Electrical behavior of T-wave polarity alternans in patients with congenital long QT syndrome

Cardiac Alternans: From Bedside to Bench and Back

Cardiac alternans arises from dynamical instabilities in the electrical and calcium cycling systems of the heart, and often precedes ventricular arrhythmias and sudden cardiac death. In this review, we integrate clinical observations with theory and experiment to paint a holistic portrait of cardiac alternans: the underlying mechanisms, arrhythmic manifestations and electrocardiographic signatures. We first summarize the cellular and tissue mechanisms of alternans that have been demonstrated both theoretically and experimentally, including 3 voltage-driven and 2 calcium-driven alternans mechanisms. Based on experimental and simulation results, we describe their relevance to mechanisms of arrhythmogenesis under different disease conditions, and their link to electrocardiographic characteristics of alternans observed in patients. Our major conclusion is that alternans is not only a predictor, but also a causal mechanism of potentially lethal ventricular and atrial arrhythmias across the full spectrum of arrhythmia mechanisms that culminate in functional reentry, although less important for anatomic reentry and focal arrhythmias.

Aging Disrupts Normal Time-of-Day Variation in Cardiac Electrophysiology

BACKGROUND

Cardiac gene expression and arrhythmia occurrence have time-of-day variation; however, daily changes in cardiac electrophysiology, arrhythmia susceptibility, and Ca²⁺ handling have not been characterized. Furthermore, how these patterns change with age is unknown.

METHODS

Hearts were isolated during the light (zeitgeber time [ZT] 4 and ZT9) and dark cycle (ZT14 and ZT21) from adult (12-18 weeks) male mice. Hearts from aged (18-20 months) male mice were isolated at ZT4 and ZT14. All hearts were Langendorff-perfused for optical mapping with voltage- and Ca²⁺-sensitive dyes (n=4-7/group). Cardiac gene and protein expression were assessed with real-time polymerase chain reaction (n=4-6/group) and Western blot (n=3-4/group).

RESULTS

Adult hearts had the shortest action potential duration (APD) and Ca²⁺ transient duration (CaTD) at ZT14 (APD₈₀: ZT4: 45.4±4.1 ms; ZT9: 45.1±8.6 ms; ZT14: 34.7±4.2 ms; ZT21: 49.2±7.6 ms, P<0.05 versus ZT4 and ZT21; and CaTD₈₀: ZT4: 70.1±3.3 ms; ZT9: 72.7±2.7 ms; ZT14: 64.3±3.3 ms; ZT21: 74.4±1.2 ms, P<0.05 versus other time points). The pacing frequency at which CaT alternans emerged was faster, and average CaT alternans magnitude was significantly reduced at ZT14 compared with the other time points. There was a trend for decreased spontaneous premature ventricular complexes and pacing-induced ventricular arrhythmias at ZT14, and the hearts at ZT14 had diminished responses to isoproterenol compared with ZT4 (ZT4: 49.5.0±5.6% versus ZT14: 22.7±9.5% decrease in APD, P<0.01). In contrast, aged hearts exhibited no difference between ZT14 and ZT4 in nearly every parameter assessed (except APD₈₀: ZT4: 39.7±1.9 ms versus ZT14: 33.8±3.1 ms, P<0.01). Gene expression of KCNA5 (potassium voltage-gated channel subfamily A member 5; encoding Kv1.5) was increased, whereas gene expression of ADRB1 (encoding β1-adrenergic receptors) was decreased at ZT14 versus ZT4 in adult hearts. No time-of-day changes in expression or phosphorylation of Ca²⁺ handling proteins (SERCA2 [sarco/endoplasmic reticulum Ca²⁺-ATPase], RyR2 [ryanodine receptor 2], and PLB [phospholamban]) was found in ex vivo perfused adult isolated hearts.

CONCLUSIONS

Isolated adult hearts have strong time-of-day variation in cardiac electrophysiology, Ca²⁺ handling, and adrenergic responsiveness, which is disrupted with age.

Complexities in cardiovascular rhythmicity: perspectives on circadian normality, ageing and disease

Biological rhythms exist in organisms at all levels of complexity, in most organs and at myriad time scales. Our own biological rhythms are driven by energy emitted by the sun, interacting via our retinas with brain stem centres, which then send out complex messages designed to synchronize the behaviour of peripheral non-light sensing organs, to ensure optimal physiological responsiveness and performance of the organism based on the time of day. Peripheral organs themselves have autonomous rhythmic behaviours that can act independently from central nervous system control but is entrainable. Dysregulation of biological rhythms either through environment or disease has far-reaching consequences on health that we are only now beginning to appreciate. In this review, we focus on cardiovascular rhythms in health, with ageing and under disease conditions.

Head-up tilt test induces T-wave alternans in long QT syndrome with KCNQ1 gene mutation: Case report CARE-compliant article

INTRODUCTION

Long QT syndrome (LQTS) is a congenital disorder characterized by a prolongation of the QT interval on electrocardiograms (ECGs) and a propensity to ventricular tachyarrhythmias, which may lead to syncope, cardiac arrest, or sudden death. T-wave alternans (TWA) refers to the periodic beat-to-beat alternation of T-wave shape, polarity and amplitude on surface ECG during regular heart rhythm. In this report, a case of long QT syndrome with KCNQ1 gene mutation induced TWA in the head-up tilt test (HUTT), which has not been reported yet.

PATIENT CONCERNS

A 6-year-old boy presented with loss of consciousness twice, 5 months in duration. The boy's ECG showed prolonged QT interval (QTc = 600 ms, QTc = QT/RR). During HUTT test, QT interval was significantly prolonged (QTc = 716 ms) based on macroscopic TWA.

DIAGNOSIS

The patient was diagnosed with 1. Long QT syndrome type 1(LQT1); 2. Vasovagal syncope (VVS) INTERVENTIONS:: Metoprolol 12.5 mg was given orally twice a day. The child was told avoid standing for a long time and strenuous exercises.

OUTCOMES

There was no syncope or arrhythmia occurred during hospitalization and follow-up for 1 year.

CONCLUSIONS

VVS may exist in patients with long QT syndrome. Increased sympathetic tone during the early stage of HUTT may induce macroscopic TWA in long QT syndrome with KCNQ1 gene mutation.

Spatially Discordant Repolarization Alternans in the Absence of Conduction Velocity Restitution

Spatially discordant alternans (SDA) of action potential duration (APD) has been widely observed in cardiac tissue and is linked to cardiac arrhythmogenesis. Theoretical studies have shown that conduction velocity restitution (CVR) is required for the formation of SDA. However, this theory is not completely supported by experiments, indicating that other mechanisms may exist. In this study, we carried out computer simulations using mathematical models of action potentials to investigate the mechanisms of SDA in cardiac tissue. We show that when CVR is present and engaged, such as fast pacing from one side of the tissue, the spatial pattern of APD in the tissue undergoes either spatially concordant alternans or SDA, independent of initial conditions or tissue heterogeneities. When CVR is not engaged, such as simultaneous pacing of the whole tissue or under normal/slow heart rates, the spatial pattern of APD in the tissue can have multiple solutions, including spatially concordant alternans and different SDA patterns, depending on heterogeneous initial conditions or pre-existing repolarization heterogeneities. In homogeneous tissue, curved nodal lines are not stable, which either evolve into straight lines or disappear. However, in heterogeneous itssue, curved nodal lines can be stable, depending on their initial locations and shapes relative to the structure of the heterogeneity. Therefore, CVR-induced SDA and non-CVR-induced SDA exhibit different dynamical properties, which may be responsible for the different SDA properties observed in experimental studies and arrhythmogenesis in different clinical settings.

Mouse Models of SCN5A-Related Cardiac Arrhythmias

Mutations of SCN5A gene, which encodes the α-subunit of the voltage-gated Na⁺ channel Na_V1.5, underlie hereditary cardiac arrhythmic syndromes such as the type 3 long QT syndrome, cardiac conduction diseases, the Brugada syndrome, the sick sinus syndrome, a trial standstill, and numerous overlap syndromes. Patch-clamp studies in heterologous expression systems have provided important information to understand the genotype-phenotype relationships of these diseases. However, they could not clarify how SCN5A mutations can be responsible for such a large spectrum of diseases, for the late age of onset or the progressiveness of some of these diseases and for the overlapping syndromes. Genetically modified mice rapidly appeared as promising tools for understanding the pathophysiological mechanisms of cardiac SCN5A-related arrhythmic syndromes and several mouse models have been established. This review presents the results obtained on these models that, for most of them, recapitulate the clinical phenotypes of the patients. This includes two models knocked out for Nav1.5 β1 and β3 auxiliary subunits that are also discussed. Despite their own limitations that we point out, the mouse models still appear as powerful tools to elucidate the pathophysiological mechanisms of SCN5A-related diseases and offer the opportunity to investigate the secondary cellular consequences of SCN5A mutations such as the expression remodeling of other genes. This points out the potential role of these genes in the overall human phenotype. Finally, they constitute useful tools for addressing the role of genetic and environmental modifiers on cardiac electrical activity.

Circadian rhythms and cardiovascular health

The functional organization of the cardiovascular system shows clear circadian rhythmicity. These and other circadian rhythms at all levels of organization are orchestrated by a central biological clock, the suprachiasmatic nuclei of the hypothalamus. Preservation of the normal circadian time structure from the level of the cardiomyocyte to the organ system appears to be essential for cardiovascular health and cardiovascular disease prevention. Myocardial ischemia, acute myocardial infarct, and sudden cardiac death are much greater in incidence than expected in the morning. Moreover, supraventricular and ventricular cardiac arrhythmias of various types show specific day-night patterns, with atrial arrhythmias--premature beats, tachycardias, atrial fibrillation, and flutter - generally being of higher frequency during the day than night--and ventricular fibrillation and ventricular premature beats more common, respectively, in the morning and during the daytime activity than sleep span. Furthermore, different circadian patterns of blood pressure are found in arterial hypertension, in relation to different cardiovascular morbidity and mortality risk. Such temporal patterns result from circadian periodicity in pathophysiological mechanisms that give rise to predictable-in-time differences in susceptibility-resistance to cyclic environmental stressors that trigger these clinical events. Circadian rhythms also may affect the pharmacokinetics and pharmacodynamics of cardiovascular and other medications. Knowledge of 24-h patterns in the risk of cardiac arrhythmias and cardiovascular disease morbidity and mortality plus circadian rhythm-dependencies of underlying pathophysiologic mechanisms suggests the requirement for preventive and therapeutic interventions is not the same throughout the day and night, and should be tailored accordingly to improve outcomes.

Nicorandil normalizes prolonged repolarisation in the first transgenic rabbit model with Long-QT syndrome 1 both in vitro and in vivo

Transgenic rabbits expressing loss-of-function pore mutants of the human gene KCNQ1 (K(v)LQT1-Y315S) have a Long QT-Syndrome 1 (LQT1) phenotype. We evaluated for the first time the effect of nicorandil, an opener of ATP-sensitive potassium channels, and of isoproterenol on cardiac action potential duration and heart rate dependent dispersion of repolarisation in transgenic LQT1 rabbits. In vivo LQT1 and littermate control were subjected to transvenous electrophysiological studies; in vitro monophasic action potentials were recorded from explanted Langendorff-perfused hearts. In vivo ventricular effective refractory periods (VERP) at the right ventricular base were significantly prolonged in LQT1 as compared to littermate control, resulting in a more pronounced VERP dispersion in LQT1. This difference in VERP dispersion between LQT1 and littermate control disappeared after infusion of nicorandil. In vitro, mean action potential durations (APD(75) and APD(90)) of LQT1 were significantly prolonged compared to littermate control at baseline. Nicorandil decreased APD(75) and APD(90) in LQT1 and littermate control at all stimulated heart rates. After adding nicorandil, the APD(90) at all hearts rates and the APD(75) at high heart rates were no longer different. Dispersion of repolarisation (∆APD(75) and ∆APD(90)) was heart rate dependently decreased after nicorandil at all tested stimulation cycle lengths only in LQT1. We demonstrated phenotypic differences of LQT1 and littermate control in vivo and in vitro. Nicorandil 20μmol/l improved repolarisation abnormalities and heterogeneities in transgenic LQT1 rabbits.

Mouse models of SCN5A-related cardiac arrhythmias

Both gain- and loss-of-function mutations in the SCN5A gene, which encodes the alpha-subunit of the cardiac voltage-gated Na+ channel Na(v)1.5, are well established to underlie hereditary arrhythmic syndromes (cardiac channelopathies) such as the type 3 long QT syndrome, cardiac conduction diseases, Brugada syndrome, sick sinus syndrome, atrial standstill and numerous overlap syndromes. Although patch-clamp studies in heterologous expression systems have provided important information to understand the genotype-phenotype relationships of these diseases, they could not clarify how mutations can be responsible for such a large spectrum of diseases, the late age of onset or the progressiveness of some of them, and for the overlapping syndromes. Genetically modified mice rapidly appeared as promising tools for understanding the pathophysiological sequence of cardiac SCN5A-related channelopathies and several mouse models have been established. Here, we review the results obtained on these models that, for most of them, convincingly recapitulate the clinical phenotypes of the patients but that also have their own limitations. Mouse models turn out to be powerful tools to elucidate the pathophysiological mechanisms of SCN5A-related diseases and offer the opportunity to investigate the cellular consequences of SCN5A mutations such as the remodelling of other gene expression that might participate in the overall phenotype and explain some of the differences among patients. Finally, they also constitute useful tools for future studies addressing as yet unanswered questions, such as the role of genetic and environmental modifiers on cardiac conduction and repolarisation.

Circadian rhythms in cardiac arrhythmias and opportunities for their chronotherapy

It is now well established that nearly all functions of the body, including those that influence the pharmacokinetics and pharmacodynamics of medications, exhibit significant 24-hour variation. The electrical properties of the heart as well as cardiac arrhythmias also vary as circadian rhythms, even though the suboptimal methods initially used for their investigation slowed their identification and thorough characterization. The application of continuous Holter monitoring of the electrical properties of the heart has revealed 24-hour variation in the occurrence of ventricular premature beats with the peak in events, in diurnally active persons, between 6 a.m. and noon. After the introduction of implantable cardioverter-defibrillators, ventricular tachycardia or fibrillation were also found to peak in the same period of the day. Even defibrillator energy requirements show circadian variation, thus supporting the need for a temporal awareness in the therapeutic approach to arrhythmias. Imbalanced autonomic tone, circulating levels of catecholamines, increased heart rate and blood pressure, all established determinants of cardiac arrhythmias, show circadian variations and underlie the genesis of the circadian pattern of cardiac arrhythmias. Arrhythmogenesis appears to be suppressed during nighttime sleep, and this can influence the evaluation of the efficacy of antiarrhythmic medications in relation to their administration time. Unfortunately, very few studies have been undertaken to assess the proper timing (chronotherapy) of antiarrhythmic medications as means to maximize efficacy and possibly reduce side effects. Further research in this field is warranted and could bring new insight and clinical advantage.

Characterization of the cardiac sodium channel SCN5A mutation, N1325S, in single murine ventricular myocytes

The N(1325)S mutation in the cardiac sodium channel gene SCN5A causes the type-3 long-QT syndrome but the arrhythmogenic trigger associated with N(1325)S has not been characterized. In this study, we investigated the triggers for cardiac events in the expanded N(1325)S family. Among 11 symptomatic patients with document triggers, six died suddenly during sleep or while sitting (bradycardia-induced trigger), three died suddenly, and two developed syncope due to stress and excitement (non-bradycardia-induced). Patch-clamping studies revealed that the late sodium current (I(Na,L)) generated by mutation N(1325)S in ventricular myocytes from TG-NS/LQT3 mice was reduced with increased pacing, which explains bradycardia-induced mortalities in the family. The non-bradycardic triggers are related to the finding that APD became prolonged and unstable at increasing rates, often with alternating repolarization phases which was corrected with verapamil. This implies that Ca2+ influx and intracellular Ca2+ ([Ca2+]i) ions are involved and that [Ca2+]i inhomogeneity may be the underlying mechanisms behind non-bradycardia LQT3 arrhythmogenesis associated with mutation N(1325)S.

T-wave alternans and the susceptibility to ventricular arrhythmias

T-wave alternans (TWA) reflects beat-to-beat fluctuations in the electrocardiographic T-wave, and is associated with dispersion of repolarization and the mechanisms for sudden cardiac arrest (SCA). This review examines the bench-to-bedside literature that, over decades, has linked alternans of repolarization in cellular, whole-heart, and human studies with spatial dispersion of repolarization, alternans of cellular action potential, and fluctuations in ionic currents that may lead to ventricular arrhythmias. Collectively, these studies provide a foundation for the clinical use of TWA to reflect susceptibility to ventricular arrhythmias in several disease states. This review then provides a contemporary evidence-based framework for the use of TWA to enhance risk stratification for SCA, identifying populations for whom TWA is best established, those for whom further studies are required, and areas for additional investigation.

Provocation of sudden heart rate oscillation with adenosine exposes abnormal QT responses in patients with long QT syndrome: a bedside test for diagnosing long QT syndrome

AIMS

As arrhythmias in the long QT syndrome (LQTS) are triggered by heart rate deceleration or acceleration, we speculated that the sudden bradycardia and subsequent tachycardia that follow adenosine injection would unravel QT changes of diagnostic value in patients with LQTS.

METHODS AND RESULTS

Patients (18 LQTS and 20 controls) received intravenous adenosine during sinus rhythm. Adenosine was injected at incremental doses until atrioventricular block or sinus pauses lasting 3 s occurred. The QT duration and morphology were studied at baseline and at the time of maximal bradycardia and subsequent tachycardia. Despite similar degree of adenosine-induced bradycardia (longest R-R 1.7+/-0.7 vs. 2.2+/-1.3 s for LQTS and controls, P=NS), the QT interval of LQT patients increased by 15.8+/-13.1%, whereas the QT of controls increased by only 1.5+/-6.7% (P<0.001). Similarly, despite similar reflex tachycardia (shortest R-R 0.58+/-0.07 vs. 0.55+/-0.07 s for LQT patients and controls, P=NS), LQTS patients developed greater QT prolongation (QTc=569+/-53 vs. 458+/-58 ms for LQT patients and controls, P<0.001). The best discriminator was the QTc during maximal bradycardia. Notched T-waves were observed in 72% of LQT patients but in only 5% of controls during adenosine-induced bradycardia (P<0.001).

CONCLUSION

By provoking transient bradycardia followed by sinus tachycardia, this adenosine challenge test triggers QT changes that appear to be useful in distinguishing patients with LQTS from healthy controls.

Noninvasive sudden death risk stratification by ambulatory ECG-based T-wave alternans analysis: evidence and methodological guidelines

Extensive experimental and clinical evidence supports the utility of T-wave alternans (TWA) as a marker of risk for ventricular fibrillation. This entity appears to reflect the fundamental arrhythmogenic property of enhanced dispersion of repolarization. This relationship probably accounts for its relative ubiquity in patients with diverse types of cardiac disease, as has been recognized with the development of analytical tools. A basic premise of this review is that ambulatory ECG monitoring of TWA as patients experience the provocative stimuli of daily activities can expose latent electrical instability in individuals at heightened risk for arrhythmias. We will discuss the literature that supports this concept and summarize the current state of knowledge regarding the use of routine ambulatory ECGs to evaluate TWA for arrhythmia risk stratification. The dynamic, nonspectral modified moving average analysis method for assessing TWA, which is compatible with ambulatory ECG monitoring, is described along with methodological guidelines for its implementation. Finally, the rationale for combined monitoring of autonomic markers along with TWA will be presented.

Ambulatory ECG monitoring of T-Wave alternans for arrhythmia risk assessment

Experimental and clinical studies indicate a basic linkage between T-wave alternans (TWA) and susceptibility to malignant arrhythmias. In a variety of clinical populations with elevated risk of ventricular tachyarrhythmias, Fast Fourier Transform (FFT)-based assessment of TWA during fixed-rate atrial pacing or bicycle ergometry has shown predictive ability for arrhythmic events. However, after more than a decade since the introduction of TWA testing in human subjects, few studies have explored its utility in ambulatory ECG (AECG) recordings. This gap probably relates to major technical obstacles associated with monitoring of ambulatory subjects, including motion artifact and the requirement of data stationarity, which mandates fixing heart rate. To circumvent these difficulties, we devised a time-domain method, "Modified Moving Average Beat Analysis" (MMA) to determine TWA level accurately in freely moving subjects. Recently, MMA analysis was employed to analyze ambulatory ECG (AECG) records of post-myocardial infarction patients who were were at low risk of arrhythmic death. An increased risk of arrhythmic death was predicted by TWA level above the 75th percentile of controls (p<.05). Thus, the predictive power of TWA obtained with MMA analysis from AECG records obtained appears promising.

Catecholamine-provoked microvoltage T wave alternans in genotyped long QT syndrome

Macrovoltage T wave alternans (TWA) has been described in congenital long QT syndrome (LQTS). Microvoltage T wave alternans (microV-TWA) at low heart rate (HR) is a marker of arrhythmogenic risk in many conditions, but its significance in LQTS has not been established. Twenty-three genotypically heterogeneous patients with LQTS and 16 control subjects were studied at rest and during phenylephrine and dobutamine provocation. Genotyping was established by PCR amplification and DNA sequencing of the three most common LQTS genes; KCNQ1/KVLQT1 (LQT1), KCNH2/HERG (LQT2), and SCN5A (LQT3). microV-TWA was determined using Fast Fourier transform. Precluded by ectopy, microV-TWA could not be assessed in 8 of 23 patients with LQTS. In the remaining 15 patients with LQTS, microV-TWA occurred at lower HR in LQTS than in controls (117 +/- 49 vs 153 +/- 37 beats/min; P < 0.05). Patients with LQTS developed microV-TWA at HR < 150 beats/min more often than controls (10/15 vs 2/16; P = 0.003). However, microV-TWA was not detected in the 3 individuals with a history of out-of-hospital cardiac arrest including a 14-year-old male with an F339del-KVLQT1 mutation (LQT1) who had dobutamine-provoked polymorphic ventricular tachycardia requiring external defibrillation. Catecholamine-provoked microV-TWA occurs at lower HR in patients with LQTS than in healthy people but does not identify high risk subjects.

Circadian rhythm in the cardiovascular system: chronocardiology

BACKGROUND

We reviewed recent progress in the study of the chronobiological aspects of the cardiovascular system.

METHODS

Medline was used as the main search tool, and the full texts of selected papers were obtained.

RESULTS

More than 300 references were found, and 52 of them, representing the major findings in this field, were included in the reference list. Results of these studies confirm that most cardiovascular physiological parameters (such as heart rate, blood pressure, electrocardiogram indices) and pathophysiological events (myocardial ischemia/infarction, sudden cardiac death) show circadian rhythms. Results also suggest that consideration of these rhythms is important for the diagnosis and treatment of cardiovascular disorders and that restoration of normal circadian rhythms may be associated with clinical improvement.

CONCLUSION

The study of circadian rhythms in the cardiovascular system is emerging as an important area of investigation because of its potential implications for patient management.

Catecholamine-induced T-wave lability in congenital long QT syndrome: a novel phenomenon associated with syncope and cardiac arrest

OBJECTIVE

To determine the effects of phenylephrine and dobutamine on repolarization lability in patients with genotyped long QT syndrome (LQTS).

PATIENTS AND METHODS

Between December 1998 and August 2000, 23 patients with genotyped LQTS (13 LQT1, 7 LQT2, and 3 LQT3) and 16 controls underwent electrocardiographic stress testing at the Mayo Clinic in Rochester, Minn. Aperiodic repolarization lability was quantified from digitized electrocardiograms recorded during catecholamine stress testing with phenylephrine and dobutamine. T-wave lability was quantified as a root-mean-square of the differences between corresponding signal values of subsequent beats. The magnitude of aperiodic T-wave lability was quantified by using a newly derived T-wave lability index (TWLI).

RESULTS

The TWLI was significantly greater in patients with LQTS than in controls (0.0945 +/- 0.0517 vs 0.0445 +/- 0.0123; P < .003). Marked T-wave lability (TWLI > or = 0.095) was detected in all 3 LQTS genotypes (10/23) but in no controls (P < .003). There was no correlation between the TWLI and the baseline corrected QT interval. All high-risk patients having either a history of out-of-hospital cardiac arrest or syncope had a TWLI of 0.095 or greater.

CONCLUSIONS

Beat-to-beat nonalternating T-wave lability occurs in LQT1, LQT2, and LQT3 patients during catecholamine provocation and is associated with a history of prior cardiac events. The quantification of this novel phenomenon may assist in identifying LQTS patients with increased risk of sudden cardiac death.