Electrophysiological mechanism of the characteristic electrocardiographic morphology of torsade de pointes tachyarrhythmias in the long-QT syndrome: detailed analysis of ventricular tridimensional activation patterns

Mechanisms underlying the spontaneous termination of torsades de pointes in an experimental model of long QT syndrome

BACKGROUND

Torsades de pointes (TdP) represent a complex polymorphic ventricular tachycardia. While the triggering mechanisms of early afterdepolarization and increased dispersion of repolarization are well investigated, the sudden self-limiting termination remains poorly understood.

OBJECTIVE

The purpose of this study was to perform analysis of TdP to investigate factors causing spontaneous termination.

METHODS

We used a large data set from Langendorff experiments in isolated rabbit hearts in which drug-induced QT prolongation, bradycardia, and hypokalemia provoke TdP. We included 427 episodes with typical TdP characteristics of polymorphic self-terminating beats and twisting QRS complexes occurring in the presence of abnormal QT prolongation due to various different QT-prolonging drugs. The use of 8 monophasic action potential catheters allowed the characterization of action potential duration, configuration, and dispersion of repolarization beyond the capabilities of the surface electrocardiogram. To identify possible mechanisms of arrhythmia termination, the initial, midpoint, and terminal 3 ventricular complexes were analyzed for each episode.

RESULTS

An abrupt decrease in spatial dispersion over the course of a TdP episode was identified as a precursor for termination of TdP. Within the last 3 beats, a sudden significant decrease in the dispersion of repolarization was observed as a predictor of termination. In parallel, there was a decrease in action potential duration (action potential duration at 90% repolarization) before termination. Also, a change in action potential configuration (action potential duration at 90% repolarization/action potential duration at 50% repolarization ratio) in terms of the loss of action potential dome with a restitution of action potential triangulation was observed.

CONCLUSION

In >400 TdP episodes, homogenization of myocardial repolarization with the recovery of an action potential configuration determines the termination of TdP episodes.

Mechanisms of torsades de pointes: an update

Torsades de Pointes (TdP) refers to a polymorphic ventricular tachycardia (VT) with undulating QRS axis that occurs in long QT syndrome (LQTS), although the term has been used to describe polymorphic ventricular tachyarrhythmias in which QT intervals are not prolonged, such as short-coupled variant of TdP currently known as short-coupled ventricular fibrillation (VF) and Brugada syndrome. Extensive works on LQTS-related TdP over more than 50 years since it was first recognized by Dessertennes who coined the French term meaning "twisting of the points", have led to current understanding of the electrophysiological mechanism that TdP is initiated by triggered activity due to early afterdepolarization (EAD) and maintained by reentry within a substrate of inhomogeneous repolarization. While a recently emerging notion that steep voltage gradients rather than EADs are crucial to generate premature ventricular contractions provides additions to the initiation mode, the research to elucidate the maintenance mechanism hasn't made much progress. The reentrant activity that produces the specific form of VT is not well characterized. We have conducted optical mapping in a rabbit model of electrical storm by electrical remodeling (QT prolongation) due to chronic complete atrioventricular block and demonstrated that a tissue-island with prolonged refractoriness due to enhanced late Na+ current (INa-L) contributes to the generation of drifting rotors in a unique manner, which may explain the ECG characteristic of TdP. Moreover, we have proposed that the neural Na+ channel NaV1.8-mediated INa-L may be a new player to form the substrate for TdP. Here we discuss TdP mechanisms by comparing the findings in electrical storm rabbits with recently published studies by others in simulation models and human and animal models of LQTS.

Primary Electrical Heart Disease-Principles of Pathophysiology and Genetics

Primary electrical heart diseases, often considered channelopathies, are inherited genetic abnormalities of cardiomyocyte electrical behavior carrying the risk of malignant arrhythmias leading to sudden cardiac death (SCD). Approximately 54% of sudden, unexpected deaths in individuals under the age of 35 do not exhibit signs of structural heart disease during autopsy, suggesting the potential significance of channelopathies in this group of age. Channelopathies constitute a highly heterogenous group comprising various diseases such as long QT syndrome (LQTS), short QT syndrome (SQTS), idiopathic ventricular fibrillation (IVF), Brugada syndrome (BrS), catecholaminergic polymorphic ventricular tachycardia (CPVT), and early repolarization syndromes (ERS). Although new advances in the diagnostic process of channelopathies have been made, the link between a disease and sudden cardiac death remains not fully explained. Evolving data in electrophysiology and genetic testing suggest previously described diseases as complex with multiple underlying genes and a high variety of factors associated with SCD in channelopathies. This review summarizes available, well-established information about channelopathy pathogenesis, genetic basics, and molecular aspects relative to principles of the pathophysiology of arrhythmia. In addition, general information about diagnostic approaches and management is presented. Analyzing principles of channelopathies and their underlying causes improves the understanding of genetic and molecular basics that may assist general research and improve SCD prevention.

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

BACKGROUND AND OBJECTIVE

Excessive prolongation of QT interval on ECGs in patients with congenital/acquired long QT syndrome and heart failure is a sign suggesting the development of early afterdepolarization (EAD), an abnormal repolarization in the action potential of ventricular cardiomyocytes. The development of EAD has been believed to be a trigger for fatal tachyarrhythmia, which can be a risk for sudden cardiac death. The role of EAD in triggering ventricular tachycardia (VT) remains unclear. The aim of this study was to elucidate the mechanism of EAD-induced triggered activity formation that leads to the VT such as Torsades de Pointes.

METHODS

We investigated the relationship between EAD and tachyarrhythmia initiation by constructing homogeneous myocardial sheet models consisting of the mid-myocardial cell version of a human ventricular myocyte model and performing simulations of excitation propagation.

RESULTS

A solitary island-like (clustering) occurrence of EADs in the homogeneous myocardial sheet could induce a focal excitation wave. However, reentrant excitation, an entity of tachyarrhythmia, was not able to be triggered regardless of the EAD cluster size when the focal excitation wave formed a repolarization potential difference boundary consisting of only a convex surface. The discontinuous distribution of multiple EAD clusters in the ventricular tissue formed a specific repolarization heterogeneity due to the repolarization potential difference, the shape of which depended on EAD cluster size and placed intervals. We found that the triggered activity was formed in such a manner that the repolarization potential difference boundary included a concave surface.

CONCLUSIONS

The formation of triggered activity that led to tachyarrhythmia required not only the occurrence of EAD onset-mediated focal excitation wave but also a repolarization heterogeneity-based specific repolarization potential difference boundary shape formed within the tissue.

Directed graph mapping shows rotors maintain non-terminating and focal sources maintain self-terminating Torsade de Pointes in canine model

Torsade de Pointes is a polymorphic ventricular tachycardia which is as yet incompletely understood. While the onset of a TdP episode is generally accepted to be caused by triggered activity, the mechanisms for the perpetuation is still under debate. In this study, we analysed data from 54 TdP episodes divided over 5 dogs (4 female, 1 male) with chronic atrioventricular block. Previous research on this dataset showed both reentry and triggered activity to perpetuate the arrhythmia. 13 of those TdP episodes showed reentry as part of the driving mechanism of perpetuating the episode. The remaining 41 episodes were purely ectopic. Reentry was the main mechanism in long-lasting episodes (>14 beats), while focal sources were responsible for maintaining shorter episodes. Building on these results, we re-analysed the data using directed graph mapping This program uses principles from network theory and a combination of positional data and local activation times to identify reentry loops and focal sources within the data. The results of this study are twofold. First, concerning reentry loops, we found that on average non-terminating (NT) episodes (≥10 s) show significantly more simultaneous reentry loops than self-terminating (ST) TdP (<10 s). Non-terminating episodes have on average 2.72 ± 1.48 simultaneous loops, compared to an average of 1.33 ± 0.66 for self-terminating episodes. In addition, each NT episode showed a presence of (bi-)ventricular loops between 10.10% and 69.62% of their total reentry duration. Compared to the ST episodes, only 1 in 4 episodes (25%) showed (bi-)ventricular reentry, lasting only 7.12% of its total reentry duration. This suggests that while focal beats trigger TdP, macro-reentry and multiple simultaneous localized reentries are the major drivers of long-lasting episodes. Second, using heatmaps, we found focal sources to occur in preferred locations, instead of being distributed randomly. This may have implications on treatment if such focal origins can be disabled reliably.

Effects of beta-blockers on ventricular repolarization documented by 24-h electrocardiography in long-QT syndrome type 2

BACKGROUND

Long QT syndrome (LQTS) is an inherited arrhythmia disorder characterized by ventricular repolarization abnormalities and a risk of sudden cardiac death. The electrophysiological components generating the high risk of arrhythmias in LQTS are prolonged repolarization, increased dispersion of repolarization, and early afterdepolarizations, which are clinically estimated as QT interval, T-wave peak to end (TPE) interval, and T2/T1-wave amplitude ratio, respectively. In experimental LQTS type 2 (LQT2) models, beta-blockers decrease dispersion of repolarization and prevent early afterdepolarizations. In clinical studies among LQT2 patients beta-blockers are more effective against exercise-induced than arousal-induced cardiac events.

OBJECTIVES AND METHODS

The aim of the study was to investigate the effects of beta-blocker therapy on QT and TPE intervals, and maximal T2/T1-wave amplitude ratios recorded by 24-h electrocardiograms (ECG) among 25 LQT2 patients.

RESULTS

Beta-blocker therapy decreased the maximal T2/T1-wave amplitude ratio from 2.9±1.1 to 1.8±0.7 (p<0.001), but did not change pause-induced T2/T1-wave amplitude ratio. Under medication abrupt maximal TPE intervals were shorter at heart rates of 75 beats/min or over, and maximal QT intervals were shorter at a heart rate of 100 beats/min.

CONCLUSIONS

Beta-blockers stabilize ventricular repolarization in LQT2 by reducing electrocardiographic early afterdepolarizations, and by reducing abrupt prolongation of electrocardiographic dispersion of repolarization and ventricular repolarization duration at elevated heart rates. The effect of beta-blockers on pause-induced electrocardiographic early afterdepolarizations is weak. The findings provide electrocardiographic explanation for the protective effects of beta-blockers against exercise-induced cardiac events in LQT2.

Interplay between temporal and spatial dispersion of repolarization in the initiation and perpetuation of torsades de pointes in the chronic atrioventricular block dog

Ventricular arrhythmias, consisting of single ectopic beats (sEB), multiple EB (mEB), and torsades de pointes (TdP, defined as ≥5 beats with QRS vector twisting around isoelectric line) can be induced in the anesthetized chronic atrioventricular block (CAVB) dog by dofetilide (I_Kr blocker). The interplay between temporal dispersion of repolarization, quantified as short-term variability (STV), and spatial dispersion of repolarization (SDR) in the initiation and perpetuation of these arrhythmias remains unclear. Five inducible (≥3 TdPs/10 min) CAVB dogs underwent one mapping experiment and were observed for 10 min from the start of dofetilide infusion (0.025 mg/kg, 5 min). An intracardiac decapolar electrogram (EGM) catheter and 30 intramural cardiac needles in the left ventricle (LV) were introduced. STV_ARI was derived from 31 consecutive activation recovery intervals (ARIs) on the intracardiac EGM, using the formula: [Formula: see text]. The mean SDR_3D in the LV was determined as the three-dimensional repolarization time differences between the intramural cardiac needles. Moments of measurement included baseline (BL) and after dofetilide infusion before first 1) sEB (occurrence at 100 ± 35 s), 2) mEB (224 ± 96 s), and 3) non-self-terminating TdP (454 ± 298 s). STV_ARI increased from 2.15 ± 0.32 ms at BL to 3.73 ± 0.99 ms* before the first sEB and remained increased without further significant progression to mEB (4.41 ± 0.45 ms*) and TdP (5.07 ± 0.84 ms*) (*P < 0.05 compared with BL). SDR_3D did not change from 31 ± 11 ms at BL to 43 ± 13 ms before sEB but increased significantly before mEB (68 ± 7 ms*) and to TdP (86 ± 9 ms*⁺) (⁺P < 0.05 compared with sEB). An increase in STV contributes to the initiation of sEB, whereas an increase in SDR is important for the perpetuation of non-self-terminating TdPs.NEW & NOTEWORTHY This study compared two well-established electrophysiological parameters, being temporal and spatial dispersion of repolarization, and provided new insights into their interplay in the arrhythmogenesis of torsades de pointes arrhythmias. Although it confirmed that an increase in temporal dispersion of repolarization contributes to the initiation of single ectopic beats, it showed that an increase in spatial dispersion of repolarization is important for the perpetuation of non-self-terminating torsades de pointes arrhythmias.

Voltage/Calcium Uncoupling Underlies Sustained Torsade de Pointes Ventricular Tachyarrhythmia in an Experimental Model of Long QT Syndrome

Background

Clinical experience showed that the majority of Torsade de Pointes (TdP) ventricular tachyarrhythmia (VT) in patients with long QT syndrome (LQTS) are self-terminating (ST), but the few that are non-self-terminating (NST) are potentially fatal. A paramount issue in clinical arrhythmology is to understand the electrophysiological mechanism of ST vs. NST TdP VT.

Methods

We investigated the electrophysiological mechanism of ST vs. NST TdP VT in the guinea pig Anthopleurin-A experimental model of LQTS, a close surrogate model of congenital LQT3. We utilized simultaneous optical recordings of membrane voltage (V_m) and intracellular calcium (Ca_i) and a robust analytical method based on spatiotemporal entropy difference (E_d) to investigate the hypothesis that early V_m/Ca_i uncoupling during TdP VT can play a primary role in perpetuation of VT episodes.

Results

We analyzed a total of 35 episodes of TdP VT from 14 guinea pig surrogate models of LQTS, including 23 ST and 12 NST VTs. E_d values for NST VT were significantly higher than E_d values for ST VT. Analysis of wave front topology during the early phase of ST VT showed the Ca_i wave front following closely V_m wave front consistent with a lower degree of E_d. In contrast, NST VT was associated with uncoupling of V_m/Ca_i wave fronts during the first 2 or 3 cycles of VT associated with early wave break propagation pattern.

Conclusions

Utilizing a robust analytical method we showed that, in comparison to ST TdP VT, NST VT was consistently predated by early uncoupling of V_m/Ca_i that destabilized wave front propagation and can explain a sustained complex reentrant excitation pattern.

Disease network delineates the disease progression profile of cardiovascular diseases

OBJECTIVE

As Electronic Health Records (EHR) data accumulated explosively in recent years, the tremendous amount of patient clinical data provided opportunities to discover real world evidence. In this study, a graphical disease network, named progressive cardiovascular disease network (progCDN), was built to delineate the progression profiles of cardiovascular diseases (CVD).

MATERIALS AND METHODS

The EHR data of 14.3 million patients with CVD diagnoses were collected for building disease network and further analysis. We applied a new designed method, progression rates (PR), to calculate the progression relationship among different diagnoses. Based on the disease network outcome, 23 disease progression pair were selected to screen for salient features.

RESULTS

The network depicted the dominant diseases in CVD development, such as the heart failure and coronary arteriosclerosis. Novel progression relationships were also discovered, such as the progression path from long QT syndrome to major depression. In addition, three age-group progCDNs identified a series of age-associated disease progression paths and important successor diseases with age bias. Furthermore, a list of important features with sufficient abundance and high correlation was extracted for building disease risk models.

DISCUSSION

The PR method designed for identifying the progression relationship could be widely applied in any EHR database due to its flexibility and robust functionality. Meanwhile, researchers could use the progCDN network to validate or explore novel disease relationships in real world data.

CONCLUSION

The first-time interrogation of such a huge CVD patients cohort enabled us to explore the general and age-specific disease progression patterns in CVD development.

Circadian Pattern of Ion Channel Gene Expression in Failing Human Hearts

BACKGROUND

Ventricular tachyarrhythmias and sudden cardiac death show a circadian pattern of occurrence in patients with heart failure. In the rodent ventricle, a significant portion of genes, including some ion channels, shows a circadian pattern of expression. However, genes that define electrophysiological properties in failing human heart ventricles have not been examined for a circadian expression pattern.

METHODS

Ventricular tissue samples were collected from patients at the time of cardiac transplantation. Two sets of samples (n=37 and 46, one set with a greater arrhythmic history) were selected to generate pseudo-time series according to their collection time. A third set (n=27) of samples was acquired from the nonfailing ventricles of brain-dead donors. The expression of 5 known circadian clock genes and 19 additional ion channel genes plausibly important to electrophysiological properties were analyzed by real-time polymerase chain reaction and then analyzed for the percentage of expression variation attributed to a 24-hour circadian pattern.

RESULTS

The 5 known circadian clock gene transcripts showed a strong circadian expression pattern. Compared with rodent hearts, the human circadian clock gene transcripts showed a similar temporal order of acrophases but with a ≈7.6 hours phase shift. Five of the ion channel genes also showed strong circadian expression. Comparable studies of circadian clock gene expression in samples recovered from nonheart failure brain-dead donors showed acrophase shifts, or weak or complete loss of circadian rhythmicity, suggesting alterations in circadian gene expression.

CONCLUSIONS

Ventricular tissue from failing human hearts display a circadian pattern of circadian clock gene expression but phase-shifted relative to rodent hearts. At least 5 ion channels show a circadian expression pattern in the ventricles of failing human hearts, which may underlie a circadian pattern of ventricular tachyarrhythmia/sudden cardiac death. Nonfailing hearts from brain-dead donors show marked differences in circadian clock gene expression patterns, suggesting fundamental deviations from circadian expression.

Adaptation of ventricular repolarization duration and dispersion during changes in heart rate induced by atrial stimulation

BACKGROUND

The duration of ventricular repolarization (VR) and its spatial and temporal heterogeneity are central elements in arrhythmogenesis. We studied the adaptation of VR duration and dispersion and their relationship in healthy human subjects during atrial pacing.

METHODS

Patients 20-50 years of age who were scheduled for ablation of supraventricular tachycardia without preexcitation but otherwise healthy were eligible. Vectorcardiography recordings with Frank leads were used for data collection. Incremental atrial pacing from a coronary sinus electrode was performed by decrements of 10ms/cycle from just above sinus rate, and then kept at a fixed heart rate (HR) just below the Wenckebach rate for ≥5min and then stopped. VR duration was measured as QT and VR dispersion as T area, T amplitude and ventricular gradient. The primary measure (T90 End) was the time to reach 90% change from baseline to the steady state value during and after pacing.

RESULTS

A complete study protocol was accomplished in 9 individuals (6 women). VR duration displayed a monophasic adaptation during HR acceleration lasting on average 20s. The median (Q1-Q3) T90 End for QT was 85s (51-104), a delay by a factor >4. All dispersion measures displayed a tri-phasic response pattern during HR acceleration and T90 End was 3-5 times shorter than for VR duration.

CONCLUSIONS

Even during close to "physiological" conditions, complex and differing response patterns in VR duration and dispersion measures followed changes in HR. Extended knowledge about these responses in disease conditions might assist in risk evaluation and finding therapeutic alternatives.

The Increment of Short-term Variability of Repolarisation Determines the Severity of the Imminent Arrhythmic Outcome

Ventricular remodelling can make the heart more susceptible to ventricular arrhythmias like torsades de pointes. Understanding the underlying mechanisms of initiation of ventricular arrhythmias and the determining factors for its severity has the potential to uncover new interventions. Beat-to-beat variation of repolarisation, quantified as short-term variability of repolarisation (STV), has been identified as an important factor contributing to arrhythmogenesis. This article provides an overview of experimental data about STV in relation to the initiation of torsades de pointes in a canine model of complete chronic atrioventricular block susceptible to torsades de pointes arrhythmias. Furthermore, it explores STV in relation to the severity of the arrhythmic outcome.

Acquired Long QT Syndrome and Electrophysiology of Torsade de Pointes

Congenital long QT syndrome (LQTS) has been the most investigated cardiac ion channelopathy. Although congenital LQTS remains the domain of cardiologists, cardiac electrophysiologists and specialised centres, the much more frequently acquired LQTS is the domain of physicians and other members of healthcare teams required to make therapeutic decisions. This paper reviews the electrophysiological mechanisms of acquired LQTS, its ECG characteristics, clinical presentation, and management. The paper concludes with a comprehensive review of the electrophysiological mechanisms of torsade de pointes.

Acquired long QT syndrome and torsade de pointes

Since its initial description by Jervell and Lange-Nielsen in 1957, the congenital long QT syndrome (LQTS) has been the most investigated cardiac ion channelopathy. Although congenital LQTS continues to remain the domain of cardiologists, cardiac electrophysiologists, and specialized centers, the by far more frequent acquired drug-induced LQTS is the domain of all physicians and other members of the health care team who are required to make therapeutic decisions. This report will review the electrophysiological mechanisms of LQTS and torsade de pointes, electrocardiographic characteristics of acquired LQTS, its clinical presentation, management, and future directions in the field.

Congenital Long QT syndrome and torsade de pointes

Since its initial description by Jervell and Lange-Nielsen in 1957, the congenital long QT syndrome (LQTS) has been the most investigated cardiac ion channelopathy. A prolonged QT interval in the surface electrocardiogram is the sine qua non of the LQTS and is a surrogate measure of the ventricular action potential duration (APD). Congenital as well as acquired alterations in certain cardiac ion channels can affect their currents in such a way as to increase the APD and hence the QT interval. The inhomogeneous lengthening of the APD across the ventricular wall results in dispersion of APD. This together with the tendency of prolonged APD to be associated with oscillations at the plateau level, termed early afterdepolarizations (EADs), provides the substrate of ventricular tachyarrhythmia associated with LQTS, usually referred to as torsade de pointes (TdP) VT. This review will discuss the genetic, molecular, and phenotype characteristics of congenital LQTS as well as current management strategies and future directions in the field.

Spatial Patterns of Excitation at Tissue and Whole Organ Level Due to Early Afterdepolarizations

Early after depolarizations (EAD) occur in many pathological conditions, such as congenital or acquired channelopathies, drug induced arrhythmias, and several other situations that are associated with increased arrhythmogenicity. In this paper we present an overview of the relevant computational studies on spatial EAD dynamics in 1D, 2D, and in 3D anatomical models and discuss the relation of EADs to cardiac arrhythmias. We also discuss unsolved problems and highlight new lines of research in this area.

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.

Perpetuation of torsade de pointes in heterogeneous hearts: competing foci or re-entry?

KEY POINTS

The underlying mechanism of torsade de pointes (TdP) remains of debate: perpetuation may be due to (1) focal activity or (2) re-entrant activity. The onset of TdP correlates with action potential heterogeneities in different regions of the heart. We studied the mechanism of perpetuation of TdP in silico using a 2D model of human cardiac tissue and an anatomically accurate model of the ventricles of the human heart. We found that the mechanism of perpetuation TdP depends on the degree of heterogeneity. If the degree of heterogeneity is large, focal activity alone can sustain a TdP, otherwise re-entrant activity emerges. This result can help to understand the relationship between the mechanisms of TdP and tissue properties and may help in developing new drugs against it.

ABSTRACT

Torsade de pointes (TdP) can be the consequence of cardiac remodelling, drug effects or a combination of both. The mechanism underlying TdP is unclear, and may involve triggered focal activity or re-entry. Recent work by our group has indicated that both cases may exist, i.e. TdPs induced in the chronic atrioventricular block (CAVB) dog model may have a focal origin or are due to re-entry. Also it was found that heterogeneities might play an important role. In the current study we have used computational modelling to further investigate the mechanisms involved in TdP initiation and perpetuation, especially in the CAVB dog model, by the addition of heterogeneities with reduced repolarization reserve in comparison with the surrounding tissue. For this, the TNNP computer model was used for computations. We demonstrated in 2D and 3D simulations that ECGs with the typical TdP morphology can be caused by both multiple competing foci and re-entry circuits as a result of introduction of heterogeneities, depending on whether the heterogeneities have a large or a smaller reduced repolarization reserve in comparison with the surrounding tissue. Large heterogeneities can produce ectopic TdP, while smaller heterogeneities will produce re-entry-type TdP.

Role of pharmacotherapy in cardiac ion channelopathies

In the last decade, there have been considerable advances in the understanding of the pathophysiology of malignant ventricular tachyarrhythmias (VT) and sudden cardiac death (SCD). Over 80% of SCD occurs in patients with organic heart disease. However, approximately 10%-15% of SCD occurs in the presence of structurally normal heart, and the majority of these patients are young. In this group of patients, changes in genes encoding cardiac ion channels produce modifications of the function of the channel resulting in an electrophysiological substrate of VT and SCD. Collectively, these disorders are referred to as cardiac ion channelopathies. The four major syndromes in this group are: the long QT syndrome (LQTS), the Brugada syndrome (BrS), the short QT syndrome (SQTS), and the catecholaminergic polymorphic ventricular tachycardia (CPVT). Each of these syndromes includes multiple subtypes with different and sometimes complex cardiac ion channel genetic abnormalities. Many are associated with other somatic and neurological abnormalities besides the risk of VT and SCD. The current management of cardiac ion channelopathies can be summarized as follows: (1) in symptomatic patients, the implantable cardioverter defibrillator (ICD) is the only viable option; (2) in asymptomatic patients, risk stratification is necessary, followed by either the ICD, pharmacotherapy, or a combination of both. A genotype-specific approach to pharmacotherapy requires a thorough understanding of the molecular-cellular basis of arrhythmogenesis in cardiac ion channelopathies as well as the specific drug profile.

Repeated Aborted Sudden Cardiac Death with Long QT Syndrome in a Patient with Anomalous Origin of the Right Coronary Artery from the Left Coronary Cusp

A 15-year-old female with a prior history of aborted cardiac death and surgical correction of anomalous origin of the right coronary artery (RCA) presented with polymorphic ventricular tachycardia. Her electrocardiogram after defibrillation was suggestive of congenital long QT syndrome (LQTS). The patient was treated with a β-blocker and remained free from ventricular arrhythmia during the follow-up of more than 6 months. Here, we present the case of a young female with repeated aborted cardiac death accompanied by anomalous origin of the RCA and congenital LQTS for the first time.

Spatiotemporal estimation of activation times of fractionated ECGs on complex heart surfaces

Identification of electrical activation or depolarization times on sparsely-sampled complex heart surfaces is of importance to clinicians and researchers in cardiac electrophysiology. We introduce a spatiotemporal approach for activation time estimation which combines prior results using spatial and temporal methods with our own progress on gradient estimation on triangulated surfaces. Results of the method applied to simulated and canine heart data suggest that improvements are possible using this novel combined approach.

Optical imaging of voltage and calcium in cardiac cells & tissues

Cardiac optical mapping has proven to be a powerful technology for studying cardiovascular function and disease. The development and scientific impact of this methodology are well-documented. Because of its relevance in cardiac research, this imaging technology advances at a rapid pace. Here, we review technological and scientific developments during the past several years and look toward the future. First, we explore key components of a modern optical mapping set-up, focusing on: (1) new camera technologies; (2) powerful light-emitting-diodes (from ultraviolet to red) for illumination; (3) improved optical filter technology; (4) new synthetic and optogenetic fluorescent probes; (5) optical mapping with motion and contraction; (6) new multiparametric optical mapping techniques; and (7) photon scattering effects in thick tissue preparations. We then look at recent optical mapping studies in single cells, cardiomyocyte monolayers, atria, and whole hearts. Finally, we briefly look into the possible future roles of optical mapping in the development of regenerative cardiac research, cardiac cell therapies, and molecular genetic advances.

Endocardial arrhythmogenic mechanisms of torsades de pointes in patients with the congenital long QT syndrome

We injected acetylcholine (Ach) into the coronary artery to ascertain whether coronary vasospasm contributed to the syncopal events or chest oppression suffered by 3 patients with long QT syndrome (LQTS). During the test, a quadripolar electrode catheter was placed in the right ventricle and the activation-recovery interval was reanalyzed from the stored data. Intracoronary Ach transiently prolonged the QT intervals in all 3 patients without inducing coronary vasospasm. The Ach-induced QT prolongation was associated with enhanced spatial and temporal dispersion of intra-ventricular repolarization. The electrophysiological abnormalities were consistent with the putative arrhythmogenic mechanisms identified in experimental studies of LQTS.

Torsades: adjacent and triggering electrocardiographic events

Torsades de pointes (TdP) is a particular variant of ventricular arrhythmia associated with the long QT syndrome. The background of the latter is essentially 2-fold: patients under treatment with QT-prolonging drugs and subjects with congenital ionopathies. A third category is composed of subjects with both of these backgrounds. The fundamental feature of TdP is its provocation by pause-related augmentation of the repolarizing TU wave. The substrate electrocardiogram (ECG) shows prominent U waves in regular rhythm. The exaggeration of the U wave voltage following a pause is more marked the longer the pause and, for a given pause, more marked the faster the prepause rate. The pause-related sequences figure frequently adjacent to that which actually triggers the attack of TdP and continues to be seen after the event, serving to advise the physician as to the diagnosis, even following cardiac resuscitation, so that preventive measures can be taken (pacing, intravenous magnesium sulfate, or infusion with isoproterenol). The U wave of the regular rhythm ECG may show amplitude instability: an especially tall U triggering a premature ventricular complexe (PVC) that then in turn generates a pause-related sequence. TU alternans is common. Because these patients may not be in a monitored bed, recognition of pause-related phenomena in a patient with a long QT requires the ECG reader of the day to alert the floor as to the running danger. The mechanism of the pause-related TU augmentation is the generation of early afterdepolarizations.

Transmural ultrasound-based visualization of patterns of action potential wave propagation in cardiac tissue

The pattern of action potential propagation during various tachyarrhythmias is strongly suspected to be composed of multiple re-entrant waves, but has never been imaged in detail deep within myocardial tissue. An understanding of the nature and dynamics of these waves is important in the development of appropriate electrical or pharmacological treatments for these pathological conditions. We propose a new imaging modality that uses ultrasound to visualize the patterns of propagation of these waves through the mechanical deformations they induce. The new method would have the distinct advantage of being able to visualize these waves deep within cardiac tissue. In this article, we describe one step that would be necessary in this imaging process—the conversion of these deformations into the action potential induced active stresses that produced them. We demonstrate that, because the active stress induced by an action potential is, to a good approximation, only nonzero along the local fiber direction, the problem in our case is actually overdetermined, allowing us to obtain a complete solution. Use of two- rather than three-dimensional displacement data, noise in these displacements, and/or errors in the measurements of the fiber orientations all produce substantial but acceptable errors in the solution. We conclude that the reconstruction of action potential-induced active stress from the deformation it causes appears possible, and that, therefore, the path is open to the development of the new imaging modality.

Targeting device therapy: genomics of sudden death

This article discusses the development and use of genomic predictors to define the population at risk for sudden cardiac death, which is usually defined as death from cardiac causes within an hour of symptom onset. The identification of genetic predictors of sudden death in heart failure is in its earliest stages. Mutations in ion channels have been shown to cause inherited forms of sudden death; there is, however, little evidence that mutations or rare single nucleotide polymorphisms (SNPs) in those genes are important causes of the common forms of sudden death. Other common variants in ion channels and related genes are associated with sudden death in the setting of acute myocardial infarction or heart failure. Ultimately, we hope to identify a handful of SNPs that modulate the risk of sudden death in heart failure and to develop an algorithm to predict risk based on genotype.

The isolated rabbit heart and Purkinje fibers as models for identifying proarrhythmic liability

INTRODUCTION

Delayed ventricular repolarization is associated with rare, but often fatal, polymorphic tachyarrhythmias named Torsades de Pointes. ICH S7B guideline recommends an integrated approach for cardiovascular preclinical evaluation of new drug candidates, including action potential assays (as a Purkinje fiber test) but also proarrhythmia models. The aim of this preliminary study was to compare the respective value of two preclinical in vitro rabbit cardiac preparations-the Purkinje fiber and the isolated perfused heart (Langendorff method)-based on effects of dofetilide, a selective IKr inhibitor.

METHODS

Transmembrane action potentials from rabbit Purkinje fibers were recorded using a conventional intracellular glass microelectrode. Electrocardiograms from rabbit isolated hearts were evaluated for QRS, QT and T wave durations (Tpeak-Tend). The pacing protocol was the same for both preparations (basal rate of 80 bpm and pacing of 40, 60 and 140 bpm). Dofetilide was tested in both systems at concentrations of 1, 3 and 10 nmol/L.

RESULTS

In Purkinje fibers dofetilide induced a concentration- and reverse use-dependent increase in action potential durations measured at 50 and 90% of repolarization. At 10 nmol/L, only 3/10 fibers showed early after depolarizations. In the isolated heart model, dofetilide also induced a similar concentration- and reverse use-dependent increase in QT-interval. From 3 nmol/L, major changes in T wave morphology, R-on-T extrasystoles and TdP were observed, mainly at low rate. Prior to arrhythmias, T wave shape and duration were markedly altered suggesting an increase in the heterogeneity of cardiac ventricular repolarization.

CONCLUSIONS

The effects of dofetilide were comparable in the two models for delayed repolarization but the isolated heart appears to be a better predictor for arrhythmias and a unique in vitro model to assess arrhythmogenic potential of QT prolonging compounds at least when associated with IKr/hERG inhibition.

Electrophysiological remodeling in heart failure

Heart failure affects nearly 6 million Americans, with a half-million new cases emerging each year. Whereas up to 50% of heart failure patients die of arrhythmia, the diverse mechanisms underlying heart failure-associated arrhythmia are poorly understood. As a consequence, effectiveness of antiarrhythmic pharmacotherapy remains elusive. Here, we review recent advances in our understanding of heart failure-associated molecular events impacting the electrical function of the myocardium. We approach this from an anatomical standpoint, summarizing recent insights gleaned from pre-clinical models and discussing their relevance to human heart failure.

Models of torsades de pointes: effects of FPL64176, DPI201106, dofetilide, and chromanol 293B in isolated rabbit and guinea pig hearts

INTRODUCTION

For studying the torsades de pointes (TdP) liability of a compound, most high and medium throughput methods use surrogate markers such as HERG inhibition and QT prolongation. In this study, we have tested whether isolated hearts may be modified to allow TdP to be the direct readout.

METHOD

Isolated spontaneously beating rabbit and guinea pig hearts were perfused according to the Langendorff method in hypokalemic (2.1 mM) solution. The in vitro lead II ECG equivalent and the incidence of TdP were monitored for 1 h. In addition, heart rate, QTc, Tp-Te, short-term variability (STV), time to arrhythmia, and time to TdP were also analyzed.

RESULTS

FPL64176, a calcium channel activator; and DPI201106, a sodium channel inactivation inhibitor, produced TdP in isolated rabbit and guinea pig hearts in a concentration dependent manner; guinea pig hearts were 3- to 5-fold more sensitive than rabbit hearts. Both compounds also increased QTc and STV. In contrast, dofetilide, an IKr inhibitor, produced no (or a low incidence of) TdP in both species, in spite of prolongation of QTc intervals. Chromanol 293B, an IKs inhibitor, did not produce TdP in rabbit hearts but elicited TdP concentration dependently in guinea pig hearts even though the compound had no effect on QTc intervals.

CONCLUSION

IKs inhibition appears to be more likely to produce TdP in isolated guinea pig hearts than IKr inhibition. Chromanol 293B did not produce TdP in rabbit hearts presumably due to a low level of IKs channels in the heart. TdP produced in this study was consistent with the notion that its production was a consequence of reduced repolarization reserve, thereby causing rhythmic abnormalities. This isolated, perfused, and spontaneously beating rabbit and guinea pig heart preparation in hypokalemic medium may be useful as a preclinical test model for studying proarrhythmic liability of compounds in new drug development.

Arrhythmia phenotype in mouse models of human long QT

Enhanced dispersion of repolarization (DR) was proposed as a unifying mechanism, central to arrhythmia genesis in the long QT (LQT) syndrome. In mammalian hearts, K(+) channels are heterogeneously expressed across the ventricles resulting in 'intrinsic' DR that may worsen in long QT. DR was shown to be central to the arrhythmia phenotype of transgenic mice with LQT caused by loss of function of the dominant mouse K(+) currents. Here, we investigated the arrhythmia phenotype of mice with targeted deletions of KCNE1 and KCNH2 genes which encode for minK/IsK and Merg1 (mouse homolog of human ERG) proteins resulting in loss of function of I(Ks) and I(Kr), respectively. Both currents are important human K(+) currents associated with LQT5 and LQT2. Loss of minK, a protein subunit that interacts with KvLQT1, results in a marked reduction of I(Ks) giving rise to the Jervell and Lange-Nielsen syndrome and the reduced KCNH2 gene reduces MERG and I(Kr). Hearts were perfused, stained with di-4-ANEPPS and optically mapped to compare action potential durations (APDs) and arrhythmia phenotype in homozygous minK (minK(-/-)) and heterozygous Merg1 (Merg(+/-)) deletions and littermate control mice. MinK(-/-) mice has similar APDs and no arrhythmias (n = 4). Merg(+/-) mice had prolonged APDs (from 20 +/- 6 to 32 +/- 9 ms at the base, p < 0.01; from 18 +/- 5 to 25 +/- 9 ms at the apex, p < 0.01; n = 8), longer refractory periods (RP) (36 +/- 14 to 63 +/- 27 at the base, p < 0.01 and 34 +/- 5 to 53 +/- 21 ms at the apex, p < 0.03; n = 8), higher DR 10.4 +/- 4.1 vs. 14 +/- 2.3 ms, p < 0.02) and similar conduction velocities (n = 8). Programmed stimulation exposed a higher propensity to VT in Merg(+/-) mice (60% vs. 10%). A comparison of mouse models of LQT based on K(+) channel mutations important to human and mouse repolarization emphasizes DR as a major determinant of arrhythmia vulnerability.

Optical measurements of intramural action potentials in isolated porcine hearts using optrodes

BACKGROUND

Measurements of intramural membrane potential (Vm) would greatly increase knowledge of cardiac arrhythmias and defibrillation. Optrodes offer the possibility for three-dimensional Vm mapping, but their signal quality has been inadequate.

OBJECTIVE

The purpose of this work was to improve optrode signal quality and use optrodes to measure intramural distribution of action potentials and shock-induced Vm changes in porcine hearts.

METHODS

Optrodes were made from seven optical fibers 225 or 325 microm in diameter. Fiber ends were polished at a 45 degrees angle, which improved light collection and allowed their insertion without a needle. Fluorescent measurements were performed in isolated porcine hearts perfused with Tyrode's solution or blood using Vm-sensitive dye RH-237 and a 200-W Hg/Xe lamp.

RESULTS

The signal-to-noise ratio for 325-microm fibers was 44 +/- 23 in blood-perfused hearts (n = 5) and 106 +/- 45 in Tyrode's-perfused hearts (n = 3), which represents an approximately four-fold improvement over previously reported data. There was close correspondence between optical and electrical measurements of activation times and action potential duration (APD). No significant intramural APD gradients were observed at cycle lengths up to 4 s and in the presence of dofetilide or d-sotalol. Application of shocks (5-50 V/cm) produced large intramural Vm changes (up to approximately 200% action potential amplitude), possibly reflecting a combined effect of tissue discontinuities and optrode geometry.

CONCLUSIONS

A substantial improvement of optrode signal quality was achieved. Optical measurements of APD and activation times matched electrical measurements. Optrode measurements revealed no significant intramural APD gradients. Application of shocks caused large intramural Vm changes that could be influenced by the optrode geometry.

Dispersion of repolarization and refractoriness are determinants of arrhythmia phenotype in transgenic mice with long QT

Enhanced dispersion of repolarization (DR) and refractoriness may be a unifying mechanism central to arrhythmia genesis in the long QT (LQT) syndrome. The role of DR in promoting arrhythmias was investigated in several strains of molecularly engineered mice: (a) Kv4.2 dominant negative transgenic (Kv4.2DN) that lacks the fast component of the transient outward current, I(to,f), have action potential (AP) and QT prolongation, but no spontaneous arrhythmias, (b) Kv1.4 targeted mice (Kv1.4-/-) that lack the slow component of I(to) (I(to,s)), have no QT prolongation and no spontaneous arrhythmias, and (c) double transgenic (Kv4.2DN x Kv1.4-/-) mice that lack both I(to,f) and I(to,s), have AP and QT prolongation, and spontaneous ventricular tachyarrhythmias. Hearts were perfused, stained with di-4-ANEPPS and optically mapped. Activation patterns and conduction velocities were similar between the strains but AP duration at 75% recovery (APD75) was longer in Kv4.2DN (28.0 +/- 2.5 ms, P < 0.01, n = 6), Kv1.4-/- (28.4 +/- 0.4 ms, P < 0.01, n = 5) and Kv4.2DN x Kv1.4-/- (34.3 +/- 2.6 ms, P < 0.01, n = 6) mice than controls (20.3 +/- 1.0 ms, n = 5). Dispersion of refractoriness between apex and base was markedly reduced in Kv4.2DN (0.3 +/- 0.5 ms, n = 6, P < 0.05) but enhanced in Kv1.4-/- (14.2 +/- 2.0 ms, n = 5, P < 0.05) and Kv4.2DN x Kv1.4(-/-) (15.0 +/- 3 ms, n = 5, P < 0.5) mice compared with controls (10 +/- 2 ms, n = 5). A premature pulse elicited ventricular tachycardia (VT) in Kv1.4-/- (n = 4/5) and Kv4.2DN x Kv1.4-/- hearts (n = 5/5) but not Kv4.2DN hearts (n = 0/6). Voltage-clamp recordings showed that I(to,f) was 30% greater in myocytes from the apex than base which may account for the absence of DR in Kv4.2DN mice. Thus, dispersion of repolarization (DR) appears to be an important determinant of arrhythmia vulnerability.

[Acquired long QT syndrome: a dominant problem?]

QT prolongation is essentially of pharmacologic origin. It is principally linked to a block of the outward potassium current Ikr, with as a consequence a prolongation of the repolarisation causing early after potentials and re-entry. The term "repolarisation reserve" expresses the variable risk of arrhythmia among individuals under the same drug blocking Ikr. This reserve can be altered under various pathologic or genetic conditions. A series of risk factors (bradycar-Torsades de pointes} were described in 1966 by Dessertenne. They are due to a perturbation of ventricular repolarisation causing QT prolongation on surface ECG. Acquired dia, electrolytic disorders, cardiac disease, neurologic disorders, nutrition troubles, female gender) can play a role as well as the metabolic processing of pharmacological agents by Cytochrome P450 and various inhibitors or inductors of this system which can influence the half life of drugs. The list of drugs involved is continuously increasing: antiarrhythmics, antihistamines, psychotropics, anti-infectious are the main categories involved. Risk prediction is difficult particularly for non cardiovascular drugs and a low risk incidence. An other risk is to exclude patients from the benefit of an efficient drug for a serious but not frequent risk, at last an industrial risk for the manufacturer when a drug is withdrawn lately when important quantities of money have already been invested for its development. The diagnosis of torsades is easy on standard ECG although QT measurement and its heart rate variation remain uneasy. The treatment of the arrhythmias is based on heart rate acceleration by Isoprenaline or intravenous pacing and on intravenous administration of magnesium.