Sudden death in noncoronary heart disease is associated with delayed paced ventricular activation

Should lethal arrhythmias in hypertrophic cardiomyopathy be predicted using non-electrophysiological methods?

While sudden cardiac death (SCD) in hypertrophic cardiomyopathy (HCM) is due to arrhythmias, the guidelines for prediction of SCD are based solely on non-electrophysiological methods. This study aims to stimulate thinking about whether the interests of patients with HCM are better served by using current, 'risk factor', methods of prediction or by further development of electrophysiological methods to determine arrhythmic risk. Five published predictive studies of SCD in HCM, which contain sufficient data to permit analysis, were analysed to compute receiver operating characteristics together with their confidence bounds to compare their formal prediction either by bootstrapping or Monte Carlo analysis. Four are based on clinical risk factors, one with additional MRI analysis, and were regarded as exemplars of the risk factor approach. The other used an electrophysiological method and directly compared this method to risk factors in the same patients. Prediction methods that use conventional clinical risk factors and MRI have low predictive capacities that will only detect 50-60% of patients at risk with a 15-30% false positive rate [area under the curve (AUC) = ∼0.7], while the electrophysiological method detects 90% of events with a 20% false positive rate (AUC = ∼0.89). Given improved understanding of complex arrhythmogenesis, arrhythmic SCD is likely to be more accurately predictable using electrophysiologically based approaches as opposed to current guidelines and should drive further development of electrophysiologically based methods.

Heterogeneous repolarization creates ventricular tachycardia circuits in healed myocardial infarction scar

Arrhythmias originating in scarred ventricular myocardium are a major cause of death, but the underlying mechanism allowing these rhythms to exist remains unknown. This gap in knowledge critically limits identification of at-risk patients and treatment once arrhythmias become manifest. Here we show that potassium voltage-gated channel subfamily E regulatory subunits 3 and 4 (KCNE3, KCNE4) are uniquely upregulated at arrhythmia sites within scarred myocardium. Ventricular arrhythmias occur in areas with a distinctive cardiomyocyte repolarization pattern, where myocyte tracts with short repolarization times connect to myocytes tracts with long repolarization times. We found this unique pattern of repolarization heterogeneity only in ventricular arrhythmia circuits. In contrast, conduction abnormalities were ubiquitous within scar. These repolarization heterogeneities are consistent with known functional effects of KCNE3 and KCNE4 on the slow delayed-rectifier potassium current. We observed repolarization heterogeneity using conventional cardiac electrophysiologic techniques that could potentially translate to identification of at-risk patients. The neutralization of the repolarization heterogeneities could represent a potential strategy for the elimination of ventricular arrhythmia circuits.

Ventricular arrhythmias after heart attack are a leading cause of death. Here the authors show, in a porcine model, that KCNE3 and KCNE4 upregulation and a unique pattern of repolarization heterogeneity in the scar facilitate reentrant ventricular tachycardia.

Brugada syndrome genetics is associated with phenotype severity

Aims 

Brugada syndrome (BrS) is associated with an increased risk of sudden cardiac death due to ventricular tachycardia/fibrillation (VT/VF) in young, otherwise healthy individuals. Despite SCN5A being the most commonly known mutated gene to date, the genotype–phenotype relationship is poorly understood and remains uncertain. This study aimed to elucidate the genotype–phenotype correlation in BrS.

Methods and results

Brugada syndrome probands deemed at high risk of future arrhythmic events underwent genetic testing and phenotype characterization by the means of epicardial arrhythmogenic substrate (AS) mapping, and were divided into two groups according to the presence or absence of SCN5A mutation. Two-hundred probands (160 males, 80%; mean age 42.6 ± 12.2 years) were included in this study. Patients harbouring SCN5A mutations exhibited a spontaneous type 1 pattern and experienced aborted cardiac arrest or spontaneous VT/VF more frequently than the other subjects. SCN5A-positive patients exhibited a larger epicardial AS area, more prolonged electrograms and more frequently observed non-invasive late potentials. The presence of an SCN5A mutation explained >26% of the variation in the epicardial AS area and was the strongest predictor of a large epicardial area.

Conclusion 

In BrS, the genetic background is the main determinant for the extent of the electrophysiological abnormalities. SCN5A mutation carriers exhibit more pronounced epicardial electrical abnormalities and a more aggressive clinical presentation. These results contribute to the understanding of the genetic determinants of the BrS phenotypic expression and provide possible explanations for the varying degrees of disease expression.

Impact of atrial programmed electrical stimulation techniques on unipolar electrogram morphology

INTRODUCTION

Intra-atrial conduction abnormalities are associated with the development of atrial fibrillation (AF) and cause morphological changes of the unipolar atrial electrogram (U-AEGM). This study examined the impact of different atrial programmed electrical stimulation (APES) protocols on U-AEGM morphology to identify the most optimal APES protocol provoking conduction abnormalities.

METHODS

APES techniques (14 protocols) were applied in 30 patients referred for an electrophysiology study, consisting of fixed rate, extra, and decremental stimuli at different frequencies. U-AEGM morphologies including width, amplitude, and fractionation for patients without (control group) and with a history of AF (AF group) were examined during APES. In addition, sinus rhythm (SR) U-AEGMs preceding different APES protocols were compared to evaluate the morphology stability over time.

RESULTS

U-AEGM morphologies during SR before the APES protocols were comparable (all P > .396). Atrial refractoriness was longer in the AF group compared to the control group (298 ± 48 vs 255 ± 33 ms; P ≤ .020), but did not differ between AF patients with and without amiodarone therapy (278 ± 48 vs 311 ± 40 ms; P ≥ .126). Compared to the initial SR morphology, U-AEGM width, amplitude, and fractionation changed significantly during the 14 different APES protocols, particularly in the AF group. In both groups, U-AEGM changes in morphology were most pronounced during fixed-rate stimulation with extra stimuli (8S1-S2 = 400-250 ms).

CONCLUSION

APES results in significant changes in U-AEGM morphology, including width, amplitude, and fractionation. The impact of APES differed between APES sequence and between patients with and without AF. These findings suggest that APES could be useful to identify AF-related conduction abnormalities in the individual patient.

Fibrosis Microstructure Modulates Reentry in Non-ischemic Dilated Cardiomyopathy: Insights From Imaged Guided 2D Computational Modeling

Aims: Patients who present with non-ischemic dilated cardiomyopathy (NIDCM) and enhancement on late gadolinium magnetic resonance imaging (LGE-CMR), are at high risk of sudden cardiac death (SCD). Further risk stratification of these patients based on LGE-CMR may be improved through better understanding of fibrosis microstructure. Our aim is to examine variations in fibrosis microstructure based on LGE imaging, and quantify the effect on reentry inducibility and mechanism. Furthermore, we examine the relationship between transmural activation time differences and reentry. Methods and Results: 2D Computational models were created from a single short axis LGE-CMR image, with 401 variations in fibrosis type (interstitial, replacement) and density, as well as presence or absence of reduced conductivity (RC). Transmural activation times (TAT) were measured, as well as reentry incidence and mechanism. Reentries were inducible above specific density thresholds (0.8, 0.6 for interstitial, replacement fibrosis). RC reduced these thresholds (0.3, 0.4 for interstitial, replacement fibrosis) and increased reentry incidence (48 no RC vs. 133 with RC). Reentries were classified as rotor, micro-reentry, or macro-reentry and depended on fibrosis micro-structure. Differences in TAT at coupling intervals 210 and 500ms predicted reentry in the models (sensitivity 89%, specificity 93%). A sensitivity analysis of TAT and reentry incidence showed that these quantities were robust to small changes in the pacing location. Conclusion: Computational models of fibrosis micro-structure underlying areas of LGE in NIDCM provide insight into the mechanisms and inducibility of reentry, and their dependence upon the type and density of fibrosis. Transmural activation times, measured at the central extent of the scar, can potentially differentiate microstructures which support reentry.

Differential pacing from two sites to diagnose risk of ventricular arrhythmia and death

BACKGROUND

QRS abnormalities may not be apparent in sinus rhythm in electrically stable cardiomyopathy patients who can have quiescent but highly arrhythmogenic substrate. Here, we test the hypothesis that differential changes in QRS construction during right-ventricular apex pacing (RVP) as opposed to atrial pacing (AP) will identify latent substrate for ventricular arrhythmias (VA) and death.

METHODS

Forty patients with cardiomyopathy free of VA underwent baseline 114-electrode body-surface electrocardiogram during AP (100 beats per minute [bpm]) and RVP (100 and 120 bpm). The filtered-averaged QRS at each electrode was deconstructed into individual intra-QRS and post-QRS ventricular myopotentials (V_MP ). The primary outcome was VA or death. Prognostic accuracy of V_MP was validated using V1 to V6 leads in another prospective cohort of 44-cardiomyopathy patients.

RESULTS

Twenty-six patients were eligible for initial analysis. After 5 ± 2 years of follow-up, eight (31%) patients had VA (VA_Pos ) while rest were uneventful (VA_Neg ). During AP₁₀₀ , VA_Pos patients expressed more V_MP than VA_Neg patients (16 ± 1 vs 12 ± 1, P = 0.02). RVP₁₀₀ and RVP₁₂₀ in VA_Pos patients introduced an additional 5.5 ± 0.5 and 6.0 ± 0.5 V_MP (P < 0.0001 vs AP₁₀₀ ). The relative change with RVP₁₂₀ versus AP₁₀₀ in VA_Neg patients exceeded VA_Pos patients by 1.2 ± 0.5 V_MP (P = 0.03). Increment in V_MP count of <8 in lead-V5 with RVP₁₂₀ compared to AP₁₀₀ best predicted VA (area under curve 0.81, P = 0.01). In the validation cohort, primary outcome occurred in 13 (33%) patients. Native QRS features and AP₁₀₀ alone failed to predict primary outcome. Patients with increment in V_MP count of <8 in lead-V5 with RVP₁₂₀ versus AP₁₀₀ had 7.9-fold increased risk of primary outcome (95% confidence interval 1.01, 61.61; P = 0.049).

CONCLUSION

Cardiomyopathy patients at risk of VA or death perturb the QRS less than low-risk patients with differential pacing. This functional response may be useful to identify arrhythmogenic substrate.

Human iPS Cell-Derived Patient Tissues and 3D Cell Culture Part 1: Target Identification and Lead Optimization

Human-induced pluripotent stem cells (HiPSCs), and new technologies to culture them into functional cell types and tissues, are now aiding drug discovery. Patient-derived HiPSCs can provide disease models that are more clinically relevant and so more predictive than the currently available animal-derived or tumor cell-derived cells. These cells, consequently, exhibit disease phenotypes close to the human pathology, particularly when cultured under conditions that allow them to recapitulate the tissue architecture in three-dimensional (3D) systems. A key feature of HiPSCs is that they can be cultured under conditions that favor formation of multicellular spheroids or organoids. By culturing and differentiating in systems mimicking the human tissue in vivo, the HiPSC microenvironment further reflects patient in vivo physiology, pathophysiology, and ultimately pharmacological responsiveness. We assess the rationale for using HiPSCs in several phases of preclinical drug discovery, specifically in disease modeling, target identification, and lead optimization. We also discuss the growing use of HiPSCs in compound lead optimization, particularly in profiling compounds for their potential metabolic liability and off-target toxicities. Collectively, we contend that both approaches, HiPSCs and 3D cell culture, when used in concert, have exciting potential for the development of novel medicines.

Prolonged Ventricular Conduction and Repolarization During Right Ventricular Stimulation Predicts Ventricular Arrhythmias and Death in Patients With Cardiomyopathy

OBJECTIVES

The goal of this study was to evaluate whether prolonged ventricular conduction (paced QRS) and repolarization (paced QTc) times observed during ventricular stimulation predict ventricular arrhythmic events and death.

BACKGROUND

Abnormal ventricular conduction and repolarization can predispose patients to ventricular arrhythmias.

METHODS

Consecutive patients with left ventricular dysfunction (ejection fraction <50%) undergoing electrophysiology studies from January 2002 until May 2014 were identified at Mayo Clinic (Rochester, Minnesota). Patients were followed up until December 2014 for occurrence of ventricular arrhythmias and death.

RESULTS

Among the 501 patients included (mean age 65 years; mean left ventricular ejection fraction 33.1%), longer paced ventricular conduction was associated with longer baseline QRS duration, longer QT interval, and lower ejection fraction. On multivariable analysis, longer paced QRS duration was associated with higher risk of ventricular arrhythmia (hazard ratio [HR]: 1.11 per 10-ms increase; 95% confidence interval [CI]: 1.07 to 1.16; p < 0.001) and all-cause death or arrhythmia (HR: 1.09; 95% CI: 1.09 to 1.13; p < 0.001). A paced QRS duration >190 ms was associated with a 3.6 times higher risk of ventricular arrhythmia (HR: 3.6; 95% CI: 2.35 to 5.53; p < 0.001) and a 2.1 times higher risk of death or arrhythmia (HR: 2.12; 95% CI: 1.53 to 2.95; p < 0.001), independent of left ventricular function or baseline QRS duration. Longer QTc interval during ventricular pacing was associated with a higher risk of ventricular arrhythmia (HR: 1.03 per 10-ms increase; 95% CI: 1.02 to 1.12; p < 0.001) independent of paced QRS duration.

CONCLUSIONS

Longer paced QRS duration and paced QTc interval predict ventricular arrhythmias in patients with cardiomyopathy. Ventricular conduction and repolarization prolongation during right ventricular pacing can determine the risk of ventricular arrhythmias.

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.

Key Challenges and Opportunities Associated with the Use of In Vitro Models to Detect Human DILI: Integrated Risk Assessment and Mitigation Plans

Drug-induced liver injury (DILI) is a major cause of late-stage clinical drug attrition, market withdrawal, black-box warnings, and acute liver failure. Consequently, it has been an area of focus for toxicologists and clinicians for several decades. In spite of considerable efforts, limited improvements in DILI prediction have been made and efforts to improve existing preclinical models or develop new test systems remain a high priority. While prediction of intrinsic DILI has improved, identifying compounds with a risk for idiosyncratic DILI (iDILI) remains extremely challenging because of the lack of a clear mechanistic understanding and the multifactorial pathogenesis of idiosyncratic drug reactions. Well-defined clinical diagnostic criteria and risk factors are also missing. This paper summarizes key data interpretation challenges, practical considerations, model limitations, and the need for an integrated risk assessment. As demonstrated through selected initiatives to address other types of toxicities, opportunities exist however for improvement, especially through better concerted efforts at harmonization of current, emerging and novel in vitro systems or through the establishment of strategies for implementation of preclinical DILI models across the pharmaceutical industry. Perspectives on the incorporation of newer technologies and the value of precompetitive consortia to identify useful practices are also discussed.

QRS prolongation after premature stimulation is associated with polymorphic ventricular tachycardia in nonischemic cardiomyopathy: Results from the Leiden Nonischemic Cardiomyopathy Study

BACKGROUND

Progressive activation delay after premature stimulation has been associated with ventricular fibrillation in nonischemic cardiomyopathy (NICM).

OBJECTIVES

The objectives of this study were (1) to investigate prolongation of the paced QRS duration (QRSd) after premature stimulation as a marker of activation delay in NICM, (2) to assess its relation to induced ventricular arrhythmias, and (3) to analyze its underlying substrate by late gadolinium enhancement cardiac magnetic resonance imaging (LGE-CMR) and endomyocardial biopsy.

METHODS

Patients with NICM were prospectively enrolled in the Leiden Nonischemic Cardiomyopathy Study and underwent a comprehensive evaluation including LGE-CMR, electrophysiology study, and endomyocardial biopsy. Patients without structural heart disease served as controls for electrophysiology study.

RESULTS

Forty patients with NICM were included (mean age 57 ± 14 years; 33 men [83%]; left ventricular ejection fraction 30% ± 13%). After the 400-ms drive train and progressively premature stimulation, the maximum increase in QRSd was larger in patients with NICM than in controls (35 ± 18 ms vs. 23 ± 12 ms; P = .005) and the coupling interval window with QRSd prolongation was wider (47 ± 23 ms vs. 31 ± 14 ms; P = .005). The maximum paced QRSd exceeded the ventricular effective refractory period, allowing for pacing before the offset of the QRS complex in 20 of 39 patients with NICM vs. 1 of 20 controls (P < .001). In patients with NICM, QRSd prolongation was associated with the inducibility of polymorphic ventricular tachycardia (16 of 39 patients) and was related to long, thick strands of fibrosis in biopsies, but not to focal enhancement on LGE-CMR.

CONCLUSION

QRSd is a simple parameter used to quantify activation delay after premature stimulation, and its prolongation is associated with the inducibility of polymorphic ventricular tachycardia and with the pattern of myocardial fibrosis in biopsies.

Late gadolinium enhancement on cardiac magnetic resonance represents the depolarizing and repolarizing electrically damaged foci causing malignant ventricular arrhythmia in hypertrophic cardiomyopathy

BACKGROUND

The presence of a myocardial scar detected by late gadolinium enhancement (LGE) on cardiac magnetic resonance (CMR) has been described as a predictor of all-cause mortality in hypertrophic cardiomyopathy (HCM). However, the detailed spatial relationship between LGE site and electrical abnormality is unclear in high-risk HCM with malignant arrhythmia.

OBJECTIVE

The purpose of this study was to elucidate the detailed relationship between the site on CMR imaging and the electrically damaged site, a potential origin of ventricular arrhythmias in patients with HCM.

METHODS

Fifty consecutive HCM patients underwent contrast-enhanced CMR. Of those patients, 18 patients with ventricular tachycardia underwent electrophysiology study including endocardial mapping of the left ventricle (LV). The LGE area was calculated at 12 different LV sites: anterior, lateral, posterior, and septal segments of the basal, middle, and apical portions. At each LV site, the bipolar electrogram, effective refractory period (ERP), and monophasic action potential were recorded.

RESULTS

LGE-positive segments demonstrated a significantly lower amplitude (4.0 ± 2.8 mV vs 7.3 ± 3.6 mV; P < .001), longer duration (54.7 ± 17.8 vs 40.6 ± 7.8 ms; P < .001), longer ERP (320 ± 42 ms vs 284 ± 37 ms; P = .001), and longer monophasic action potential duration measured at 90% repolarization (321 ± 19 ms vs 283 ± 25 ms; P < .001) than did LGE-negative segments. The LGE area negatively correlated with the amplitude (r = -0.59; P < .001) and positively correlated with the duration (r = 0.64; P < .001), ERP (r = 0.44; P < .001), and action potential duration measured at 90% repolarization (r = 0.63; P < .001). All the observed VTs originated from LGE-positive segments.

CONCLUSION

The spatial distribution of LGE significantly correlates with depolarizing and repolarizing electrical damage in high-risk HCM with malignant ventricular arrhythmia.

Drug screening using a library of human induced pluripotent stem cell-derived cardiomyocytes reveals disease-specific patterns of cardiotoxicity

BACKGROUND

Cardiotoxicity is a leading cause for drug attrition during pharmaceutical development and has resulted in numerous preventable patient deaths. Incidents of adverse cardiac drug reactions are more common in patients with preexisting heart disease than the general population. Here we generated a library of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) from patients with various hereditary cardiac disorders to model differences in cardiac drug toxicity susceptibility for patients of different genetic backgrounds.

METHODS AND RESULTS

Action potential duration and drug-induced arrhythmia were measured at the single cell level in hiPSC-CMs derived from healthy subjects and patients with hereditary long QT syndrome, familial hypertrophic cardiomyopathy, and familial dilated cardiomyopathy. Disease phenotypes were verified in long QT syndrome, hypertrophic cardiomyopathy, and dilated cardiomyopathy hiPSC-CMs by immunostaining and single cell patch clamp. Human embryonic stem cell-derived cardiomyocytes (hESC-CMs) and the human ether-a-go-go-related gene expressing human embryonic kidney cells were used as controls. Single cell PCR confirmed expression of all cardiac ion channels in patient-specific hiPSC-CMs as well as hESC-CMs, but not in human embryonic kidney cells. Disease-specific hiPSC-CMs demonstrated increased susceptibility to known cardiotoxic drugs as measured by action potential duration and quantification of drug-induced arrhythmias such as early afterdepolarizations and delayed afterdepolarizations.

CONCLUSIONS

We have recapitulated drug-induced cardiotoxicity profiles for healthy subjects, long QT syndrome, hypertrophic cardiomyopathy, and dilated cardiomyopathy patients at the single cell level for the first time. Our data indicate that healthy and diseased individuals exhibit different susceptibilities to cardiotoxic drugs and that use of disease-specific hiPSC-CMs may predict adverse drug responses more accurately than the standard human ether-a-go-go-related gene test or healthy control hiPSC-CM/hESC-CM screening assays.

Surface fragmented QRS in a patient with hypertrophic cardiomyopathy and malignant arrhythmias: Is there an association?

An 18- year old woman with hypertrophic cardiomyopathy, aborted sudden cardiac death and implanted with an implantable cardioverter defibrillator (ICD), developed progressive fragmentation of her surface 12-lead electrocardiogram (ECG). During the follow-up, she presented with multiple appropriate ICD discharges. Here, we discuss the possible association between surface fragmented ECG and the risk of ventricular arrhythmias in patients with hypertrophic cardiomyopathy.

Pacing for drug-refractory or drug-intolerant hypertrophic cardiomyopathy

BACKGROUND

Hypertrophic cardiomyopathy (HCM) is a genetic disease with an autosomal-dominant inheritance for which negative inotropes are the most widely used initial therapies. Observational studies and small randomised trials have suggested symptomatic and functional benefits using pacing and several theories have been put forward to explain why. Pacing, although not the primary treatment for HCM, could be beneficial to patients with relative or absolute contraindications to surgery or alcohol ablation. Several randomised controlled trials comparing pacing to other therapeutic modalities have been conducted but no Cochrane-style systematic review has been done.

OBJECTIVES

To assess the effects of pacing in drug-refractory or drug-intolerant hypertrophic cardiomyopathy patients.

SEARCH METHODS

We searched the following on the 14/4/2010: CENTRAL (The Cochrane Library 2010, Issue 1), MEDLINE OVID (from 1950 onwards ), EMBASE OVID (from 1980 onwards ), Web of Science with Conference Proceedings (from 1970 onwards). No language restrictions were applied.

SELECTION CRITERIA

Randomised controlled trials of either parallel or crossover design that assess the beneficial and harmful effects of pacing for hypertrophic cardiomyopathy were included. When crossover studies were identified, we considered data only from the first phase.

DATA COLLECTION AND ANALYSIS

Data from included studies were extracted onto a pre-formed data extraction paper by two authors independently. Data was then entered into Review Manager 5.1 for analysis. Risk of bias was assessed using the guidance provided in the Cochrane Handbook. For dichotomous data, relative risk was calculated; and for continuous data, the mean differences were calculated. Where appropriate data were available, meta-analysis was performed. Where meta-analysis was not possible, a narrative synthesis was written. A QUROUM flow chart was provided to show the flow of papers.

MAIN RESULTS

Five studies (reported in 10 papers) were identified. However, three of the five studies provided un-usable data. Thus the data from only two studies (reported in seven papers) with 105 participants were included for this review. There was insufficient data to compare results on all-cause mortality, cost effectiveness, exercise capacity, Quality of life and Peak O2 consumption.When comparing active pacing versus placebo pacing on exercise capacity, one study showed that exercise time decreased from (13.1 ± 4.4) minutes to (12.6 ± 4.3) minutes in the placebo group and increased from (12.1 ± 5.6) minutes to (12.9 ± 4.2) minutes in the treatment group (MD 0.30; 95% CI -1.54 to 2.14). Statistically significant data from the same study showed that left ventricular outflow tract obstruction decreased from (71 ± 32) mm Hg to (52 ± 34) mm Hg in the placebo group and from (70 ± 24) mm Hg to (33 ± 27) mm Hg in the active pacing group (MD -19.00; 95% CI -32.29 to -5.71). This study was also able to show that New York Heart Association (NYHA) functional class decreased from (2.5 ± 0.5) to (2.2 ± 0.6) in the inactive pacing group and decreased from (2.6 ± 0.5) to (1.7 ± 0.7) in the placebo group (MD -0.50; 95% CI -0.78 to -0.22).When comparing active pacing versus trancoronary ablation of septal hypertrophy (TASH), data from one study showed that NYHA functional class decreased from (3.2 ± 0.7) to (1.5 ± 0.5) in the TASH group and decreased from (3.0 ± 0.1) to (1.9 ± 0.6) in the pacemaker group. This study also showed that LV wall thickness remained unchanged in the active pacing group compared to reduction from (22 ± 4) mm to (17 ± 3) mm in the TASH group (MD 0.60; 95% CI -5.65 to 6.85) and that LV outflow tract obstruction decreased from (80 ± 35.5) mm Hg in the TASH group to (49.3 ± 37.7) mm Hg in the pacemaker group.

AUTHORS' CONCLUSIONS

Trials published to date lack information on clinically relevant end-points. Existing data is derived from small trials at high risk of bias, which concentrate on physiological measures. Their results are inconclusive. Further large and high quality trials with more appropriate outcomes are warranted.

Ventricular arrhythmogenesis following slowed conduction in heptanol-treated, Langendorff-perfused mouse hearts

Arrhythmogenic effects of slowed action potential conduction produced by the gap junction and sodium-channel inhibitor heptanol (0.1-2 mM) were explored in Langendorff-perfused mouse hearts. Monophasic action potential recordings showed that 2 mM heptanol induced ventricular tachycardia in the absence of triggered activity arising from early or after-depolarizations during regular 8 Hz pacing and programmed electrical stimulation (PES). It also increased activation latencies and ventricular effective refractory periods (VERPs), but did not alter action potential duration (APD), thereby reducing local critical intervals for re-excitation given by APD(90) - VERP. Bipolar electrogram recordings showed that 2 mM heptanol increased electrogram duration (EGD) and ratios of EGDs obtained at the longest to those obtained at the shortest S1S2 intervals studied during PES, suggesting increased dispersion of conduction velocities. These findings show, for the first time in the mouse heart, that slowed conduction induces reversible arrhythmogenic effects despite repolarization abnormalities expected to reduce arrhythmogenicity.

Prevention of ventricular fibrillation episodes in Brugada syndrome by catheter ablation over the anterior right ventricular outflow tract epicardium

BACKGROUND

The underlying electrophysiological mechanism that causes an abnormal ECG pattern and ventricular tachycardia/ventricular fibrillation (Vt/VF) in patients with the Brugada syndrome (BrS) remains unelucidated. However, several studies have indicated that the right ventricular outflow tract (RVOT) is likely to be the site of electrophysiological substrate. We hypothesized that in patients with BrS who have frequent recurrent VF episodes, the substrate site is the RVOT, either over the epicardium or endocardium; abnormal electrograms would be identified at this location, which would serve as the target site for catheter ablation.

METHODS AND RESULTS

We studied 9 symptomatic patients with the BrS (all men; median age 38 years) who had recurrent VF episodes (median 4 episodes) per month, necessitating implantable cardioverter defibrillator discharge. Electroanatomic mapping of the right ventricle, both endocardially and epicardially, and epicardial mapping of the left ventricle were performed in all patients during sinus rhythm. All patients had typical type 1 Brugada ECG pattern and inducible Vt/VF; they were found to have unique abnormal low voltage (0.94±0.79 mV), prolonged duration (132±48 ms), and fractionated late potentials (96±47 ms beyond QRS complex) clustering exclusively in the anterior aspect of the RVOT epicardium. Ablation at these sites rendered Vt/VF noninducible (7 of 9 patients [78%]; 95% confidence interval, 0.40 to 0.97, P=0.015) and normalization of the Brugada ECG pattern in 89% (95% confidence interval, 0.52 to 0.99; P=0.008). Long-term outcomes (20±6 months) were excellent, with no recurrent Vt/VF in all patients off medication (except 1 patient on amiodarone).

CONCLUSIONS

The underlying electrophysiological mechanism in patients with BrS is delayed depolarization over the anterior aspect of the RVOT epicardium. Catheter ablation over this abnormal area results in normalization of the Brugada ECG pattern and prevents Vt/VF, both during electrophysiological studies as well as spontaneous recurrent Vt/VF episodes in patients with BrS.

Delayed conduction and its implications in murine Scn5a(+/-) hearts: independent and interacting effects of genotype, age, and sex

We explored for relationships between SCN5A haploinsufficiency, implicated in clinical arrhythmogenicity, and right ventricular (RV) conduction disorders in Langendorff-perfused, male and female, and young (3 months) and old (>12 month old) Scn5a^+/− and wild type (WT) hearts. The investigated conditions of genotype, age, and sex affected latencies but not repolarization time courses of RV monophasic action potentials. This prompted examination of the patterns of RV epicardial activation, its dispersion, and their interrelationships as possible arrhythmic mechanisms using a 64-channel, multi-electrode array. Mean ventricular activation times (T*_MEAN), spatial dispersions (D*_S) between recording channels/cardiac cycle, and maximum activation times (T*_MAX) representing the slowest possible conduction in any given heart were all higher in old male Scn5a^+/− compared with young male and old female Scn5a^+/− and old male WT. Temporal dispersions (D*_T) of recording channels were similarly higher in old male Scn5a^+/− compared with old male WT. All groupings of D*_T, D*_S, and T*_MAX nevertheless linearly correlated with T*_MEAN, with indistinguishable slopes. The variates explored thus influence D*_T, D*_S, and T*_MAX through actions on T*_MEAN. These findings in turn correlated with increased levels of fibrosis in young male, young female, and old male Scn5a^+/− compared with the corresponding WTs. We thus demonstrate for the first time independent and interacting effects of genotype, age, and sex on epicardial conduction and its dispersions at least partially attributable to fibrotic change, resulting in the greatest effects in old male Scn5a^+/− in an absence of alterations in repolarization time courses. This directly implicates altered depolarization in the clinical arrhythmogenicity associated with Scn5a^+/−.

Structural heterogeneity alone is a sufficient substrate for dynamic instability and altered restitution

BACKGROUND

Marked changes in ventricular APD restitution and associated alternans rhythm have been demonstrated in structural heart disease (SHD). However, whether this is due to structural heterogeneity or regional variation in cellular properties remains uncertain. In this study, we address the hypothesis that the structural heterogeneity associated with SHD is sufficient to alter dynamic restitution and increase the probability of electric instability.

METHODS AND RESULTS

Activation was simulated in a 14x14 mm(2) domain in the presence and absence (control) of a central region containing nonuniform discontinuities resembling patchy fibrosis. A modified LR1 cardiac activation model was used in a bidomain formulation with isotropic conductivities. Bipolar stimulation was imposed above the central region with coupling intervals decreasing progressively from 500 ms and then maintained at 105 ms. Structural discontinuities had little effect on electric activation at low stimulus rates, but activation time and APD distributions became highly nonuniform within and adjacent to the discontinuous region at high rates. Discordant APD alternans occurred in both "fibrosis" and control, but at lower stimulus rates and with markedly greater extent in the former. Tortuous conduction through the discontinuous region resulted in large fluctuations of diastolic intervals giving rise to regional electric instability, which modulates dynamic conduction velocity and APD restitution. This led to heterogeneous conduction block and reentry not observed in control.

CONCLUSIONS

We show that structural discontinuities can amplify discordant alternans and provide a rate-dependent substrate for reentry. This work provides new insights into the mechanisms by which fibrosis may contribute to arrhythmogenesis.

Atrial arrhythmogenesis in wild-type and Scn5a+/delta murine hearts modelling LQT3 syndrome

Long QT(3) (LQT3) syndrome is associated with abnormal repolarisation kinetics, prolonged action potential durations (APD) and QT intervals and may lead to life-threatening ventricular arrhythmias. However, there have been few physiological studies of its effects on atrial electrophysiology. Programmed electrical stimulation and burst pacing induced atrial arrhythmic episodes in 16 out of 16 (16/16) wild-type (WT) and 7/16 genetically modified Scn5a+/Δ (KPQ) Langendorff-perfused murine hearts modelling LQT3 (P < 0.001 for both), and in 14/16 WT and 1/16 KPQ hearts (P < 0.001 for both; Fisher’s exact test), respectively. The arrhythmogenic WT hearts had significantly larger positive critical intervals (CI), given by the difference between atrial effective refractory periods (AERPs) and action potential durations at 90% recovery (APD₉₀), compared to KPQ hearts (8.1 and 3.2 ms, respectively, P < 0.001). Flecainide prevented atrial arrhythmias in all arrhythmogenic WT (P < 0.001) and KPQ hearts (P < 0.05). It prolonged the AERP to a larger extent than it did the APD₉₀ in both WT and KPQ groups, giving negative CIs. Quinidine similarly exerted anti-arrhythmic effects, prolonged AERP over corresponding APD₉₀ in both WT and KPQ groups. These findings, thus, demonstrate, for the first time, inhibitory effects of the KPQ mutation on atrial arrhythmogenesis and its modification by flecainide and quinidine. They attribute these findings to differences in the CI between WT and mutant hearts, in the presence or absence of these drugs. Thus, prolongation of APD₉₀ over AERP gave positive CI values and increased atrial arrhythmogenicity whereas lengthening of AERP over APD₉₀ reduced such CI values and produced the opposite effect.

Anti-arrhythmic effects of cyclopiazonic acid in Langendorff-perfused murine hearts

We investigated the effects of reducing sarcoplasmic reticular (SR) Ca(2+) stores using the Ca(2+)-ATPase inhibitor cyclopiazonic acid (CPA) in Langendorff-perfused mouse hearts exposed to different pro-arrhythmic agents all known to produce Ca(2+)-mediated arrhythmogenesis. CPA (100 and 150 nM) produced progressive (beginning over approximately 1 min) and significant (P<0.0001) reductions in peak amplitudes of Ca(2+) transients evoked by regular stimulation in isolated Fluo-3 loaded myocytes from F/F(0)=3.2+/-0.16 (n=12 cells) to 1.62+/-0.012 (n=6 cells) and 1.53+/-0.06 (n=12 cells), respectively, consistent with previous reports describing reductions of store Ca(2+) in other cell systems. The corresponding effects of CPA were then examined in intact hearts exposed to isoproterenol (100 nM), elevated extracellular [Ca(2+)] (5mM) and caffeine (1mM). All three agents produced ventricular tachycardia either when added alone or simultaneously with CPA during programmed electrical stimulation. However, arrhythmogenicity was not observed when such agents were added approximately 10 min after introduction of CPA. CPA thus antagonized this Ca(2+)-mediated arrhythmogenesis but only under circumstances of SR Ca(2+) depletion. These alterations in arrhythmogenic tendency took place despite an absence of alterations in electrogram and monophasic action potential characteristics. This was in sharp contrast to previous observations in murine, DeltaKPQ-Scn5a (LQT3) and KCNE1(-/-) (LQT5), systems where re-entry has been implicated in arrhythmogenesis.

Scn3b knockout mice exhibit abnormal ventricular electrophysiological properties

We report for the first time abnormalities in cardiac ventricular electrophysiology in a genetically modified murine model lacking the Scn3b gene (Scn3b^−/−). Scn3b^−/− mice were created by homologous recombination in embryonic stem (ES) cells. RT-PCR analysis confirmed that Scn3b mRNA was expressed in the ventricles of wild-type (WT) hearts but was absent in the Scn3b^−/− hearts. These hearts also showed increased expression levels of Scn1b mRNA in both ventricles and Scn5a mRNA in the right ventricles compared to findings in WT hearts. Scn1b and Scn5a mRNA was expressed at higher levels in the left than in the right ventricles of both Scn3b^−/− and WT hearts. Bipolar electrogram and monophasic action potential recordings from the ventricles of Langendorff-perfused Scn3b^−/− hearts demonstrated significantly shorter ventricular effective refractory periods (VERPs), larger ratios of electrogram duration obtained at the shortest and longest S₁–S₂ intervals, and ventricular tachycardias (VTs) induced by programmed electrical stimulation. Such arrhythmogenesis took the form of either monomorphic or polymorphic VT. Despite shorter action potential durations (APDs) in both the endocardium and epicardium, Scn3b^−/− hearts showed ΔAPD₉₀ values that remained similar to those shown in WT hearts. The whole-cell patch-clamp technique applied to ventricular myocytes isolated from Scn3b^−/− hearts demonstrated reduced peak Na⁺ current densities and inactivation curves that were shifted in the negative direction, relative to those shown in WT myocytes. Together, these findings associate the lack of the Scn3b gene with arrhythmic tendencies in intact perfused hearts and electrophysiological features similar to those in Scn5a^+/− hearts.

Slow and discontinuous conduction conspire in Brugada syndrome: a right ventricular mapping and stimulation study

BACKGROUND

Brugada syndrome (BrS) is associated with lethal arrhythmias, which are linked to specific ST-segment changes (type-1 BrS-ECG) and the right ventricle (RV). The pathophysiological basis of the arrhythmias and type-1 BrS-ECG is unresolved. We studied the electrophysiological characteristics of the RV endocardium in BrS.

METHODS AND RESULTS

RV endocardial electroanatomical mapping and stimulation studies were performed in controls (n=12) and BrS patients with a type-1 (BrS-1, n=10) or type-2 BrS-ECG (BrS-2, n=12) during the studies. BrS-1 patients had prominent impairment of RV endocardial impulse propagation when compared with controls, as represented by: (1) prolonged activation-duration during sinus rhythm (86+/-4 versus 65+/-3 ms), (2) increased electrogram fractionation (1.36+/-0.04 versus 1.15+/-0.01 deflections per electrogram), (3) longer electrogram duration (83+/-3 versus 63+/-2 ms), (4) activation delays on premature stimulation (longitudinal: 160+/-26 versus 86+/-9 ms; transversal: 112+/-5 versus 58+/-6 ms), and (5) abnormal transversal conduction velocity restitution (42+/-8 versus 18+/-2 ms increase in delay at shortest coupling intervals). Wider and more fractionated electrograms were also found in BrS-2 patients. Repolarization was not different between groups.

CONCLUSIONS

BrS-1 and BrS-2 patients are characterized by wide and fractionated electrograms at the RV endocardium. BrS-1 patients display additional conduction slowing during sinus rhythm and premature stimulation along with abnormal transversal conduction velocity restitution. These patients may thus exhibit a substrate for slow and discontinuous conduction caused by abnormal active membrane processes and electric coupling. Our findings support the emerging notion that BrS is not solely attributable to abnormal electrophysiological properties but requires the conspiring effects of conduction slowing and tissue discontinuities.

Arrhythmogenic actions of the Ca2+ channel agonist FPL-64716 in Langendorff-perfused murine hearts

The experiments explored the extent to which alterations in L-type Ca(2+) channel-mediated Ca(2+) entry triggers Ca(2+)-mediated arrhythmogenesis in Langendorff-perfused murine hearts through use of the specific L-type Ca(2+) channel modulator FPL-64716 (FPL). Introduction of FPL (1 microm) resulted in a gradual development (>10 min) of diastolic electrical events and alternans in spontaneously beating hearts from which monophasic action potentials were recorded. In regularly paced hearts, they additionally led to non-sustained and sustained ventricular tachycardia (nsVT and sVT). Programmed electrical stimulation (PES) resulted in nsVT and sVT after 5-10 and >10 min perfusion, respectively. Pretreatments with nifedipine, diltiazem and cyclopiazonic acid abolished arrhythmogenic tendency induced by subsequent introduction of FPL, consistent with its dependence upon both extracellular Ca(2+) entry and the degree of filling of the sarcoplasmic reticular Ca(2+) store. Values for action potential duration at 90% repolarization when any of these agents were applied to FPL-treated hearts became indistinguishable from those shown by untreated control hearts, in contrast to earlier reports of their altering in long QT syndrome type 3 and hypokalaemic murine models for re-entrant arrhythmogenesis. These arrhythmic effects instead correlated with alterations in Ca(2+) homeostasis at the single-cell level found in investigations of the effects of both FPL and the same agents in regularly stimulated fluo-3 loaded myocytes. These findings are compatible with a prolonged extracellular Ca(2+) entry that potentially results in an intracellular Ca(2+) overload and produces the cardiac arrhythmogenecity following addition of FPL.

Physiological consequences of the P2328S mutation in the ryanodine receptor (RyR2) gene in genetically modified murine hearts

Aim

To explore the physiological consequences of the ryanodine receptor (RyR2)-P2328S mutation associated with catecholaminergic polymorphic ventricular tachycardia (CPVT).

Methods

We generated heterozygotic (RyR2^p/s) and homozygotic (RyR2^s/s) transgenic mice and studied Ca²⁺ signals from regularly stimulated, Fluo-3-loaded, cardiac myocytes. Results were compared with monophasic action potentials (MAPs) in Langendorff-perfused hearts under both regular and programmed electrical stimulation (PES).

Results

Evoked Ca²⁺ transients from wild-type (WT), heterozygote (RyR2^p/s) and homozygote (RyR2^s/s) myocytes had indistinguishable peak amplitudes with RyR2^s/s showing subsidiary events. Adding 100 nm isoproterenol produced both ectopic peaks and subsidiary events in WT but not RyR2^p/s and ectopic peaks and reduced amplitudes of evoked peaks in RyR2^s/s. Regularly stimulated WT, RyR2^p/s and RyR2^s/s hearts showed indistinguishable MAP durations and refractory periods. RyR2^p/s hearts showed non-sustained ventricular tachycardias (nsVTs) only with PES. Both nsVTs and sustained VTs (sVTs) occurred with regular stimuli and PES with isoproterenol treatment. RyR2^s/s hearts showed higher incidences of nsVTs before but mainly sVTs after introduction of isoproterenol with both regular stimuli and PES, particularly at higher pacing frequencies. Additionally, intrinsically beating RyR2^s/s showed extrasystolic events often followed by spontaneous sVT.

Conclusion

The RyR2-P2328S mutation results in marked alterations in cellular Ca²⁺ homeostasis and arrhythmogenic properties resembling CPVT with greater effects in the homozygote than the heterozygote demonstrating an important gene dosage effect.

Mouse models of human arrhythmia syndromes

Sudden cardiac death stemming from ventricular arrhythmogenesis is one of the major causes of mortality in the developed world. Congenital and acquired forms of long QT syndrome (LQTS) are in turn associated with life threatening arrhythmias. Over the past decade our understanding of arrhythmogenic mechanisms in the setting of these diseases has increased greatly due to the creation of a number of animal models. Of these, the genetically amenable mouse has proved to be a particularly powerful tool. This review summarizes the congenital and acquired LQTS and describes the various mouse models that have been created to further probe arrhythmogenic mechanisms.

Sudden Cardiac Death: Epidemiology, Mechanisms, and Therapy

Sudden cardiac death is a major public health problem affecting 500,000 patients annually in the United States alone. The major risk factor for sudden cardiac death is the presence of coronary artery disease, usually in the setting of reduced ejection fraction. Globally, the incidence is expected to rise sharply as the prevalence of coronary artery disease and heart failure continue to increase. However, sudden cardiac death is a heterogeneous condition and may be caused by acute ischemia, structural defects, myocardial scar, and/or genetic mutations. Sudden death may occur even in a grossly normal heart. Beta-blockers can reduce the risk of sudden cardiac death, while implantable cardioverter defibrillators are effective at terminating malignant arrhythmias. Ejection fraction remains the major criterion to stratify patients for defibrillator implantation but this strategy alone is insensitive and nonspecific. Novel clinical, electrophysiologic, and genetic markers have been identified that may increase precision in patient selection for primary prevention therapy. This review discusses the epidemiology, mechanisms, etiologies, therapies, treatment guidelines, and future directions in the management of sudden cardiac death.

Spatiotemporal electrocardiographic characterization of ventricular depolarization and repolarization abnormalities in long QT syndrome

BACKGROUND AND PURPOSE

If only a standard electrocardiogram (ECG) is available, at least 25% of patients with long QT syndrome (LQTS) may be missed. Our goal is to quantify abnormal electrical activity and to develop an ECG decision rule for the patients with LQTS.

METHODS

One hundred forty-one subjects were included in this study (71 patients with LQTS and 70 healthy subjects). A 12-lead digital ECG was recorded for each subject and analyzed using the CAVIAR (comparative analysis of ECG-VCG and their interpretation with auto-reference to the patient) method.

RESULTS

A decision tree involving criteria based on 3 spatiotemporal ECG measurements-the QT interval and the maximum amplitude of the T wave, both corrected from heart rate, and the loss of planarity of the end of QRS-identified patients with LQTS from healthy subjects with a sensitivity of 89%, a specificity of 96%, and a total accuracy of 92%.

CONCLUSIONS

This study suggests that 3-dimensional ECG analysis may improve the detection of patients with LQTS.

Electrophysiologic manifestations of ventricular tachyarrhythmias provoking appropriate defibrillator interventions in high-risk patients with hypertrophic cardiomyopathy

INTRODUCTION

Our objective was to determine features of ventricular tachyarrhythmias triggering appropriate implantable cardioverter-defibrillator (ICD) interventions in hypertrophic cardiomyopathy (HCM).

METHODS AND RESULTS

The study cohort was 68 high-risk HCM patients who received ICDs for primary sudden cardiac death prevention from 1995 to 2003. All episodes of sustained ventricular tachyarrhythmias identified by stored intracardiac electrograms were analyzed. Nine patients had 51 episodes of sustained ventricular tachyarrhythmic events that required device therapy (mean follow-up, 3.4 +/- 2.2 years; cumulative event rate, 3.2% per year): five had 47 episodes of monomorphic ventricular tachycardia (VT); four each had one episode of ventricular fibrillation (VF). Sinus tachycardia or atrial fibrillation was the initiating rhythm in five of nine patients and in 43 of 51 episodes of events. Of the 17 episodes of monomorphic VT detected in the VT zone, 16 (94%) were terminated by antitachycardia pacing. Thirty episodes of monomorphic VT were detected in the VF zone and were terminated by defibrillation.

CONCLUSION

Sustained monomorphic VT is common in a high-risk cohort with HCM. Sinus tachycardia is often the initiating rhythm, suggesting that high sympathetic drive may be proarrhythmic when a susceptible substrate is present. Antitachycardia pacing is highly effective in terminating VT in this patient population.

Effects of flecainide and quinidine on arrhythmogenic properties of Scn5a+/- murine hearts modelling the Brugada syndrome

Brugada syndrome (BrS) is associated with a loss of Na+ channel function and an increased incidence of rapid polymorphic ventricular tachycardia (VT) and sudden cardiac death. A programmed electrical stimulation (PES) technique assessed arrhythmic tendency in Langendorff-perfused wild-type (WT) and genetically modified (Scn5a+/-) 'loss-of-function' murine hearts in the presence and absence of flecainide and quinidine, and the extent to which Scn5a+/- hearts model the human BrS. Extra-stimuli (S2), applied to the right ventricular epicardium, followed trains of pacing stimuli (S1) at progressively reduced S1-S2 intervals. These triggered VT in 16 out of 29 untreated Scn5a+/- and zero out of 31 WT hearts. VT occurred in 11 out of 16 (10 microM) flecainide-treated WT and nine out of the 13 initially non-arrhythmogenic Scn5a+/- hearts treated with (1.0 microM) flecainide. Quinidine (10 microM) prevented VT in six out of six flecainide-treated WT and 13 out of the 16 arrhythmogenic Scn5a+/- hearts in parallel with its clinical effects. Paced electrogram fractionation analysis demonstrated increased electrogram durations, expressed as electrogram duration (EGD) ratios, with shortening S1-S2 intervals in arrhythmogenic Scn5a+/- hearts, and prolonged ventricular effective refractory periods (VERPs) in non-arrhythmogenic Scn5a+/- hearts. Flecainide increased EGD ratios in WT (at 10 microM) and non-arrhythmogenic Scn5a+/- hearts (at 1.0 microM), whereas quinidine (10 microM) reduced EGD ratios and prolonged VERPs in WT and arrhythmogenic Scn5a+/- hearts. However, epicardial and endocardial monophasic action potential recordings consistently demonstrated positive gradients of repolarization in WT, arrhythmogenic and non-arrhythmogenic Scn5a+/- hearts under all pharmacological conditions. Together, these findings demonstrate proarrhythmic effects of flecainide in WT and Scn5a+/- murine hearts that recapitulate its clinical effects. They further attribute the arrhythmogenic phenomena observed here to re-entrant substrates resulting from delayed epicardial activation despite an absence of transmural heterogeneities of repolarization, in sharp contrast to recent characterizations in 'gain-of-function' Scn5a+/Delta murine hearts modelling the long-QT(3) syndrome.

Effects of flecainide and quinidine on arrhythmogenic properties of Scn5a+/Delta murine hearts modelling long QT syndrome 3

Long QT3 (LQT3) syndrome is associated with incomplete Na+ channel inactivation, abnormal repolarization kinetics and prolonged cardiac action potential duration (APD). Electrophysiological effects of flecainide and quinidine were compared in Langendorff-perfused wild-type (WT), and genetically modified (Scn5a+/Delta) murine hearts modelling LQT3. Extra stimuli (S2) following trains of pacing stimuli (S1) applied to the right ventricular epicardium triggered ventricular tachycardia (VT) in 16 out of 28 untreated Scn5a+/Delta and zero out of 12 WT hearts. Paced electrogram fractionation analysis then demonstrated increased electrogram durations (EGD), expressed as EGD ratios, in arrhythmogenic Scn5a+/Delta hearts, and prolonged ventricular effective refractory periods in initially non-arrhythmogenic Scn5a+/Delta hearts. Nevertheless, comparisons of epicardial and endocardial monophasic action potential recordings demonstrated negative transmural repolarization gradients in both groups, giving DeltaAPD(90) values at 90% repolarization of -20.88 +/- 1.93 ms (n = 11) and -16.91 +/- 1.43 ms (n = 23), respectively. Flecainide prevented initiation of VT in 13 out of 16 arrhythmogenic Scn5a+/Delta hearts, reducing EGD ratio and restoring DeltaAPD90 to + 7.55 +/- 2.24 ms (n = 9) (P < 0.05). VT occurred in four out of eight non-arrhythmogenic Scn5a+/Delta hearts in the presence of quinidine, which increased EGD ratio but left DeltaAPD90 unchanged. In contrast (P < 0.05), WT hearts had positive DeltaAPD90 values (+ 11.72 +/- 2.17 ms) (n = 20). Flecainide then increased arrhythmic tendency and EGD ratio but conserved DeltaAPD90; reduced EGD ratios and unaltered DeltaAPD90 values accompanied the lower arrhythmogenicity associated with quinidine treatment. In addition to the changes in EGD ratio shown by WT hearts, these findings attribute arrhythmogenesis and its modification by flecainide and quinidine to alterations in DeltaAPD90 in Scn5a+/Delta hearts. This is consistent with a hypothesis in which incomplete Na+ channel inactivation in Scn5a+/Delta hearts generates functional substrates dependent on altered refractoriness that cause abnormalities in activation and conduction of subsequent cardiac impulses. Any spatial heterogeneities between the epicardial and endocardial layers would thus cause fragmentation of the activation wavefront and contribute to electrogram spreading.

Paced electrogram fractionation analysis of arrhythmogenic tendency in DeltaKPQ Scn5a mice

INTRODUCTION

Gain-of-function mutations within Scn5a, including the DeltaKPQ 1505-1507 deletion in the inactivation domain compromising myocardial repolarization, are implicated in human long QT 3 syndrome (LQT3), associated with ventricular arrhythmogenesis and sudden death.

METHODS AND RESULTS

Patch clamp studies on isolated ventricular Scn5a+/Delta myocytes from DeltaKPQ mice produced by homologous recombination in embryonic stem (ES) cells confirmed such altered electrophysiological properties of the mutant channel. Programmed electrical stimulation (PES) with decremental pacing from the basal right ventricular epicardial surface and paced electrogram fractionation analysis (PEFA) of electrograms recorded from the basal left ventricular epicardial surface of Langendorff-perfused whole heart preparations demonstrated ventricular tachycardia (VT) in 8 of 9 Scn5a+/Delta mutant (but no Scn5a+/+ (wild-type (WT)) controls; n = 17), with increased electrogram durations (EGD) and more dispersed conduction curves. Isoproterenol (100 nM) was without effect on tachycardic Scn5a+/Delta hearts (n = 9) yet propranolol (1 microM) prevented VT in all isoproterenol-infused WT control (n = 4) but no Scn5a+/Delta hearts (n = 4). Furthermore propranolol itself increased EGD and dispersion in Scn5a+/Delta hearts. In contrast, mexiletine (10 microM) suppressed VTs in 4 of 5 Scn5a+/Delta hearts without altering EGD or dispersion.

CONCLUSION

Beta-adrenoreceptor blockade does not confer an antiarrhythmic effect and may even enhance arrhythmogenesis by increasing reentrant substrate in Scn5a+/Delta hearts while mexiletine protects against VT without modifying conduction characteristics. Together these findings permit a scheme where VT in LQT3 is initiated by triggered mechanisms but propagated by reentry.

Sudden death is associated with a widened paced QRS complex in noncoronary cardiac disease

INTRODUCTION

Recent experimental and clinical trials have provided evidence that increased duration of right ventricular electrogram in response to premature extrastimuli correlates with the risk of ventricular fibrillation in noncoronary heart disease. The aim of the present study was to investigate the duration of the surface QRS complex at short coupling intervals of extrastimuli as a new indicator for major arrhythmic events.

METHODS

32 patients all with nonischemic heart diseases and well preserved left ventricular function in sinusrhythm were included into the study. Fifteen had witnessed sudden death due to ventricular fibrillation or polymorphic ventricular tachycardia (VF/VT group). The control group comprised seventeen patients without a history of ventricular arrhythmias (control group). All subjects underwent programmed ventricular stimulation and QRS-durations S1-S2-S3 directly above the ventricular refractory period were analyzed.

RESULTS

Both groups had a comparable basic QRS complex of 85 +/- 9 (VF/VT) vs. 87 +/- 13 ms (control), p = 0.83. The stimulated QRS complex S3 was significantly wider in the VF/VT group compared to the control group at pacing rates of 500 and 430 ms (500 ms: 256 +/- 22 vs. 235 +/- 32 ms, p = 0.04; 430 ms: 258 +/- 23 vs. 226 +/- 27 ms, p = 0.001). No differences with regard to the ventricular effective refractory period and the ventriculoatrial conduction could be observed beween the groups.

CONCLUSIONS

Our results indicate that the duration of the paced QRS complex may be a valuable parameter to predict arrhythmic risk in patients with nonischemic heart disease. Further prospective studies in larger trials are necessary to corroborate this investigation.

Molecular Basis of Arrhythmias

The characterization of single gene disorders has provided important insights into the molecular pathogenesis of cardiac arrhythmias. Primary electricalal diseases including long-QT syndrome, short-QT syndrome, Brugada syndrome, and catecholaminergic polymorphic ventricular tachycardia have been associated with mutations in a variety of ion channel subunit genes that promote arrhythmogenesis. Pathological remodeling of ionic currents and network properties of the heart critical for normal electrical propagation plays a critical role in the initiation and maintenance of acquired arrhythmias. This review focuses on the molecular and cellular basis of electrical activity in the heart under normal and pathophysiological conditions to provide insights into the fundamental mechanisms of inherited and acquired cardiac arrhythmias. Improved understanding of the basic biology of cardiac arrhythmias holds the promise of identifying new molecular targets for the treatment of cardiac arrhythmias.

Caffeine-induced arrhythmias in murine hearts parallel changes in cellular Ca(2+) homeostasis

Heart failure leading to ventricular arrhythmogenesis is a major cause of clinical mortality and has been associated with a leak of sarcoplasmic reticular Ca(2+) into the cytosol due to increased open probabilities in cardiac ryanodine receptor Ca(2+)-release channels. Caffeine similarly increases such open probabilities, and so we explored its arrhythmogenic effects on intact murine hearts. A clinically established programmed electrical stimulation protocol adapted for studies of isolated intact mouse hearts demonstrated that caffeine (1 mM) increased the frequency of ventricular tachycardia from 0 to 100% yet left electrogram duration and latency unchanged during programmed electrical stimulation, thereby excluding slowed conduction as a cause of arrhythmogenesis. We then used fluorescence measurements of intracellular Ca(2+) concentration in isolated mouse ventricular cells to investigate parallel changes in Ca(2+) homeostasis associated with these arrhythmias. Both caffeine (1 mM) and FK506 (30 microM) reduced electrically evoked cytosolic Ca(2+) transients yet increased the frequency of spontaneous Ca(2+)-release events. Diltiazem (1 microM) but not nifedipine (1 microM) pretreatment suppressed these increases in frequency. Identical concentrations of diltiazem but not nifedipine correspondingly suppressed the arrhythmogenic effects of caffeine in whole hearts. These findings thus directly implicate spontaneous Ca(2+) waves in triggered arrhythmogenesis in intact hearts.

What causes sudden death in heart failure?

Patients with heart failure experience a number of changes in the electrical function of the heart that predispose to potentially lethal cardiac arrhythmias. Action potential prolongation, the result of functional downregulation of K currents, and aberrant Ca2+ handling is a recurrent theme. Significant alterations in conduction and activation of a number of initially adaptive but ultimately maladaptive signaling cascades contribute to the generation of a highly arrhythmogenic substrate. We review the changes in active and passive membrane properties, neurohumoral signaling, and genetic determinants that predispose to sudden arrhythmic death in patients with heart failure and highlight the critical unanswered questions that are ripe for future investigation.

Numerical Simulation of Paced Electrogram Fractionation: Relating Clinical Observations to Changes in Fibrosis and Action Potential Duration

Simulating paced electrogram fractionation.

INTRODUCTION

Paced electrogram fractionation analysis (PEFA) may identify a re-entrant substrate in patients at risk of ventricular fibrillation (VF) by detecting prolonged, fractionated ventricular electrograms ("fractionation") in response to premature extrastimuli. Numerical simulations of action potential (AP) propagation through human myocardium following such premature stimulation were performed to study the relationship between electrogram fractionation, fibrosis, and changes in AP currents.

METHODS AND RESULTS

Activation in a resistive monodomain 2 cm2 sheet of myocardium containing nonconducting fibrous tissue was modeled using standard numerical methods for solutions of partial differential equations using the Priebe-Beukelmann (PB) AP equations. Myocardial fibrosis significantly influenced electrogram morphology. High densities of closely spaced fibrous septa caused functional block and altered propagation paths at short coupling intervals, and produced large increases in electrogram duration similar to those associated with increased risk of VF in clinical studies. Prolongation of the cardiac AP using the heart failure variant of the PB model further increased the amount of fractionation and thereby replicated clinical recordings more closely than did fibrosis alone. Increasing AP dispersion by a variable reduction in the potassium current I(Kr) simulated results seen in patients with the long QT syndrome with an abrupt increase in electrogram duration, while a uniform reduction in I(Kr) alone did not result in fractionated electrograms. In contrast, increases in cytosolic Ca2+ and Ca2+ buffering by troponin to simulate HCM had little effect on fractionation.

CONCLUSIONS

These results relate the effects of fibrosis, AP abnormalities, and dispersion of AP duration to the characteristic electrograms recorded in patients at risk of sudden death.

Hypertrophic cardiomyopathy in childhood

Hypertrophic cardiomyopathy (HCM) is a heterogeneous and relatively common genetic cardiac disease that has been the subject of intense scrutiny and investigation for over 40 years. HCM is an important cause of disability and death in patients of all ages, although unexpected sudden death in the young is perhaps the most devastating component of the natural history. Therefore, while HCM is uncommon in pediatric cardiology practice, it is nevertheless a disease of great importance to young people and those clinicians charged with their care. Due to marked heterogeneity in clinical expression, natural history and prognosis, diagnostic and management strategies often represent a dilemma (and even the source of controversy) to both primary care clinicians and cardiovascular specialists. Consequently, it is timely to place perspective and clarify many of these relevant clinical issues, and profile the rapidly evolving concepts regarding HCM, especially as they may impact on this disease in childhood.

Nifedipine and diltiazem suppress ventricular arrhythmogenesis and calcium release in mouse hearts

Ventricular arrhythmogenesis leading to sudden cardiac death remains responsible for significant mortality in conditions such as cardiac failure and the long-QT syndrome (LQTS). Arrhythmias may be accentuated by beta-adrenergic stimulation and, accordingly, the present study explored the possible effects of beta-adrenergic stimulation and L-type Ca(2+) channel blockade on ventricular arrhythmogenesis and Ca(2+) handling using the mouse heart as an experimental system. Studies in whole, Langendorff-perfused hearts using programmed electrical stimulation protocols adapted from clinical practice demonstrated sustained ventricular tachycardia following addition of 0.1 microM isoprenaline (n=15), whilst no arrhythmias were observed in the absence of the drug (n=15). Arrhythmias were suppressed by nifedipine or diltiazem pre-treatment (both 1 microM) (n=8 and 4 respectively) and were also induced by elevating external [Ca(2+)] (n=3). At the cellular level, 0.1 microM isoprenaline significantly increased normalized fluorescence (F/F(0)) in field-stimulated fluo-3-loaded mouse ventricular myocytes imaged using confocal microscopy, reflecting increases in sarcoplasmic reticulum Ca(2+) release (n=8). Elevated external [Ca(2+)] also increased F/F(0) (n=4) whilst 0.1 microM nifedipine or 0.1 microM diltiazem significantly decreased F/F(0) (n=13 and 6 respectively). Pre-treatment with 0.1 microM nifedipine or 0.1 microM diltiazem suppressed the increases in F/F(0) induced by 0.1 microM isoprenaline alone (n=14 and 6 respectively). The findings thus paralleled suppression of isoprenaline-induced arrhythmias seen with nifedipine or diltiazem at the whole-heart level. Taken together, the findings may have implications for the use of L-type Ca(2+) channel blockade in conditions associated with beta-adrenergically driven ventricular arrhythmias such as cardiac failure and LQTS.

Electrogram prolongation and nifedipine-suppressible ventricular arrhythmias in mice following targeted disruption of KCNE1

Mutations in KCNE1, the gene encoding the beta subunit of the slowly activating delayed rectifier potassium current (IKs) channel protein, may lead to the long QT syndrome (LQTS), a condition associated with enhanced arrhythmogenesis. Mice with homozygous deletion of the coding sequence of KCNE1 have inner ear defects strikingly similar to those seen in the corresponding human condition. The present study demonstrated and assessed the mechanism of ventricular arrhythmias in Langendorff-perfused whole heart preparations from homozygous KCNE1-/- mice compared to wild-type mice of the same age. The effects of programmed electrical stimulation with decremental pacing from the basal right ventricular epicardial surface upon electrogram waveforms recorded from the basal left ventricle were assessed and quantified using techniques of paced electrogram fractionation analysis for the first time in an experimental system. All KCNE1-/-(n = 10) but not wild-type (n = 14) mouse hearts empirically demonstrated marked pacing-induced ventricular arrhythmogenicity. This correlated with significant increases in electrogram dispersion, consistent with a wider spread in conduction velocities, in parallel with clinical findings from LQTS patients with potassium channel mutations. In contrast, introduction of 100 nM isoprenaline induced arrhythmogenicity in both KCNE1-/- (n = 7) and wild-type (n = 6) hearts during pacing. Furthermore, pretreatment with 1 muM nifedipine exerted a strong anti-arrhythmic effect in the KCNE1-/- hearts (n = 12) that persisted even in the presence of 100 nM isoprenaline (n = 6). Our findings associate KCNE1-/- with an arrhythmogenic phenotype that shows an increased dispersion of conduction velocities, and whose initiation is prevented by nifedipine, a finding that in turn may have therapeutic applications in conditions such as LQTS.