Molecular aspects of arrhythmias associated with cardiomyopathies

Management of ventricular arrhythmias in heart failure: Current perspectives

Congestive heart failure (HF) is a progressive affliction defined as the inability of the heart to sufficiently maintain blood flow. Ventricular arrhythmias (VAs) are common in patients with HF, and conversely, advanced HF promotes the risk of VAs. Management of VA in HF requires a systematic, multimodality approach that comprises optimization of medical therapy and use of implantable cardioverter-defibrillator and/or device combined with cardiac resynchronization therapy. Catheter ablation is one of the most important strategies with the potential to abolish or decrease the number of recurrences of VA in this population. It can be a curative strategy in arrhythmia-induced cardiomyopathy and may even save lives in cases of an electrical storm. Additionally, modulation of the autonomic nervous system and stereotactic radiotherapy have been introduced as novel methods to control refractory VAs. In patients with end-stage HF and refractory VAs, an institution of the mechanical circulatory support device and cardiac transplant may be considered. This review aims to provide an overview of current evidence regarding management strategies of VAs in HF with an emphasis on interventional treatment.

Antiarrhythmic Action ofβ-Blockers: Potential Mechanisms

Sympathetic nervous system overactivity has been linked to ventricular tachyarrhythmias and sudden death. It has been hypothesized that the extent and nature of the arrhythmogenic effect of sympathetic stimulation depends on the underlying myocardial substrate, the mechanism of the arrhythmia, and the integrated effects of sympathetic stimulation in the particular individual circumstance. Multiple direct and indirect mechanisms of adrenergic action on the heart may benefit from the known antiarrhythmic actions of β-blocker therapy and other interventions that decrease sympathetic tone. The antiarrhythmic mechanism of β-blockade (and possibly α-blockade) will depend on the specific mechanism of the individual arrhythmia and will differ for those arrhythmias caused by tachycardia and ischemia, those caused by reentry and promoted by decreased conduction velocity and shortened refractoriness, and those caused by early or delayed afterdepolarizations, usually in the context of prolonged action potential duration. Antagonism of cardiac adrenergic activity by β-blockade in particular is the best-established drug therapy to prevent ventricular arrhythmias.

Cardionomics: a new integrative approach for screening cardiotoxicity of drug candidates

BACKGROUND

Despite the FDA guidelines for studies to be performed to rule out potential cardiac toxicity, many drugs have nevertheless entered the market only to be later withdrawn from the market owing to cardiac toxicity. Cardiac toxicity may result from drugs causing impaired function or death of cardiomyocytes, valvular damage, myocardial ischemia and/or ventricular arrhythmias. Negative cardiovascular events have been implicated in 28% of drug withdrawals in the USA. The significance for patients, regulators and the pharmaceutical industry is immense.

OBJECTIVE

We address whether a more rigorous and integrative approach is needed for cardiovascular safety screening of all new drug candidates. Furthermore, we will present a cardionomics approach that looks at several in vitro and in vivo models that can be applied to all drugs independent of category, therapeutic area or class.

METHODS

We present examples of drugs demonstrating cardiac toxicity and provide an in-depth review of how calcium homeostasis may be a unifying theme in clinically observed cardiotoxic events. We introduce a cardionomics approach that detects clinical cardiac toxicity early in the drug discovery process, thus, preventing costly late attrition.

CONCLUSION

The consequences of a failure to detect potential cardiovascular safety issues before clinical launch can have an enormous cost for the pharmaceutical industry, when major drugs are withdrawn due to lawsuits as well as loss of time and resources. An integrated cardionomics approach may reduce the risk of drug withdrawals as a result of unexpected clinical cardiac safety issues.

Prevention of ventricular arrhythmias with sarcoplasmic reticulum Ca2+ ATPase pump overexpression in a porcine model of ischemia reperfusion

BACKGROUND

Ventricular arrhythmias are life-threatening complications of heart failure and myocardial ischemia. Increased diastolic Ca2+ overload occurring in ischemia leads to afterdepolarizations and aftercontractions that are responsible for cellular electric instability. We inquired whether sarcoplasmic reticulum Ca2+ ATPase pump (SERCA2a) overexpression could reduce ischemic ventricular arrhythmias by modulating Ca2+ overload.

METHODS AND RESULTS

SERCA2a overexpression in pig hearts was achieved by intracoronary gene delivery of adenovirus in the 3 main coronary arteries. Homogeneous distribution of the gene was obtained through the left ventricle. After gene delivery, the left anterior descending coronary artery was occluded for 30 minutes to induce myocardial ischemia followed by reperfusion. We compared this model with a model of permanent coronary artery occlusion. Twenty-four-hour ECG Holter recordings showed that SERCA2a overexpression significantly reduced the number of episodes of ventricular tachycardia after reperfusion, whereas no significant difference was found in the occurrence of sustained or nonsustained ventricular tachycardia and ventricular fibrillation in pigs undergoing permanent occlusion.

CONCLUSIONS

We show that Ca2+ cycling modulation using SERCA2a overexpression reduces ventricular arrhythmias after ischemia-reperfusion. Strategies that modulate postischemic Ca2+ overload may have clinical promise for the treatment of ventricular arrhythmias.

NF-kappaB-dependent transcriptional regulation of the cardiac scn5a sodium channel by angiotensin II

Angiotensin II (ANG II) increases oxidative stress and is associated with increased risk of sudden cardiac death. The cardiac Na(+) channel promoter contains elements that confer redox sensitivity. We tested the hypothesis that ANG II-mediated oxidative stress may modulate Na(+) channel current through altering channel transcription. In H9c2 myocytes treated for 48 h with ANG II (100 nmol/l) or H(2)O(2) (10 micromol/l) showed delayed macroscopic inactivation, increased late current, and 59.6% and 53.8% reductions in Na(+) current, respectively (P < or = 0.01). By quantitative real-time RT-PCR, the cardiac Na(+) channel (scn5a) mRNA abundance declined by 47.3% (P < 0.01) in H9c2 myocytes treated for 48 h with 100 nmol/l ANG II. A similar change occurred with 20 micromol/l H(2)O(2) (46.9%, P < 0.01) after 48 h. Comparable effects were seen in acutely isolated ventricular myocytes. The effects of ANG II could be inhibited by prior treatment of H9c2 cells with scavengers of reactive oxygen species or an inhibitor of the NADPH oxidase. Mutation of the scn5a promoter NF-kappaB binding site prevented decreased activity in response to ANG II and H(2)O(2). Gel shift and chromosomal immunoprecipitation assays confirmed that nuclear factor (NF)-kappaB bound to the scn5a promoter in response to ANG II and H(2)O(2). Overexpression of the p50 subunit of NF-kappaB in H9c2 cells reduced scn5a mRNA (77.3%, P < 0.01). In conclusion, ANG II can decrease scn5a transcription and current. This effect appears to be through production of H(2)O(2) resulting in NF-kappaB binding to the Na(+) channel promoter.

Cardiac-restricted angiotensin-converting enzyme overexpression causes conduction defects and connexin dysregulation

Renin-angiotensin (RAS) system activation is associated with an increased risk of sudden death. Previously, we used cardiac-restricted angiotensin-converting enzyme (ACE) overexpression to construct a mouse model of RAS activation. These ACE 8/8 mice die prematurely and abruptly. Here, we have investigated cardiac electrophysiological abnormalities that may contribute to early mortality in this model. In ACE 8/8 mice, surface ECG voltages are reduced. Intracardiac electrograms showed atrial and ventricular potential amplitudes of 11% and 24% compared with matched wild-type (WT) controls. The atrioventricular (AV), atrio-Hisian (AH), and Hisian-ventricular (HV) intervals were prolonged 2.8-, 2.6-, and 3.9-fold, respectively, in ACE 8/8 vs. WT mice. Various degrees of AV nodal block were present only in ACE 8/8 mice. Intracardiac electrophysiology studies demonstrated that WT and heterozygote (HZ) mice were noninducible, whereas 83% of ACE 8/8 mice demonstrated ventricular tachycardia with burst pacing. Atrial connexin 40 (Cx40) and connexin 43 (Cx43) protein levels, ventricular Cx43 protein level, atrial and ventricular Cx40 mRNA abundances, ventricular Cx43 mRNA abundance, and atrial and ventricular cardiac Na(+) channel (Scn5a) mRNA abundances were reduced in ACE 8/8 compared with WT mice. ACE 8/8 mice demonstrated ventricular Cx43 dephosphorylation. Atrial and ventricular L-type Ca(2+) channel, Kv4.2 K(+) channel alpha-subunit, and Cx45 mRNA abundances and the peak ventricular Na(+) current did not differ between the groups. In isolated heart preparations, a connexin blocker, 1-heptanol (0.5 mM), produced an electrophysiological phenotype similar to that seen in ACE 8/8 mice. Therefore, cardiac-specific ACE overexpression resulted in changes in connexins consistent with the phenotype of low-voltage electrical activity, conduction defects, and induced ventricular arrhythmia. These results may help explain the increased risk of arrhythmia in states of RAS activation such as heart failure.

Reducing the Risk of Sudden Death in Heart Failure With β-Blockers

BACKGROUND

Heart failure (HF) is a serious cardiovascular syndrome that affects nearly 5 million people in the United States. A review of clinical data demonstrates that sudden cardiac death (SCD) accounts for approximately one-third of all HF deaths. This fatal outcome typically involves an unexpected electrical event leading to sustained cardiac arrhythmias resulting in cardiovascular collapse.

METHODS AND RESULTS

A systematic review of the literature was performed to serve as the basis for this review. Factors contributing directly to incidence of SCD in the HF population may include significant remodeling of the left ventricle (hypertrophy, dilation, and fibrosis) in addition to increased sympathetic activation. Using specific therapies to limit these mechanisms are beneficial in the HF patient by preventing SCD. Beta-blockers play a key role in the prevention of SCD for patients with HF by limiting the effects of circulating norepinephrine and by reducing left ventricular remodeling.

CONCLUSIONS

This review outlines the potential mechanisms and contributing factors of SCD in patients with HF and the impact of beta-blocker usage in the prevention of this fatal outcome for this growing patient population.

Transient-outward K+ channel inhibition facilitates L-type Ca2+ current in heart

BACKGROUND

Transient outward current (I(to)) and L-type calcium current (I(Ca)) are important repolarization currents in cardiac myocytes. These two currents often undergo disease-related remodeling while other currents are spared, suggesting a functional coupling between them. Here, we investigated the effects of I(to) channel blockers, 4-aminopyridine (4-AP) and heteropodatoxin-2 (HpTx2), on I(Ca) in cardiac ventricular myocytes.

METHODS AND RESULTS

I(Ca) was recorded in enzymatically dissociated mouse and guinea pig ventricular myocytes using the whole-cell voltage clamp method. In mouse ventricular myocytes, 4-AP (2 mM) significantly facilitated I(Ca) by increasing current amplitude and slowing inactivation. These effects were not voltage-dependent. Similar facilitating effects were seen when equimolar Ba2+ was substituted for external Ca2+, indicating that Ca2+ influx is not required. Measurements of Ca2+/calmodulin-dependent protein kinase (CaMKII) activity revealed significant increases in cells treated with 4-AP. Pretreatment of cells with 10 microM KN93, a specific inhibitor of CaMKII, abolished the effects of 4-AP on I(Ca.) To test the requirement of I(to), we studied guinea pig ventricular myocytes, which do not express I(to) channels. In these cells, 2 mM 4-AP had no effect on I(Ca) amplitude or kinetics. In both cell types, Ca2+-induced I(Ca) facilitation, a CaMKII-dependent process, was observed. However, 4-AP abolished Ca2+-induced I(Ca) facilitation exclusively in mouse ventricular myocytes.

CONCLUSION

4-AP, an I(to) blocker, facilitates L-type Ca2+ current through a mechanism involving the I(to) channel and CaMKII activation. These data indicate a functional association of I(Ca) and I(to) in cardiac myocytes.

Atorvastatin Therapy Increases Heart Rate Variability, Decreases QT Variability, and Shortens QTc Interval Duration in Patients With Advanced Chronic Heart Failure

BACKGROUND

Although statins decrease the incidence of ventricular arrhythmias in patients with atherosclerotic heart disease, their potential antiarrhythmic effects in heart failure remain undefined.

METHODS AND RESULTS

Of 80 heart failure patients enrolled, 40 were randomized to receive atorvastatin (statin group); the remaining 40 served as controls. At baseline and after 3 months, we measured heart rate variability (HRV), QT variability (QTV), and QTc interval using interactive high-resolution electrocardiogram analysis. The 2 groups did not differ in baseline HRV standard deviation of normal-to-normal intervals (SDNN) (RR): 24.6 +/- 2.8 ms in statin group versus 24.8 +/- 3.1 ms in controls, P = .72; square root of the mean of squared differences between successive intervals (rMSSD) (RR): 21.2 +/- 2.7 ms versus 21.7 +/- 2.9 ms, P = .43), QTV SDNN (QT): 6.4 +/- 1.5 ms versus 6.4+/-1.7, P = .96; rMSSD QT): 9.0 +/- 2.4 ms versus 8.7 +/- 2.9 ms, P = .65, and QTc interval 450 +/- 30 ms versus 446 +/- 27 ms, P = .59. At 3 months, the statin group displayed higher HRV SDNN RR): 27.2 +/- 4.9 ms versus 24.4 +/- 2.8 ms in controls, P = .003; rMSSD RR: 24.7 +/- 4.2 ms versus 21.3 +/- 5.6 ms, P = .004, lower QTV SDNN (QT): 5.1 +/- 1.9 ms versus 6.5 +/- 2.1, P = .004; rMSSD (QT): 6.6 +/- 2.8 ms versus 8.8 +/- 3.1 ms, P = .002, and shorter QTc interval 437 +/- 29 ms versus 450 +/- 25 ms, P = .03 than the control group.

CONCLUSIONS

Atorvastatin increases HRV, decreases QTV, and shortens QTc interval, and may thereby reduce the risk of arrhythmias in patients with advanced heart failure.

Atrial contractile dysfunction, fibrosis, and arrhythmias in a mouse model of cardiomyopathy secondary to cardiac-specific overexpression of tumor necrosis factor-{alpha}

Transgenic mice overexpressing the inflammatory cytokine TNF-alpha in the heart develop a progressive heart failure syndrome characterized by biventricular dilatation, decreased ejection fraction, decreased survival compared with non-transgenic littermates, and earlier pathology in males. TNF-alpha mice (TNF1.6) develop atrial arrhythmias on ambulatory telemetry monitoring that worsen with age and are more severe in males. We performed in vivo electrophysiological testing in transgenic and control mice, ex vivo optical mapping of voltage in the atria of isolated perfused TNF1.6 hearts, and in vitro studies on isolated atrial muscle and cells to study the mechanisms that lead to the spontaneous arrhythmias. Programmed stimulation induces atrial arrhythmias (n = 8/32) in TNF1.6 but not in control mice (n = 0/37), with a higher inducibility in males. In the isolated perfused hearts, programmed stimulation with single extra beats elicits reentrant atrial arrhythmias (n = 6/6) in TNF1.6 but not control hearts due to slow heterogeneous conduction of the premature beats. Lowering extracellular Ca(2+) normalizes conduction and prevents the arrhythmias. Atrial muscle and cells from TNF1.6 compared with control mice exhibit increased collagen deposition, decreased contractile function, and abnormal systolic and diastolic Ca(2+) handling. Thus abnormalities in action potential propagation and Ca(2+) handling contribute to the initiation of atrial arrhythmias in this mouse model of heart failure.

Molecular remodeling in the failing human heart

Heart failure is a progressive and fatal disease process. Arrhythmias and progressive cardiac dysfunction account for most of the morbidity and mortality in patients with heart failure. In general, the physiologic mechanisms responsible for progressive myocyte dysfunction, remodeling, and arrhythmias involve signaling mechanisms that alter myocardial gene expression. These changes in gene expression are complex and involve contractile proteins, ion channels, Ca++ handling, apoptosis, cell metabolism, the extracellular matrix, signal transduction pathways, and growth factors.

A benefit–risk assessment of class III antiarrhythmic agents

The prevalence of arrhythmia in the population is increasing as more people survive for longer with cardiovascular disease. It was once thought that antiarrhythmic therapy could save life, however, it is now evident that antiarrhythmic therapy should be administrated with the purpose of symptomatic relief. Since many patients experience a decrease in physical performance as well as a diminished quality of life during arrhythmia there is still a need for antiarrhythmic drug therapy. The development of new antiarrhythmic agents has changed the focus from class I to class III agents since it became evident that with class I drug therapy the prevalence of mortality is considerably higher. This review focuses on the benefits and risks of known and newer class III antiarrhythmic agents. The benefits discussed include the ability to maintain sinus rhythm in persistent atrial fibrillation patients, and reducing the need for implantable cardioverter defibrillator shock/antitachycardia therapy, since no class III antiarrhythmic agents have proven survival benefit. The risks discussed mainly focus on pro-arrhythmia as torsade de pointes ventricular tachycardia.

Management of arrhythmias in heart failure

Arrhythmias continue to contribute significantly to morbidity and mortality in heart failure. Implantable defibrillators have assumed an increasingly important role in preventing sudden death and are recommended for patients who have been resuscitated from cardiac arrest, have unexplained syncope, or exhibit inducible ventricular tachycardia in the setting of prior myocardial infarction. The extension of survival conferred by implantable defibrillators is likely to be limited in patients with advanced heart failure. Ongoing trials will help define the use of these devices in heart failure populations, in whom atrial fibrillation is common and rate control and anticoagulation are of major importance. Among pharmaceutical options, amiodarone and dofetilide are the major agents for maintenance of sinus rhythm. The complexity of coexistent heart failure and arrhythmia management warrants close collaboration between heart failure and arrhythmia specialists.

Significance and control of cardiac arrhythmias in patients with congestive cardiac failure

A wide spectrum of ventricular and supraventricular tachyarrhythmias occurs in the setting of congestive cardiac failure. However, the two most clinically significant are atrial fibrillation and ventricular tachycardia and fibrillation. In the past there has been much emphasis on premature ventricular contractions and more recently, on nonsustained ventricular tachycardia. For the most part, these arrhythmias are asymptomatic in heart failure. They are markers of sudden arrhythmic death but their suppression by antiarrhythmic drugs have not resulted in a reduction of total mortality. Two approaches have been used to this end. The first is the use of beta-adrenergic blocking drugs and antiarrhythmic agents such as amiodarone. Beta-blockers have been shown to significantly reduce sudden death as well as total mortality, while the effects of amiodarone have been less decisive. The prospective role of the implantable cardioverter defibrillator (ICD) is undergoing critical evaluation in patients with cardiac failure at high risk for sudden death. The elective role of the ICD is well established as first-line therapy in patients with heart failure resuscitated from sudden death and in those with sustained ventricular tachycardia in conjunction with conventional therapies for cardiac decompensation. The prevalence of atrial fibrillation rises as a function of severity of cardiac failure, but it is also in known that persistent atrial fibrillation with an uncontrolled ventricular response may induce heart failure. Controlled ventricular response may prevent congestive heart failure and improve left ventricular function. The two most common causes of atrial fibrillation in cardiac failure in Europe and America are ischemic heart disease and hypertension, while mitral valve disease remains the prevalent cause elsewhere. The choice of antiarrhythmic drugs for maintaining sinus rhythm is critical in the prevention of heart failure aggravation and proarrhythmic reactions of antiarrhythmic drugs. Amiodarone and dofetilide are most widely used in this context.

Management of arrhythmias in heart failure

Arrhythmias often complicate the management of heart failure and contribute to mortality and morbidity. Implantable cardioverter defibrillators are the best protection from death caused by ventricular arrhythmias, but their benefit will probably be less in heart failure populations than has been observed in trials that have not focused on heart failure populations. Implantable cardioverter defibrillators are first-line therapy for high-risk patients who have been resuscitated from sustained ventricular tachycardia or ventricular fibrillation, who have inducible ventricular tachycardia in the setting of previous myocardial infarction, or who have unexplained syncope. Amiodarone is the major pharmacologic option for treatment of symptomatic arrhythmias. In selecting therapy, the severity of heart failure and cause of heart failure are important considerations. Atrial fibrillation occurs with increasing frequency as the severity of heart failure increases. Anticoagulation and rate control are important. Attempted maintenance of sinus rhythm with amiodarone or dofetilide is a reasonable consideration for selected patients, although the benefit of treatment strategies that seek to maintain sinus rhythm has not been demonstrated. Ongoing trials will provide further guidance for arrhythmia management.

Implantable cardioverter defibrillators in heart failure

Implantable cardioverter defibrillators provide effective and reliable treatment of spontaneous VT and VF. These devices can be expected to decrease the risk for arrhythmic death in patients with heart failure but do not improve overall survival when death from severe pump dysfunction is imminent. Appropriate patient selection is a major aspect of arrhythmia management. Future devices will incorporate features that have the potential to reduce atrial arrhythmias, improve ventricular function, monitor hemodynamics, and prevent sudden arrhythmic death.

Electrical remodeling in pressure-overload cardiac hypertrophy: role of calcineurin

BACKGROUND

Myocyte hypertrophy accompanies many forms of heart disease, but its contribution to electrical remodeling is unknown.

METHODS AND RESULTS

We studied mouse hearts subjected to pressure overload by surgical thoracic aortic banding. In unbanded control hearts, action potential duration (APD) was significantly longer in subendocardial myocytes compared with subepicardial myocytes. Hypertrophy-associated APD prolongation was significantly greater in subendocardial myocytes compared with subepicardial myocytes, indicating stress-induced amplification of repolarization dispersion. To investigate the underlying basis, we performed voltage-clamp recordings on dissociated myocytes. Under control unoperated conditions, subendocardial myocytes exhibited significantly less transient outward current (I(to)) than did subepicardial cells. Hypertrophy was not associated with significant changes in I(to), sustained current, or inward rectifier current densities, but peak L-type Ca(2+) current density (I(Ca,L)) increased 26% (P<0.05). Recovery from I(Ca,L) inactivation was accelerated in hypertrophied myocytes. Inhibition of calcineurin with cyclosporin A prevented increases in heart mass and myocyte size but was associated with an intermediate APD. The hypertrophy-associated increase in I(Ca,L) and the accelerated recovery from inactivation were blocked by cyclosporin A.

CONCLUSIONS

These data reveal regional variation in the electrophysiological response within the left ventricle by way of a mechanism involving upregulated Ca(2+) current and calcineurin. Furthermore, these results reveal partial uncoupling of electrophysiological and structural remodeling in hypertrophy.