Molecular Basis of Arrhythmias

Arrhythmogenic Manifestations of Chagas Disease: Perspectives From the Bench to Bedside

Chagas cardiomyopathy caused by infection with the intracellular parasite Trypanosoma cruzi is the most common and severe expression of human Chagas disease. Heart failure, systemic and pulmonary thromboembolism, arrhythmia, and sudden cardiac death are the principal clinical manifestations of Chagas cardiomyopathy. Ventricular arrhythmias contribute significantly to morbidity and mortality and are the major cause of sudden cardiac death. Significant gaps still exist in the understanding of the pathogenesis mechanisms underlying the arrhythmogenic manifestations of Chagas cardiomyopathy. This article will review the data from experimental studies and translate those findings to draw hypotheses about clinical observations. Human- and animal-based studies at molecular, cellular, tissue, and organ levels suggest 5 main pillars of remodeling caused by the interaction of host and parasite: immunologic, electrical, autonomic, microvascular, and contractile. Integrating these 5 remodeling processes will bring insights into the current knowledge in the field, highlighting some key features for future management of this arrhythmogenic disease.

Cancer as an Individual Risk Factor for Heart Failure: A Review of Literature

The intricate relationship between cancer and cardiovascular diseases (CVD), notably heart failure (HF), is gaining attention in the medical field. This literature review explores the intricate interplay between cancer and CVD, particularly HF, emphasizing their significant impact on global mortality and comorbidity. While preventive measures have contributed to reducing their incidence, challenges persist in predicting and managing cancer-related complications. This review article delves into various risk factors associated with both cancer and HF, including lifestyle factors, genetic predispositions, and immune system dysregulation. It highlights emerging evidence suggesting a direct interaction between cancer and HF, with studies indicating an elevated risk of mortality from cancer in patients with HF and vice versa. Pathological mechanisms such as inflammation, oxidative stress, and tissue hypoxia are implicated in cancer-induced cardiac dysfunction, underscoring the need for comprehensive clinical investigations and ethical considerations in patient care. The review also discusses the potential role of biomarkers in risk assessment, early detection of cardiotoxicity, and understanding common pathophysiological links between cancer and HF, paving the way for multifaceted preventive and therapeutic approaches.

Cytoskeletal disarray increases arrhythmogenic vulnerability during sympathetic stimulation in a model of hypertrophic cardiomyopathy

Familial hypertrophic cardiomyopathy (FHC) patients are advised to avoid strenuous exercise due to increased risk of arrhythmias. Mice expressing the human FHC-causing mutation R403Q in the myosin heavy chain gene (MYH6) recapitulate the human phenotype, including cytoskeletal disarray and increased arrhythmia susceptibility. Following in vivo administration of isoproterenol, mutant mice exhibited tachyarrhythmias, poor recovery and fatigue. Arrhythmias were attenuated with the β-blocker atenolol and protein kinase A inhibitor PKI. Mutant cardiac myocytes had significantly prolonged action potentials and triggered automaticity due to reduced repolarization reserve and connexin 43 expression. Isoproterenol shortened cycle length, and escalated electrical instability. Surprisingly isoproterenol did not increase CaV1.2 current. We found alterations in CaV1.2-β1 adrenergic receptor colocalization assessed using super-resolution nanoscopy, and increased CaV1.2 phosphorylation in mutant hearts. Our results reveal for the first time that altered ion channel expression, co-localization and β-adrenergic receptor signaling associated with myocyte disarray contribute to electrical instability in the R403Q mutant heart.

Overview of Cardiac Arrhythmias and Treatment Strategies

Maintenance of normal cardiac rhythm requires coordinated activity of ion channels and transporters that allow well-ordered propagation of electrical impulses across the myocardium. Disruptions in this orderly process provoke cardiac arrhythmias that may be lethal in some patients. Risk of common acquired arrhythmias is increased markedly when structural heart disease caused by myocardial infarction (due to fibrotic scar formation) or left ventricular dysfunction is present. Genetic polymorphisms influence structure or excitability of the myocardial substrate, which increases vulnerability or risk of arrhythmias in patients. Similarly, genetic polymorphisms of drug-metabolizing enzymes give rise to distinct subgroups within the population that affect specific drug biotransformation reactions. Nonetheless, identification of triggers involved in initiation or maintenance of cardiac arrhythmias remains a major challenge. Herein, we provide an overview of knowledge regarding physiopathology of inherited and acquired cardiac arrhythmias along with a summary of treatments (pharmacologic or non-pharmacologic) used to limit their effect on morbidity and potential mortality. Improved understanding of molecular and cellular aspects of arrhythmogenesis and more epidemiologic studies (for a more accurate portrait of incidence and prevalence) are crucial for development of novel treatments and for management of cardiac arrhythmias and their consequences in patients, as their incidence is increasing worldwide.

Functional evaluation of the tachycardia patient-derived iPSC cardiomyocytes carrying a novel pathogenic SCN5A variant

Tachycardia is characterized by high beating rates that can lead to life-threatening fibrillations. Mutations in several ion-channel genes were implicated with tachycardia; however, the complex genetic contributors and their modes of action are still unclear. Here, we investigated the influence of an SCN5A gene variant on tachycardia phenotype by deriving patient-specific iPSCs and cardiomyocytes (iPSC-CM). Two tachycardia patients were genetically analyzed and revealed to inherit a heterozygous p.F1465L variant in the SCN5A gene. Gene expression and immunocytochemical analysis in iPSC-CMs generated from patients did not show any significant changes in mRNA levels of SCN5A or gross NaV1.5 cellular mislocalization, compared to healthy-derived iPSC-CMs. Electrophysiological and contraction imaging analysis in patient iPSC-CMs revealed intermittent fibrillation-like states, occasional arrhythmic events, and sustained high-paced contractions that could be selectively reduced by flecainide treatment. The patch-clamp analysis demonstrated a negative shift in the voltage-dependent activation at the patient-derived iPSC-CMs compared to the healthy control line, suggestive of a gain-of-function activity associated with the SCN5A^+/p.F1465L variant. Our patient-derived iPSC-CM model recapitulated the clinically relevant characteristics of tachycardia associated with a novel pathogenic SCN5A^+/p.F1465L variant leading to altered Na⁺ channel kinetics as the likely mechanism underlying high excitability and tachycardia phenotype.

Timing of food intake in mice unmasks a role for the cardiomyocyte circadian clock mechanism in limiting QT-interval prolongation

Cardiac electrophysiological studies demonstrate that restricting the feeding of mice to the light cycle (time restricted feeding or TRF) causes a pronounced change in heart rate and ventricular repolarization as measured by the RR- and QT-interval, respectively. TRF slows heart rate and shifts the peak (acrophase) of the day/night rhythms in the RR- and QT-intervals from the light to the dark cycle. This study tested the hypothesis that these changes in cardiac electrophysiology are driven by the cardiomyocyte circadian clock mechanism. We determined the impact that TRF had on RR- and QT-intervals in control mice or mice that had the cardiomyocyte circadian clock mechanism disrupted by inducing the deletion of Bmal1 in adult cardiomyocytes (iCSΔBmal1^-/- mice). In control and iCSΔBmal1^-/- mice, TRF increased the RR-intervals measured during the dark cycle and shifted the acrophase of the day/night rhythm in the RR-interval from the light to the dark cycle. Compared to control mice, TRF caused a larger prolongation of the QT-interval measured from iCSΔBmal1^-/- mice during the dark cycle. The larger QT-interval prolongation in the iCSΔBmal1^-/- mice caused an increased mean and amplitude in the day/night rhythm of the QT-interval. There was not a difference in the TRF-induced shift in the day/night rhythm of the QT-interval measured from control or iCSΔBmal1^-/- mice. We conclude that the cardiomyocyte circadian clock does not drive the changes in heart rate or ventricular repolarization with TRF. However, TRF unmasks an important role for the cardiomyocyte circadian clock to prevent excessive QT-interval prolongation, especially at slow heart rates.

Electrophysiological Response to Acehytisine Was Modulated by Aldosterone in Rats with Aorto-Venocaval Shunts

Aldosterone induces cardiac electrical and structural remodeling, which leads to the development of heart failure and/or atrial fibrillation (AF). However, it remains unknown whether aldosterone-induced remodeling may modulate the efficacy of anti-AF drugs. In this study, we aimed to jeopardize the structural and functional remodeling by aldosterone in rats with aorto-venocaval shunts (AVS rats) and evaluate the effect of acehytisine in this model. An AVS operation was performed on rats (n = 6, male) and it was accompanied by the intraperitoneal infusion of aldosterone (AVS + Ald) at 2.0 µg/h for 28 d. The cardiopathy was characterized by echocardiography, electrophysiologic and hemodynamic testing, and morphometric examination in comparison with sham-operated rats (n = 3), sham + Ald (n = 6), and AVS (n = 5). Aldosterone accelerated the progression from asymptomatic heart failure to overt heart failure and induced sustained AF resistant to electrical fibrillation in one out of six rats. In addition, it prolonged PR, QT interval and Wenckebach cycle length. Acehytisine failed to suppress AF in the AVS + Ald rats. In conclusion, aldosterone jeopardized electrical remodeling and blunted the electrophysiological response to acehytisine on AF.

SCN5A compound heterozygosity mutation in Brugada syndrome: Functional consequences and the implication for pharmacological treatment

AIMS

SCN5A gene encodes the α-subunit of Na_v1.5, mainly found in the human heart. SCN5A variants are the most common genetic alterations associated with Brugada syndrome (BrS). In rare cases, compound heterozygosity is observed; however, its functional consequences are poorly understood. We aimed to analyze the functional impact of de novo Na_v1.5 mutations in compound heterozygosity in distinct alleles (G400R and T1461S positions) previously found in a patient with BrS. Moreover, we evaluated the potential benefits of quinidine to improve the phenotype of mutant Na⁺ channels in vitro.

MATERIALS AND METHODS

The functional properties of human wild-type and Na_v1.5 variants were evaluated using whole-cell patch-clamp and immunofluorescence techniques in transiently expressed human embryonic kidney (HEK293) cells.

KEY FINDINGS

Both variants occur in the highly conservative positions of SCN5A. Although all variants were expressed in the cell membrane, a significant reduction in the Na⁺ current density (except for G400R alone, which was undetected) was observed along with abnormal biophysical properties, once the variants were expressed in homozygosis and heterozygosis. Interestingly, the incubation of transfected cells with quinidine partially rescued the biophysical properties of the mutant Na⁺ channel.

SIGNIFICANCE

De novo compound heterozygosis mutations in SNC5A disrupt the Na⁺ macroscopic current. Quinidine could partially reverse the in vitro loss-of-function phenotype of Na⁺ current. Thus, our data provide, for the first time, a detailed biophysical characterization of dysfunctional Na⁺ channels linked to compound heterozygosity in BrS as well as the benefits of the pharmacological treatment using quinidine on the biophysical properties of Na_v1.5.

Integrating proteomic, lipidomic and metabolomic data to construct a global metabolic network of lethal ventricular tachyarrhythmias (LVTA) induced by aconitine

Lethal ventricular tachyarrhythmias (LVTA)-related sudden cardiac death (SCD) is one of the major causes of death worldwide. However, the mechanisms underlying LVTA induced by myocardial ion channel diseases (MICDs) are not yet fully understood. Here, we produced an LVTA rat model induced by aconitine, to mimic MICDs-elicited LVTA, and constructed a global pathway network via integrating proteomic and lipidomic data, and our previously published metabolomic data. Results showed that both proteome and lipidome were disturbed during the LVTA process. Most of the differentially expressed proteins and lipid species were correlated. Proteomic data indicated disturbance of energy metabolism (e.g. fatty acid β-oxidation and the tricarboxylic acid cycle) and activation of the protein kinase C and nicotinamide adenine dinucleotide phosphate (NAPDH) oxidase pathway; these alterations led to lowered ATP and elevated ROS, respectively. Altered levels of the Ca²⁺ handling proteins suggested aberrant intracellular Ca²⁺ homeostasis, which might also be secondary to the shortage of ATP and oxidative stress. Significantly, the disrupted pathways implied by proteomic data were largely confirmed by lipidomic and metabolomic data. Collectively, we have constructed a metabolic pathway network of aconitine-induced LVTA using a multi-omics strategy, which confers great promise for the deeper interpretation of the mechanisms underlying LVTA. SIGNIFICANCE: In this study, we integrated proteomics, lipidomics and metabolomics to explore the pathophysiological processes of LVTA induced by aconitine. It is innovative to try to integrate these three omics in a study exploring the relative mechanisms. Here, based on the DEPs and differentially abundant lipid species (DALPs) between the LVTA groups and the controls, and the different metabolites discovered previously from the same model, we have successfully constructed a global metabolic network. Taken together, the multi-omics integration strategies used in this study show the potential for a new interpretation of the pathophysiological processes of LVTA induced by different conditions and open the possibility to explore deeper and broader mechanisms of other diseases.

The Emergence of Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes (hiPSC-CMs) as a Platform to Model Arrhythmogenic Diseases

There is a need for improved in vitro models of inherited cardiac diseases to better understand basic cellular and molecular mechanisms and advance drug development. Most of these diseases are associated with arrhythmias, as a result of mutations in ion channel or ion channel-modulatory proteins. Thus far, the electrophysiological phenotype of these mutations has been typically studied using transgenic animal models and heterologous expression systems. Although they have played a major role in advancing the understanding of the pathophysiology of arrhythmogenesis, more physiological and predictive preclinical models are necessary to optimize the treatment strategy for individual patients. Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) have generated much interest as an alternative tool to model arrhythmogenic diseases. They provide a unique opportunity to recapitulate the native-like environment required for mutated proteins to reproduce the human cellular disease phenotype. However, it is also important to recognize the limitations of this technology, specifically their fetal electrophysiological phenotype, which differentiates them from adult human myocytes. In this review, we provide an overview of the major inherited arrhythmogenic cardiac diseases modeled using hiPSC-CMs and for which the cellular disease phenotype has been somewhat characterized.

Dysfunctional Nav1.5 channels due to SCN5A mutations

The voltage-gated sodium channel 1.5 (Nav1.5), encoded by the SCN5A gene, is responsible for the rising phase of the action potential of cardiomyocytes. The sodium current mediated by Nav1.5 consists of peak and late components (I_Na-P and I_Na-L). Mutant Nav1.5 causes alterations in the peak and late sodium current and is associated with an increasingly wide range of congenital arrhythmias. More than 400 mutations have been identified in the SCN5A gene. Although the mechanisms of SCN5A mutations leading to a variety of arrhythmias can be classified according to the alteration of I_Na-P and I_Na-L as gain-of-function, loss-of-function and both, few researchers have summarized the mechanisms in this way before. In this review article, we aim to review the mechanisms underlying dysfunctional Nav1.5 due to SCN5A mutations and to provide some new insights into further approaches in the treatment of arrhythmias. Impact statement The field of ion channelopathy caused by dysfunctional Nav1.5 due to SCN5A mutations is rapidly evolving as novel technologies of electrophysiology are introduced and our understanding of the mechanisms of various arrhythmias develops. In this review, we focus on the dysfunctional Nav1.5 related to arrhythmias and the underlying mechanisms. We update SCN5A mutations in a precise way since 2013 and presents novel classifications of SCN5A mutations responsible for the dysfunction of the peak (I_Na-P) and late (I_Na-L) sodium channels based on their phenotypes, including loss-, gain-, and coexistence of gain- and loss-of function mutations in I_Na-P, I_Na-L, respectively. We hope this review will provide a new comprehensive way to better understand the electrophysiological mechanisms underlying arrhythmias from cell to bedside, promoting the management of various arrhythmias in practice.

Have the Findings from Clinical Risk Prediction and Trials Any Key Messages for Safety Pharmacology?

Anti-arrhythmic drugs are a mainstay in the management of symptoms related to arrhythmias, and are adjuncts in prevention and treatment of life-threatening ventricular arrhythmias. However, they also have the potential for pro-arrhythmia and thus the prediction of arrhythmia predisposition and drug response are critical issues. Clinical trials are the latter stages in the safety testing and efficacy process prior to market release, and as such serve as a critical safeguard. In this review, we look at some of the lessons to be learned from approaches to arrhythmia prediction in patients, clinical trials of drugs used in the treatment of arrhythmias, and the implications for the design of pre-clinical safety pharmacology testing.

Hormones and sex differences: changes in cardiac electrophysiology with pregnancy

Disruption of cardiac electrical activity resulting in palpitations and syncope is often an early symptom of pregnancy. Pregnancy is a time of dramatic and dynamic physiological and hormonal changes during which numerous demands are placed on the heart. These changes result in electrical remodelling which can be detected as changes in the electrocardiogram (ECG). This gestational remodelling is a very under-researched area. There are no systematic large studies powered to determine changes in the ECG from pre-pregnancy, through gestation, and into the postpartum period. The large variability between patients and the dynamic nature of pregnancy hampers interpretation of smaller studies, but some facts are consistent. Gestational cardiac hypertrophy and a physical shift of the heart contribute to changes in the ECG. There are also electrical changes such as an increased heart rate and lengthening of the QT interval. There is an increased susceptibility to arrhythmias during pregnancy and the postpartum period. Some changes in the ECG are clearly the result of changes in ion channel expression and behaviour, but little is known about the ionic basis for this electrical remodelling. Most information comes from animal models, and implicates changes in the delayed-rectifier channels. However, it is likely that there are additional roles for sodium channels as well as changes in calcium homoeostasis. The changes in the electrical profile of the heart during pregnancy and the postpartum period have clear implications for the safety of pregnant women, but the field remains relatively undeveloped.

Quantitative analysis of the Ca2+ -dependent regulation of delayed rectifier K+ current IKs in rabbit ventricular myocytes

KEY POINTS

[Ca²⁺ ]_i enhanced rabbit ventricular slowly activating delayed rectifier K⁺ current (I_Ks ) by negatively shifting the voltage dependence of activation and slowing deactivation, similar to perfusion of isoproterenol. Rabbit ventricular rapidly activating delayed rectifier K⁺ current (I_Kr ) amplitude and voltage dependence were unaffected by high [Ca²⁺ ]_i . When measuring or simulating I_Ks during an action potential, I_Ks was not different during a physiological Ca²⁺ transient or when [Ca²⁺ ]_i was buffered to 500 nm.

ABSTRACT

The slowly activating delayed rectifier K⁺ current (I_Ks ) contributes to repolarization of the cardiac action potential (AP). Intracellular Ca²⁺ ([Ca²⁺ ]_i ) and β-adrenergic receptor (β-AR) stimulation modulate I_Ks amplitude and kinetics, but details of these important I_Ks regulators and their interaction are limited. We assessed the [Ca²⁺ ]_i dependence of I_Ks in steady-state conditions and with dynamically changing membrane potential and [Ca²⁺ ]_i during an AP. I_Ks was recorded from freshly isolated rabbit ventricular myocytes using whole-cell patch clamp. With intracellular pipette solutions that controlled free [Ca²⁺ ]_i , we found that raising [Ca²⁺ ]_i from 100 to 600 nm produced similar increases in I_Ks as did β-AR activation, and the effects appeared additive. Both β-AR activation and high [Ca²⁺ ]_i increased maximally activated tail I_Ks , negatively shifted the voltage dependence of activation, and slowed deactivation kinetics. These data informed changes in our well-established mathematical model of the rabbit myocyte. In both AP-clamp experiments and simulations, I_Ks recorded during a normal physiological Ca²⁺ transient was similar to I_Ks measured with [Ca²⁺ ]_i clamped at 500-600 nm. Thus, our study provides novel quantitative data as to how physiological [Ca²⁺ ]_i regulates I_Ks amplitude and kinetics during the normal rabbit AP. Our results suggest that micromolar [Ca²⁺ ]_i , in the submembrane or junctional cleft space, is not required to maximize [Ca²⁺ ]_i -dependent I_Ks activation during normal Ca²⁺ transients.

KChIP2 is a core transcriptional regulator of cardiac excitability

Arrhythmogenesis from aberrant electrical remodeling is a primary cause of death among patients with heart disease. Amongst a multitude of remodeling events, reduced expression of the ion channel subunit KChIP2 is consistently observed in numerous cardiac pathologies. However, it remains unknown if KChIP2 loss is merely a symptom or involved in disease development. Using rat and human derived cardiomyocytes, we identify a previously unobserved transcriptional capacity for cardiac KChIP2 critical in maintaining electrical stability. Through interaction with genetic elements, KChIP2 transcriptionally repressed the miRNAs miR-34b and miR-34c, which subsequently targeted key depolarizing (I_Na) and repolarizing (I_to) currents altered in cardiac disease. Genetically maintaining KChIP2 expression or inhibiting miR-34 under pathologic conditions restored channel function and moreover, prevented the incidence of reentrant arrhythmias. This identifies the KChIP2/miR-34 axis as a central regulator in developing electrical dysfunction and reveals miR-34 as a therapeutic target for treating arrhythmogenesis in heart disease.

DOI:http://dx.doi.org/10.7554/eLife.17304.001

The heart pumps blood throughout the body to provide oxygen and nourishment. To do so, proteins in the heart create electrical signals that tell the heart muscles to contract in a coordinated manner. Heart disease can cause cells to lose control of the production or activity of these proteins, creating disorganized electrical signals called arrhythmias that interfere with the heart’s ability to pump. Sometimes these arrhythmias lead to sudden death.

Researchers do not know exactly what triggers these changes in the heart’s normal electrical rhythms. This has made it difficult to develop strategies to prevent these disruptions or to fix them when they occur.

By studying rat and human heart cells, Nassal et al. now show that a protein called KChIP2 stops working properly during heart disease. Most importantly, because of the decreased level of KChIP2 in heart disease, KChIP2 loses the ability to restrict the production of two microRNA molecules – a role that KChIP2 was not previously known to perform. This loss of activity sets off a cascade of signals that worsens the balance of electrical activity in the heart cells, creating arrhythmias.

Treatments that restored proper levels of the fully working KChIP2 protein to the heart cells or that blocked the signals set off by a lack of KChIP2 returned the electrical activity of the cells back to normal. This also stopped the development of arrhythmias. Further studies are now needed to investigate whether these treatments have the same effects in living mammals. If effective, this could ultimately lead to new treatments for heart diseases and arrhythmias.

DOI:http://dx.doi.org/10.7554/eLife.17304.002

Using Zebrafish for High-Throughput Screening of Novel Cardiovascular Drugs

Cardiovascular diseases remain a major challenge for modern drug discovery. The diseases are chronic, complex, and the result of sophisticated interactions between genetics and environment involving multiple cell types and a host of systemic factors. The clinical events are often abrupt, and the diseases may be asymptomatic until a highly morbid event. Target selection is often based on limited information, and though highly specific agents are often identified in screening, their final efficacy is often compromised by unanticipated systemic responses, a narrow therapeutic index, or substantial toxicities. Our understanding of complexity of cardiovascular disease has grown dramatically over the past 2 decades, and the range of potential disease mechanisms now includes pathways previously thought only tangentially involved in cardiac or vascular disease. Despite these insights, the majority of active cardiovascular agents derive from a remarkably small number of classes of agents and target a very limited number of pathways. These agents have often been used initially for particular indications and then discovered serendipitously to have efficacy in other cardiac disorders or in a manner unrelated to their original mechanism of action. In this review, the rationale for in vivo screening is described, and the utility of the zebrafish for this approach and for complementary work in functional genomics is discussed. Current limitations of the model in this setting and the need for careful validation in new disease areas are also described. An overview is provided of the complex mechanisms underlying most clinical cardiovascular diseases, and insight is offered into the limits of single downstream pathways as drug targets. The zebrafish is introduced as a model organism, in particular for cardiovascular biology. Potential approaches to overcoming the hurdles to drug discovery in the face of complex biology are discussed, including in vivo screening of zebrafish genetic disease models.

Effect of vitamin D deficiency on the development of postoperative atrial fibrillation in coronary artery bypass patients

Introduction: Various factors may be responsible for the development of postoperative atrial fibrillation (POAF) in coronary artery bypass graft (CABG) patients. In our study, we demonstrated the effect of vitamin D deficiency on the development of POAF. Methods: In this prospective case control study, patients undergoing elective, isolated CABG were considered. A total of 15 patients (16.6%) who developed POAF during the first five days after surgery made up the POAF group. Seventy-five patients that had a sinus rhythm in the same period were the non-POAF group. The two groups were compared statistically in terms of laboratory, clinical, echocardiographic, operative, and postoperative parameters. Results: All patients were in sinus rhythm at discharge. The baseline characteristics of the study groups were comparable. The POAF group had a lower vitamin D level than the non-POAF group (9.0 ± 5.0 and 15.0 ± 8.4 ng/mL, respectively; P=0.007). In the POAF group, the patients' left atrium diameter and incidence of hypertension (HT) were higher than those of the non-POAF group. Conclusion: Incidence of POAF was significantly higher in patients with vitamin D deficiency or insufficiency than the patients with vitamin D level in normal range. Therefore vitamin D deficiency or insufficiency may be a predictor of POAF in patients with CABG.

Changes in Myocardial Composition and Conduction Properties in Rat Heart Failure Model Induced by Chronic Volume Overload

Volume overload leads to development of eccentric cardiac hypertrophy and heart failure. In our previous report, we have shown myocyte hypertrophy with no fibrosis and decrease in gap junctional coupling via connexin43 in a rat model of aorto-caval fistula at 21 weeks. Here we set to analyze the electrophysiological and protein expression changes in the left ventricle and correlate them with phenotypic severity based upon ventricles to body weight ratio. ECG analysis showed increased amplitude and duration of the P wave, prolongation of PR and QRS interval, ST segment elevation and decreased T wave amplitude in the fistula group. Optical mapping showed a prolongation of action potential duration in the hypertrophied hearts. Minimal conduction velocity (CV) showed a bell-shaped curve, with a significant increase in the mild cases and there was a negative correlation of both minimal and maximal CV with heart to body weight ratio. Since the CV is influenced by gap junctional coupling as well as the autonomic nervous system, we measured the amounts of tyrosine hydroxylase (TH) and choline acetyl transferase (ChAT) as a proxy for sympathetic and parasympathetic innervation, respectively. At the protein level, we confirmed a significant decrease in total and phosphorylated connexin43 that was proportional to the level of hypertrophy, and similarly decreased levels of TH and ChAT. Even at a single time-point, severity of morphological phenotype correlates with progression of molecular and electrophysiological changes, with the most hypertrophied hearts showing the most severe changes that might be related to arrhythmogenesis.

Inhibition of late sodium current attenuates ionic arrhythmia mechanism in ventricular myocytes expressing LaminA-N195K mutation

BACKGROUND

Lamin A and C are nuclear filament proteins encoded by the LMNA gene. Mutations in the LMNA gene cause many congenital diseases known as laminopathies, including Emery-Dreifuss muscular dystrophy, Hutchinson-Gilford progeria syndrome, and familial dilated cardiomyopathy (DCM) with conduction disease. A missense mutation (N195K) in the A-type lamins results in familial DCM and sudden arrhythmic death.

OBJECTIVE

The purpose of this study was to investigate the ion current mechanism of arrhythmia and DCM caused by the LaminA-N195K variant.

METHODS

A homozygous mouse line expressing the Lmna-N195K mutation (Lmna^N195K/N195K) that exhibited arrhythmia, DCM, and sudden death was used. Using whole cell patch-clamp technique, we measured action potential duration (APD), Na⁺ currents (I_Na) in ventricular myocytes isolated from Lmna^N195K/N195K, and wild-type mice.

RESULTS

Both peak and late I_Na were significantly (P <.05) increased in Lmna^N195K/N195K ventricular myocytes. Similarly, Lmna^N195K/N195K ventricular myocytes exhibited significant (P <.005) prolongation of APD (time to 50% [APD₅₀] and 90% [APD₉₀] repolarization) and triggered activity. Acute application of ranolazine inhibited late I_Na, shortened APD, and abolished triggered activity in Lmna^N195K/N195K ventricular myocytes.

CONCLUSION

Inhibition of late I_Na may be an effective therapy in preventing arrhythmia in patients with LmnaN195K mutation-related DCM.

Chronic cyclic vagus nerve stimulation has beneficial electrophysiological effects on healthy hearts in the absence of autonomic imbalance

Cardiovascular disease degrades the regulatory function of the autonomic nervous system. Cyclic vagus nerve stimulation (VNS) is an already FDA-approved therapy for drug-resistant epilepsy and depression, and has been shown to normalize autonomic function and improve objective measures of heart function and subjective measures of heart failure symptoms. However, it remains unclear whether VNS may induce negative effects in patients with potentially healthy hearts where VNS can be used for epileptic patients. Hence, this study aims to investigate the effects of VNS on the hearts of healthy rats with normal autonomic balance. Sprague-Dawley rats were implanted with stimulators and randomized to either Sham or VNS groups. Rats in VNS group received 10 weeks of chronic intermittent VNS via stimulation of the right cervical vagus nerve. Echocardiography was performed at Baseline (prior to VNS), Week 2, and Week 9. After 10 weeks, high-resolution optical mapping was performed in ex vivo perfused hearts to evaluate the electrophysiological remodeling that occurs in the heart as a result of the VNS therapy. Chronic VNS modified the electrophysiological properties of healthy rat hearts by reducing the action potential duration at 50% (APD50) and 80% (APD80) repolarization. Chronic VNS also affected the restitution properties of the heart at the APD50 level and increased myocardial conduction velocity (CV). VNS did not induce any significant changes to ventricular ejection fraction (EF) and spatial dispersion of APD, thus indicating that VNS did not negatively affect cardiac function. VNS also reduced the susceptibility to ventricular arrhythmias (ventricular fibrillation [VF] and ventricular tachycardia [VT]) during ex vivo programmed electrical stimulation. In summary, chronic application of cyclic VNS induces changes to the electrophysiological properties of healthy rat hearts. The observed decrease in APD and increase in CV suggest that the beneficial effects of VNS do not require the presence of existing autonomic imbalance.

The problem of police-related cardiac arrest

The term "positional asphyxia" was originally used to describe the situation in which the upper airways becomes compromised by sharp angulation of the head or neck, or where the chest wall is splinted and the diaphragm is prevented from moving because of an unusual position of the body. The term was redefined in the early 1980s to describe sudden death during physical restraint of an individual who is in a prone position. A large percent of reported victims were overweight males. Most were in early middle age and manifesting psychotic behavior at the time of death. Most were reported to have unremarkable autopsies, save for the finding, in many cases, of cocaine or methamphetamine (more recently synthetic cannabinoids and cathinones as well). As no cause of death was apparent (other than non-specific signs such as pulmonary edema), it became common practice to attribute death to force exerted on the decedent's back. When experimental studies with human volunteers disproved this notion, the term "restraint asphyxia" was substituted for positional asphyxia, but with nearly the exact same meaning. No experimental study has ever determined the actual amount of force necessary to cause asphyxia by force applied to the back (although the range of required static force is known), nor the duration for which it must be applied. This review discusses the epidemiology and the evidence for and against the theory of "restraint/positional" asphyxia. It also considers alternative theories of causation, including the findings of studies suggesting that cardiac channelopathies/cardiomyopathies may explain many cases of ARD.

Spironolactone Regulates HCN Protein Expression Through Micro-RNA-1 in Rats With Myocardial Infarction

Emerging evidence has shown that aldosterone blockers reduced the incidence of ventricular arrhythmias in patients with myocardial infarction (MI). However, the mechanism remains unknown. In this study, we investigated the mechanism by which spironolactone, a classic aldosterone blocker, regulates hyperpolarization-activated cyclic nucleotide-gated channel (HCN) protein expression in ischemic rat myocardium after MI. Eighteen rats surviving 24 hours after MI were randomly assigned into 3 groups: MI, spironolactone, and spironolactone + antagomir-1. Six sham-operated rats had a suture loosely tied around the left coronary artery, without ligation. The border zone of the myocardial infarct was collected from each rat at 1 week after MI. HCN2 and HCN4 protein and messenger RNA (mRNA) level were measured in addition to miRNA-1 levels. Spironolactone significantly increased miRNA-1 levels and downregulated HCN2 and HCN4 protein and mRNA levels. miRNA-1 suppression with antagomir-1 increased HCN2 and HCN4 protein levels; however, HCN2 and HCN4 mRNA levels were not affected. These results suggested that spironolactone could increase miRNA-1 expression in ischemic rat myocardium after MI and that the upregulation of miRNA-1 expression partially contributed to the posttranscriptional repression of HCN protein expression, which may contribute to the effect of spironolactone to reduce the incidence of MI-associated ventricular arrhythmias.

Variability of Action Potentials Within and Among Cardiac Cell Clusters Derived from Human Embryonic Stem Cells

Electrophysiological variability in cardiomyocytes derived from pluripotent stem cells continues to be an impediment for their scientific and translational applications. We studied the variability of action potentials (APs) recorded from clusters of human embryonic stem cell-derived cardiomyocytes (hESC-CMs) using high-resolution optical mapping. Over 23,000 APs were analyzed through four parameters: APD₃₀, APD₈₀, triangulation and fractional repolarization. Although measures were taken to reduce variability due to cell culture conditions and rate-dependency of APs, we still observed significant variability in APs among and within the clusters. However, similar APs were found in spatial locations with close proximity, and in some clusters formed distinct regions having different AP characteristics that were reflected as separate peaks in the AP parameter distributions, suggesting multiple electrophysiological phenotypes. Using a recently developed automated method to group cells based on their entire AP shape, we identified distinct regions of different phenotypes within single clusters and common phenotypes across different clusters when separating APs into 2 or 3 subpopulations. The systematic analysis of the heterogeneity and potential phenotypes of large populations of hESC-CMs can be used to evaluate strategies to improve the quality of pluripotent stem cell-derived cardiomyocytes for use in diagnostic and therapeutic applications and in drug screening.

Molecular motions that shape the cardiac action potential: Insights from voltage clamp fluorometry

Very recently, voltage-clamp fluorometry (VCF) protocols have been developed to observe the membrane proteins responsible for carrying the ventricular ionic currents that form the action potential (AP), including those carried by the cardiac Na(+) channel, NaV1.5, the L-type Ca(2+) channel, CaV1.2, the Na(+)/K(+) ATPase, and the rapid and slow components of the delayed rectifier, KV11.1 and KV7.1. This development is significant, because VCF enables simultaneous observation of ionic current kinetics with conformational changes occurring within specific channel domains. The ability gained from VCF, to connect nanoscale molecular movement to ion channel function has revealed how the voltage-sensing domains (VSDs) control ion flux through channel pores, mechanisms of post-translational regulation and the molecular pathology of inherited mutations. In the future, we expect that this data will be of great use for the creation of multi-scale computational AP models that explicitly represent ion channel conformations, connecting molecular, cell and tissue electrophysiology. Here, we review the VCF protocol, recent results, and discuss potential future developments, including potential use of these experimental findings to create novel computational models.

Effects of droperidol and ondansetron on dispersion of ventricular repolarization: A randomized double-blind clinical study in anesthetized adult patients

BACKGROUND AND OBJECTIVE

Droperidol and ondansetron prolong QT interval, a circumstance that has raised some concerns regarding the possibility of inducing torsades de pointes (TdP). However drug-induced spatial dispersion of ventricular repolarization has been shown to be the principal arrhythmogenic substrate for TdP. The aim of this study is to explore the effects of droperidol and ondansetron on the dispersion of repolarization, measured using the T peak-to-end interval (Tp-e) and Tp-e/QT and Tp-e/RR(1/2) ratios in surgical anesthetized patients.

METHODS

A randomized, double-blind study carried out on sixty-three adult patients without cardiac disease or factors favoring QT prolongation and undergoing non-cardiac surgery were randomly assigned to the droperidol or ondansetron group. Under propofol anesthesia, a 12-lead EKG was obtained, and 1.25mg droperidol or 4mg ondansetron was injected. Five minutes later, a new 12-lead EKG was recorded. EKG analyses were independently performed by two cardiologists blinded to the state of the traces or group allocation. QT, RR and Tp-e intervals were measured by averaging five successive beats in leadII (QT) or V5 (Tp-e). The mean value for each measurement was calculated for statistical analysis.

RESULTS

Thirty-two patients (19 women) received droperidol, and 31 (22 women) ondansetron. Droperidol and ondansetron prolonged the QTcF interval (Fridericia formula) by 6.8 and 7.2ms (mean values) respectively, but neither droperidol nor ondansetron increased the Tp-e interval or Tp-e/QT and Tp-e/RR(1/2) ratios.

CONCLUSION

At antiemetic doses, neither ondansetron (4mg) nor droperidol (1.25mg) increases the dispersion of ventricular repolarization in healthy adult patients anesthetized with propofol.

Ion Channels in the Heart

Optimal cardiac function depends on proper timing of excitation and contraction in various regions of the heart, as well as on appropriate heart rate. This is accomplished via specialized electrical properties of various components of the system, including the sinoatrial node, atria, atrioventricular node, His-Purkinje system, and ventricles. Here we review the major regionally determined electrical properties of these cardiac regions and present the available data regarding the molecular and ionic bases of regional cardiac function and dysfunction. Understanding these differences is of fundamental importance for the investigation of arrhythmia mechanisms and pharmacotherapy.

Evidence of heterogeneity by race/ethnicity in genetic determinants of QT interval

BACKGROUND

QT interval (QT) prolongation is an established risk factor for ventricular tachyarrhythmia and sudden cardiac death. Previous genome-wide association studies in populations of the European descent have identified multiple genetic loci that influence QT, but few have examined these loci in ethnically diverse populations.

METHODS

Here, we examine the direction, magnitude, and precision of effect sizes for 21 previously reported SNPs from 12 QT loci, in populations of European (n = 16,398), African (n = 5,437), American Indian (n = 5,032), Hispanic (n = 1,143), and Asian (n = 932) descent as part of the Population Architecture using Genomics and Epidemiology (PAGE) study. Estimates obtained from linear regression models stratified by race/ethnicity were combined using inverse-variance weighted meta-analysis. Heterogeneity was evaluated using Cochran's Q test.

RESULTS

Of 21 SNPs, 7 showed consistent direction of effect across all 5 populations, and an additional 9 had estimated effects that were consistent across 4 populations. Despite consistent direction of effect, 9 of 16 SNPs had evidence (P < 0.05) of heterogeneity by race/ethnicity. For these 9 SNPs, linkage disequilibrium plots often indicated substantial variation in linkage disequilibrium patterns among the various racial/ethnic groups, as well as possible allelic heterogeneity.

CONCLUSIONS

These results emphasize the importance of analyzing racial/ethnic groups separately in genetic studies. Furthermore, they underscore the possible utility of trans-ethnic studies to pinpoint underlying casual variants influencing heritable traits such as QT.

Reduction in dynamin-2 is implicated in ischaemic cardiac arrhythmias

Ischaemic cardiac arrhythmias cause a large proportion of sudden cardiac deaths worldwide. The ischaemic arrhythmogenesis is primarily because of the dysfunction and adverse remodelling of sarcolemma ion channels. However, the potential regulators of sarcolemma ion channel turnover and function in ischaemic cardiac arrhythmias remains unknown. Our previous studies indicate that dynamin-2 (DNM2), a cardiac membrane-remodelling GTPase, modulates ion channels membrane trafficking in the cardiomyocytes. Here, we have found that DNM2 plays an important role in acute ischaemic arrhythmias. In rat ventricular tissues and primary cardiomyocytes subjected to acute ischaemic stress, the DNM2 protein and transcription levels were markedly down-regulated. This DNM2 reduction was coupled with severe ventricular arrhythmias. Moreover, we identified that the down-regulation of DNM2 within cardiomyocytes increases the action potential amplitude and prolongs the re-polarization duration by depressing the retrograde trafficking of Nav1.5 and Kir2.1 channels. These effects are likely to account for the DNM2 defect-induced arrhythmogenic potentials. These results suggest that DNM2, with its multi-ion channel targeting properties, could be a promising target for novel antiarrhythmic therapies.

Sex differences in the mechanisms underlying long QT syndrome

Sexual dimorphism is a well-established phenomenon, but its degree varies tremendously among species. Since the early days of Einthoven's development of the three-lead galvanometer ECG, we have known there are marked differences in QT intervals of men and women. It required over a century to appreciate the profound implications of sex-based electrophysiological differences in QT interval on the panoply of sex differences with respect to arrhythmia risk, drug sensitivity, and treatment modalities. Little is known about the fundamental mechanism responsible for sex differences in electrical substrate of the human heart, in large part due to the lack of tissue availability. Animal models are an important research tool, but species differences in the sexual dimorphism of the QT interval, the ionic currents underlying the cardiac repolarization, and effects of sex steroids make it difficult to interpolate animal to human sex differences. In addition, in some species, different strains of the same animal model yield conflicting data. Each model has its strengths, such as ease of genetic manipulation in mice or size in dogs. However, many animals do not reproduce the sexual dimorphism of QT seen in humans. To match sex linked prolongation of QT interval and arrhythmogenic phenotype, the current data suggest that the rabbit may be best suited to provide insight into sex differences in humans. In the future, emerging technologies such as induced pluripotent stem cell derived cardiac myocyte systems may offer the opportunity to study sex differences in a controlled hormonal situation in the context of a sex specific human model system.

Identification and functional analysis of a new putative caveolin-3 variant found in a patient with sudden unexplained death

Background

Sudden cardiac death (SCD) is the clinical outcome of a lethal arrhythmia that can develop on the background of unrecognized channelopathies or cardiomyopathies. Several susceptibility genes have been identified for the congenital forms of these cardiac diseases, including caveolin-3 (Cav-3) gene. In the heart Cav-3 is the main component of caveolae, plasma membrane domains that regulate multiple cellular processes highly relevant for cardiac excitability, such as trafficking, calcium homeostasis, signal transduction and cellular response to injury. Here we characterized a new putative Cav-3 variant, Cav-3 V82I, found in a patient with SCD.

Results

In heterologous systems Cav-3 V82I was expressed at significantly higher level than Cav-3 WT and accumulated within the cells. Cells expressing Cav-3 V82I exhibited a decreased activation of extracellular-signal-regulated kinases (ERKs) and were more vulnerable to sub-lethal osmotic stress.

Conclusion

Considering that abnormal loss of myocytes can play a mechanistic role in lethal cardiac diseases, we suggest that the detrimental effect of Cav-3 V82I variant on cell viability may participate in determining the susceptibility to cardiac death.

Calcium channel autoantibodies predicted sudden cardiac death and all-cause mortality in patients with ischemic and nonischemic chronic heart failure

The purpose of this study was to evaluate whether CC-AAbs levels could predict prognosis in CHF patients. A total of 2096 patients with CHF (841 DCM patients and 1255 ICM patients) and 834 control subjects were recruited. CC-AAbs were detected and the relationship between CC-AAbs and patient prognosis was analyzed. During a median follow-up time of 52 months, there were 578 deaths. Of these, sudden cardiac death (SCD) occurred in 102 cases of DCM and 121 cases of ICM. The presence of CC-AAbs in patients was significantly higher than that of controls (both P < 0.001). Multivariate analysis revealed that positive CC-AAbs could predict SCD (HR 3.191, 95% CI 1.598–6.369 for DCM; HR 2.805, 95% CI 1.488–5.288 for ICM) and all-cause mortality (HR 1.733, 95% CI 1.042–2.883 for DCM; HR 2.219, 95% CI 1.461–3.371 for ICM) in CHF patients. A significant association between CC-AAbs and non-SCD (NSCD) was found in ICM patients (HR = 1.887, 95% CI 1.081–3.293). Our results demonstrated that the presence of CC-AAbs was higher in CHF patients versus controls and corresponds to a higher incidence of all-cause death and SCD. Positive CC-AAbs may serve as an independent predictor for SCD and all-cause death in these patients.

Minimally invasive approach to the child with palpitations

Palpitations are a common reason for referral to a pediatric cardiologist. Although generally benign, palpitations are a significant cause for concern in the individual and their family. Similarly, palpitations may be the initial presentation of significant heart disease, resulting in heightened concern in the referring physician. Although emphasis is usually placed on excluding arrhythmia as the cause for palpitations, there are a variety of noncardiac causes for palpitations. The patient history and physical examination are the key components of the evaluation and guide subsequent investigations. In many cases, an immediate diagnosis cannot be made and additional testing may be required; this often includes further monitoring for episodes, cardiac imaging and ambulatory monitoring. Current technologies for ambulatory monitoring during symptoms include Holter monitoring and a variety of patient-activated event recorders, including implantable loop recorders. Each presents its own unique advantages and disadvantages to aid diagnosis in the management of a child with palpitations. The primary focus for the clinician is to determine whether the etiology is benign in nature or whether there is underlying heart disease that may carry a more serious prognosis.

Adenylyl cyclase 6 deletion increases mortality during sustained β-adrenergic receptor stimulation

Sustained β-adrenergic receptor stimulation is associated with cardiomyopathy, an affect thought to result from cAMP-associated cardiac injury. Using a murine line with adenylyl cyclase 6 gene deletion (AC6KO), we tested the hypothesis that AC6 deletion, by limiting cAMP production, would attenuate cardiomyopathy in the setting of sustained β-adrenergic receptor stimulation. During 7d isoproterenol infusion, there was unexpected higher mortality in AC6KO mice compared to wild type control mice (p<0.0001). However, left ventricular function was similarly impaired in isoproterenol-infused control and AC6KO mice. There were no group differences in left ventricular hypertrophy, apoptosis, and fibrosis. Telemetric electrocardiography showed progressive prolongation of PR interval (p<0.0001), QRS duration (p<0.0005), and QTc (p<0.0001), as well as reduction in heart rate (p<0.0001), in AC6KO mice during isoproterenol infusion. These defective electrophysiological properties in isoproterenol-infused AC6KO mice were associated with decreased longitudinal ventricular conduction velocity (p<0.05) and reduced phosphorylation of connexin 43 at S368 in left ventricular samples (p=0.006). Taken together, these data demonstrate that limiting cAMP production does not prevent sustained β-adrenergic receptor stimulation-induced cardiomyopathy. Moreover, AC6 deletion impairs electrophysiological properties and increases mortality during sustained β-adrenergic receptor stimulation. Decreased connexin 43 phosphorylation and impaired ventricular conduction may be of mechanistic importance for the defective electrophysiological properties.

Electromechanical relationship in hypertrophic cardiomyopathy

We examined whether there is a relationship between repolarization abnormalities on electrocardiography (EKG) and deformation abnormalities by echocardiography. Analysis of baseline EKGs and mechanical (echo-based deformation) changes was performed in 128 patients with a clinical diagnosis of hypertrophic cardiomyopathy (HCM). Patients with left ventricular hypertrophy (LVH) or repolarization abnormalities had higher septal thickness when compared to patients with normal EKG. Patients with EKG evidence of LVH or QTc prolongation had lower systolic velocity, systolic strain, systolic strain rate, late diastolic velocity, and late diastolic strain rate than patients with a normal EKG. Patients with strain pattern or ST depression/T-wave inversion had lower systolic velocity, systolic strain, systolic strain rate, early diastolic velocity, and late diastolic velocity when compared to patients with normal EKGs. LVH and repolarization abnormalities on surface EKG are markers of impaired systolic and diastolic mechanics in HCM.

The arrhythmogenic consequences of increasing late INa in the cardiomyocyte

This review presents the roles of cardiac sodium channel NaV1.5 late current (late INa) in generation of arrhythmic activity. The assumption of the authors is that proper Na(+) channel function is necessary to the maintenance of the transmembrane electrochemical gradient of Na(+) and regulation of cardiac electrical activity. Myocyte Na(+) channels' openings during the brief action potential upstroke contribute to peak INa and initiate excitation-contraction coupling. Openings of Na(+) channels outside the upstroke contribute to late INa, a depolarizing current that persists throughout the action potential plateau. The small, physiological late INa does not appear to be critical for normal electrical or contractile function in the heart. Late INa does, however, reduce the net repolarizing current, prolongs action potential duration, and increases cellular Na(+) loading. An increase of late INa, due to acquired conditions (e.g. heart failure) or inherited Na(+) channelopathies, facilitates the formation of early and delayed afterpolarizations and triggered arrhythmias, spontaneous diastolic depolarization, and cellular Ca(2+) loading. These in turn increase the spatial and temporal dispersion of repolarization time and may lead to reentrant arrhythmias.

Genetics of congenital and drug-induced long QT syndromes: current evidence and future research perspectives

The long QT syndrome (LQTS) is a condition characterized by abnormal prolongation of the QT interval with an associated risk of ventricular arrhythmias and sudden cardiac death. Congenital forms of LQTS arise due to rare and highly penetrant mutations that segregate in a Mendelian fashion. Over the years, multiple mutations in genes encoding ion channels and ion channel binding proteins have been reported to underlie congenital LQTS. Drugs are by far the most common cause of acquired forms of LQTS. Emerging evidence suggests that drug-induced LQTS also has a significant heritable component. However, the genetic substrate underlying drug-induced LQTS is presently largely unknown. In recent years, advances in next-generation sequencing technology and molecular biology techniques have significantly enhanced our ability to identify genetic variants underlying both monogenic diseases and more complex traits. In this review, we discuss the genetic basis of congenital and drug-induced LQTS and focus on future avenues of research in the field. Ultimately, a detailed characterization of the genetic substrate underlying congenital and drug-induced LQTS will enhance risk stratification and potentially result in the development of tailored genotype-based therapies.

Relationship of delayed enhancement by magnetic resonance to myocardial perfusion by positron emission tomography in hypertrophic cardiomyopathy

BACKGROUND

Presence of delayed enhancement (DE) on cardiac magnetic resonance (CMR) is associated with worse clinical outcomes in hypertrophic cardiomyopathy. We investigated the relationship between DE on CMR and myocardial ischemia in hypertrophic cardiomyopathy.

METHODS AND RESULTS

Hypertrophic cardiomyopathy patients (n=47) underwent CMR for assessment of DE and vasodilator stress ammonia positron emission tomography to quantify myocardial blood flow and coronary flow reserve. The summed difference score for regional myocardial perfusion was also assessed. Patients in the DE group (n=35) had greater left ventricular wall thickness (2.09±0.44 versus 1.78±0.34 cm; P=0.03). Stress myocardial blood flow (2.25±0.46 versus 1.78±0.43 mL/min per gram; P=0.01) and coronary flow reserve (2.78±0.32 versus 2.01±0.52; P<0.001) were significantly lower in DE-positive patients. Summed difference score (7.3±6.6 versus 0.9±1.4; P<0.0001) was significantly higher in patients with DE. A coronary flow reserve <2.00 was seen in 18 patients (51%) with DE but in none of the DE-negative patients (P<0.0001). CMR and positron emission tomography showed visually concordant DE and regional myocardial perfusion abnormalities in 31 patients and absence of DE and perfusion defects in 9 patients. Four DE-positive patients demonstrated normal regional myocardial perfusion, and 3 DE-negative patients had (apical) regional myocardial perfusion abnormalities.

CONCLUSIONS

We found a close relationship between DE by CMR and microvascular function in most of the patients studied. However, a small proportion of patients had DE in the absence of perfusion abnormalities, suggesting that microvascular dysfunction and ischemia are not the sole causes of DE in hypertrophic cardiomyopathy patients.

A KCNQ1 mutation causes a high penetrance for familial atrial fibrillation

BACKGROUND

Atrial fibrillation (AF) is the most common cardiac arrhythmia, and its incidence is expected to grow. A genetic predisposition for AF has long been recognized, but its manifestation in these patients likely involves a combination of rare and common genetic variants. Identifying genetic variants that associate with a high penetrance for AF would represent a significant breakthrough for understanding the mechanisms that associate with disease.

METHOD AND RESULTS

Candidate gene sequencing in 5 unrelated families with familial AF identified the KCNQ1 missense mutation p.Arg231His (R231H). In addition to AF, several of the family members have abnormal QTc intervals, syncope or experienced sudden cardiac arrest or death. KCNQ1 encodes the voltage-gated K(+) channel that conducts the slowly activating delayed rectifier K(+) current in the heart. Functional and computational analyses suggested that R231H increases KCNQ1 current (I(KCNQ1)) to shorten the atrial action potential (AP) duration. R231H is predicted to minimally affect ventricular excitability, but it prevented the increase in I(KCNQ1) following PKA activation. The unique properties of R231H appeared to be caused by a loss in voltage-dependent gating.

CONCLUSIONS

The R231H variant causes a high penetrance for interfamilial early-onset AF. Our study indicates R231H likely shortens atrial refractoriness to promote a substrate for reentry. Additionally, R231H might cause abnormal ventricular repolarization by disrupting PKA activation of IKCNQ1 . We conclude genetic variants, which increase IKs during the atrial AP, decrease the atrial AP duration, and/or shorten atrial refractoriness, present a high risk for interfamilial AF.