome but also, and mainly, due to the QT-prolonging effects of several acquired conditions, such as ischaemia and heart failure, as well as multiple medications from widely different categories. Propensity of repolarization disturbances to arrhythmia appears to be inherent in the function of and electrophysiology of the myocardium. In the present review the issue of QT prolongation will be addressed in terms of pathophysiology, arrhythmogenesis, treatment and risk stratification approaches. Although already discussed in literature, it is hoped that the mechanistic approach of the present review will assist in improved understanding of the underlying changes in electrophysiology, as well as the rationale for current diagnostic and therapeutic approaches.

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.