Atrial cellular electrophysiological changes in patients with ventricular dysfunction may predispose to AF

Failure of intravenous nifekalant cardioversion as an independent predictor for persistent atrial fibrillation recurrence after catheter ablation

AIMS

Nifekalant is a class III antiarrhythmic drug that exerts antiarrhythmic effects by inhibiting rapid rectifying potassium channels and extending the effective refractory period of cardiomyocytes. It has a high success rate in converting atrial fibrillation (AF) to sinus rhythm. Whether the failure of intravenous nifekalant cardioversion is an independent predictor for persistent AF recurrence after catheter ablation has not been reported.

METHODS

A total of 92 patients with drug-refractory persistent AF were retrospectively enrolled. After all ablations, intravenous nifekalant was administrated. Patients were assigned to the success group (group 1) and failure group (group 2) based on nifekalant cardioversion results and followed for 12 months to note any episode of atrial arrhythmia recurrence.

RESULTS

Each group included 46 patients. After 12 months of follow-up, nine (19.6%) patients from group 1 and 23 (50.0%) patients from group 2 had a recurrence of atrial tachyarrhythmia (P = 0.002). AF duration and type 2 diabetes were strongly associated with failure of intravenous nifekalant cardioversion. Univariable Cox proportional hazard regression showed that failure of intravenous nifekalant cardioversion, AF duration, and type 2 diabetes were potential risk factors. Multivariable Cox proportional hazard regression showed that failure of nifekalant cardioversion was statistically associated with AF recurrence (adjusted RR = 2.257, 95% CI: 1.006-5.066, P = 0.048). Failure of intravenous nifekalant cardioversion could bring a positive effect on the prognostic differentiation when added into the multivariable model (0.767 ± 0.042 vs. 0.774 ± 0.045, P = 0.025).

CONCLUSION

Failure of nifekalant cardioversion is an independent predictor for persistent AF recurrence after catheter ablation.

Resting membrane potential is less negative in trabeculae from right atrial appendages of women, but action potential duration does not shorten with age

AIMS

The incidence of atrial fibrillation (AF) increases with age. Women have a lower risk. Little is known on the impact of age, sex and clinical variables on action potentials (AP) recorded in right atrial tissue obtained during open heart surgery from patients in sinus rhythm (SR) and in longstanding AF. We here investigated whether age or sex have an impact on the shape of AP recorded in vitro from right atrial tissue.

METHODS

We performed multivariable analysis of individual AP data from trabeculae obtained during heart surgery of patients in SR (n = 320) or in longstanding AF (n = 201). AP were recorded by sharp microelectrodes at 37 °C at 1 Hz. Impact of clinical variables were modeled using a multivariable mixed model regression.

RESULTS

In SR, AP duration at 90% repolarization (APD₉₀) increased with age. Lower ejection fraction and higher body mass index were associated with smaller action potential amplitude (APA) and maximum upstroke velocity (V_max). The use of beta-blockers was associated with larger APD₉₀. In tissues from women, resting membrane potential was less negative and APA as well as V_max were smaller. Besides shorter APD₂₀ in elderly patients, effects of age and sex on atrial AP were lost in AF.

CONCLUSION

The higher probability to develop AF at advanced age cannot be explained by a shortening in APD₉₀. Less negative RMP and lower upstroke velocity might contribute to lower incidence of AF in women, which may be of clinical relevance.

Atrial fibrillation in the presence and absence of heart failure enhances expression of genes involved in cardiomyocyte structure, conduction properties, fibrosis, inflammation, and endothelial dysfunction

BACKGROUND

Little is known about genome-wide changes in the atrial transcriptome as a cause or consequence of atrial fibrillation (AF), and the effect of its common and clinically relevant comorbidity-heart failure (HF).

OBJECTIVE

The purpose of this study was to explore candidate disease processes for AF by investigating gene expression changes in atrial tissue samples from patients with and without AF, stratified by HF.

METHODS

RNA sequencing was performed in right and left atrial appendage tissue in 195 patients undergoing open heart surgery from centers participating in the CATCH-ME consortium (no history of AF, n = 91; paroxysmal AF, n = 53; persistent/permanent AF, n = 51). Analyses were stratified into patients with/without HF (n = 75/120) and adjusted for age, sex, atrial side, and a combination of clinical characteristics.

RESULTS

We identified 35 genes associated with persistent AF compared to patients without a history of AF, both in the presence or absence of HF (false discovery rate <0.05). These were mostly novel associations, including 13 long noncoding RNAs. Genes were involved in regulation of cardiomyocyte structure, conduction properties, fibrosis, inflammation, and endothelial dysfunction. Gene set enrichment analysis identified mainly inflammatory gene sets to be enriched in AF patients without HF, and gene sets involved in cellular respiration in AF patients with HF.

CONCLUSION

Analysis of atrial gene expression profiles identified numerous novel genes associated with persistent AF, in the presence or absence of HF. Interestingly, no consistent transcriptional changes were associated with paroxysmal AF, suggesting that AF-induced changes in gene expression predominate other changes.

Arrhythmogenic Remodeling in the Failing Heart

Chronic heart failure is a clinical syndrome with multiple etiologies, associated with significant morbidity and mortality. Cardiac arrhythmias, including ventricular tachyarrhythmias and atrial fibrillation, are common in heart failure. A number of cardiac diseases including heart failure alter the expression and regulation of ion channels and transporters leading to arrhythmogenic electrical remodeling. Myocardial hypertrophy, fibrosis and scar formation are key elements of arrhythmogenic structural remodeling in heart failure. In this article, the mechanisms responsible for increased arrhythmia susceptibility as well as the underlying changes in ion channel, transporter expression and function as well as alterations in calcium handling in heart failure are discussed. Understanding the mechanisms of arrhythmogenic remodeling is key to improving arrhythmia management and the prevention of sudden cardiac death in patients with heart failure.

Heart failure-induced atrial remodelling promotes electrical and conduction alternans

Heart failure (HF) is associated with an increased propensity for atrial fibrillation (AF), causing higher mortality than AF or HF alone. It is hypothesized that HF-induced remodelling of atrial cellular and tissue properties promotes the genesis of atrial action potential (AP) alternans and conduction alternans that perpetuate AF. However, the mechanism underlying the increased susceptibility to atrial alternans in HF remains incompletely elucidated. In this study, we investigated the effects of how HF-induced atrial cellular electrophysiological (with prolonged AP duration) and tissue structural (reduced cell-to-cell coupling caused by atrial fibrosis) remodelling can have an effect on the generation of atrial AP alternans and their conduction at the cellular and one-dimensional (1D) tissue levels. Simulation results showed that HF-induced atrial electrical remodelling prolonged AP duration, which was accompanied by an increased sarcoplasmic reticulum (SR) Ca²⁺ content and Ca²⁺ transient amplitude. Further analysis demonstrated that HF-induced atrial electrical remodelling increased susceptibility to atrial alternans mainly due to the increased sarcoplasmic reticulum Ca²⁺-ATPase (SERCA) Ca²⁺ reuptake, modulated by increased phospholamban (PLB) phosphorylation, and the decreased transient outward K⁺ current (I_to). The underlying mechanism has been suggested that the increased SR Ca²⁺ content and prolonged AP did not fully recover to their previous levels at the end of diastole, resulting in a smaller SR Ca²⁺ release and AP in the next beat. These produced Ca²⁺ transient alternans and AP alternans, and further caused AP alternans and Ca²⁺ transient alternans through Ca²⁺→AP coupling and AP→Ca²⁺ coupling, respectively. Simulation of a 1D tissue model showed that the combined action of HF-induced ion channel remodelling and a decrease in cell-to-cell coupling due to fibrosis increased the heart tissue’s susceptibility to the formation of spatially discordant alternans, resulting in an increased functional AP propagation dispersion, which is pro-arrhythmic. These findings provide insights into how HF promotes atrial arrhythmia in association with atrial alternans.

Atrial Fibrillation (AF) is the most common arrhythmia in adults, especially in the elderly, with the increased incidence of stroke being a major complication that increases morbidity and mortality. The occurrence of AF is often accompanied by heart failure (HF). AF and HF are also known to have the bidirectional relationship that AF worsens HF and HF promotes AF. HF can induce atrial remodelling, including electrical remodelling, atrial fibrosis, stretch and dilatation, and oxidative stress, in which many factors are associated with arrhythmogenic atrial alternans. HF-induced atrial remodelling varies during various stages and complications of HF, but possible mechanisms underlying their pro-susceptibility to alternans have not been completely elucidated. In this study, we investigated the effects of HF-induced atrial remodelling with prolonged action potential duration (APD) and decreased cell-to-cell coupling on susceptibility to atrial alternans. Simulation results showed that HF-induced an increase in sarcoplasmic reticulum Ca²⁺-ATPase (SERCA) Ca²⁺ reuptake caused by increased phospholamban phosphorylation and a decrease in transient outward K⁺ current played significant roles in the genesis of Ca²⁺ transient alternans and action potential alternans at the single-cell level. The HF-induced decline of cell-to-cell coupling and APD prolongation promoted the genesis of spatially discordant alternans in atrial tissue. This provides insights into how HF facilitates atrial arrhythmia in relation to atrial alternans.

Triggered Ca2+ Waves Induce Depolarization of Maximum Diastolic Potential and Action Potential Prolongation in Dog Atrial Myocytes

BACKGROUND

We have identified a novel form of abnormal Ca2+ wave activity in normal and failing dog atrial myocytes which occurs during the action potential (AP) and is absent during diastole. The goal of this study was to determine if triggered Ca2+ waves affect cellular electrophysiological properties.

METHODS

Simultaneous recordings of intracellular Ca2+ and APs allowed measurements of maximum diastolic potential and AP duration during triggered calcium waves (TCWs) in isolated dog atrial myocytes. Computer simulations then explored electrophysiological behavior arising from TCWs at the tissue scale.

RESULTS

At 3.3 to 5 Hz, TCWs occurred during the AP and often outlasted several AP cycles. Maximum diastolic potential was reduced, and AP duration was significantly prolonged during TCWs. All electrophysiological responses to TCWs were abolished by SEA0400 and ORM10103, indicating that Na-Ca exchange current caused depolarization. The time constant of recovery from inactivation of Ca2+ current was 40 to 70 ms in atrial myocytes (depending on holding potential) so this current could be responsible for AP activation during depolarization induced by TCWs. Modeling studies demonstrated that the characteristic properties of TCWs are potentially arrhythmogenic by promoting both conduction block and reentry arising from the depolarization induced by TCWs.

CONCLUSIONS

Triggered Ca2+ waves activate inward NCX and dramatically reduce atrial maximum diastolic potential and prolong AP duration, establishing the substrate for reentry which could contribute to the initiation and maintenance of atrial arrhythmias.

[The effect of angiotensin receptors and neprilysin inhibitors on myocardial remodeling in patients with chronic heart failure and atrial fibrillation]

AIM

To evaluate the effect of angiotensin-neprilysin receptor inhibitors on myocardial remodeling in patients with chronic heart failure and atrial fibrillation.

MATERIALS AND METHODS

We studied dynamics of the parameters of ultrasound structural and functional parameters of the left atrium and left ventricle of the heart was during 3-month therapy with sacubitryl-valsartan in a group of 15 patients with a combination of chronic heart failure due to dilated and paroxysmal paroxysmal forms of atrial fibrillation.

RESULTS

Showed a statistically significant positive effect of the use of angiotensin receptors and neprilysin inhibitors on the parameters of remodeling of the left atrium (according to transthoracic and transesophageal echocardiography), left ventricle, as well as levels of natriuretic peptides ANP and NT-pro-BNP.

CONCLUSION

The use of ARNI may be promising in terms of treatment and prevention of AF in patients with heart failure.

Profibrotic, Electrical, and Calcium-Handling Remodeling of the Atria in Heart Failure Patients With and Without Atrial Fibrillation

Atrial fibrillation (AF) and heart failure (HF) are common cardiovascular diseases that often co-exist. Animal models have suggested complex AF-promoting atrial structural, electrical, and Ca²⁺-handling remodeling in the setting of HF, but data in human samples are scarce, particularly regarding Ca²⁺-handling remodeling. Here, we evaluated atrial remodeling in patients with severe left ventricular (LV) dysfunction (HFrEF), long-standing persistent ('chronic') AF (cAF) or both (HFrEF-cAF), and sinus rhythm controls with normal LV function (Ctl) using western blot in right-atrial tissue, sharp-electrode action potential (AP) measurements in atrial trabeculae and voltage-clamp experiments in isolated right-atrial cardiomyocytes. Compared to Ctl, expression of profibrotic markers (collagen-1a, fibronectin, periostin) was higher in HFrEF and HFrEF-cAF patients, indicative of structural remodeling. Connexin-43 expression was reduced in HFrEF patients, but not HFrEF-cAF patients. AP characteristics were unchanged in HFrEF, but showed classical indices of electrical remodeling in cAF and HFrEF-cAF (prolonged AP duration at 20% and shorter AP duration at 50% and 90% repolarization). L-type Ca²⁺ current (I_Ca,L) was significantly reduced in HFrEF, cAF and HFrEF-cAF, without changes in voltage-dependence. Potentially proarrhythmic spontaneous transient-inward currents were significantly more frequent in HFrEF and HFrEF-cAF compared to Ctl, likely resulting from increased sarcoplasmic reticulum (SR) Ca²⁺ load (integrated caffeine-induced current) in HFrEF and increased ryanodine-receptor (RyR2) single-channel open probability in HFrEF and HFrEF-cAF. Although expression and phosphorylation of the SR Ca²⁺-ATPase type-2a (SERCA2a) regulator phospholamban were unchanged in HFrEF and HFrEF-cAF patients, protein levels of SERCA2a were increased in HFrEF-cAF and sarcolipin expression was decreased in both HFrEF and HFrEF-cAF, likely increasing SR Ca²⁺ uptake and load. RyR2 protein levels were decreased in HFrEF and HFrEF-cAF patients, but junctin levels were higher in HFrEF and relative Ser2814-RyR2 phosphorylation levels were increased in HFrEF-cAF, both potentially contributing to the greater RyR2 open probability. These novel insights into the molecular substrate for atrial arrhythmias in HF-patients position Ca²⁺-handling abnormalities as a likely trigger of AF in HF patients, which subsequently produces electrical remodeling that promotes the maintenance of the arrhythmia. Our new findings may have important implications for the development of novel treatment options for AF in the context of HF.

Calcium in the Pathophysiology of Atrial Fibrillation and Heart Failure

Atrial fibrillation (AF) is commonly associated with heart failure. A bidirectional relationship exists between the two-AF exacerbates heart failure causing a significant increase in heart failure symptoms, admissions to hospital and cardiovascular death, while pathological remodeling of the atria as a result of heart failure increases the risk of AF. A comprehensive understanding of the pathophysiology of AF is essential if we are to break this vicious circle. In this review, the latest evidence will be presented showing a fundamental role for calcium in both the induction and maintenance of AF. After outlining atrial electrophysiology and calcium handling, the role of calcium-dependent afterdepolarizations and atrial repolarization alternans in triggering AF will be considered. The atrial response to rapid stimulation will be discussed, including the short-term protection from calcium overload in the form of calcium signaling silencing and the eventual progression to diastolic calcium leak causing afterdepolarizations and the development of an electrical substrate that perpetuates AF. The role of calcium in the bidirectional relationship between heart failure and AF will then be covered. The effects of heart failure on atrial calcium handling that promote AF will be reviewed, including effects on both atrial myocytes and the pulmonary veins, before the aspects of AF which exacerbate heart failure are discussed. Finally, the limitations of human and animal studies will be explored allowing contextualization of what are sometimes discordant results.

Atrial Fibrillation in Heart Failure: a Therapeutic Challenge of Our Times

Atrial fibrillation (AF) and heart failure (HF) are growing cardiovascular disease epidemics worldwide. There has been an exponential increase in the prevalence of AF and HF correlating with an increased burden of cardiac risk factors and improved survival rates in patients with structural heart disease. AF is associated with adverse prognostic outcomes in HF and is most evident in mild-to-moderate left ventricular (LV) dysfunction where the loss of "atrial kick" translates into poorer quality of life and increased mortality. In the absence of underlying structural heart disease, arrhythmia can independently contribute to the development of cardiomyopathy. Together, these 2 conditions carry a high risk of thromboembolism due to stasis, inflammation and cellular dysfunction. Stroke prevention with oral anticoagulation (OAC) remains a mainstay of treatment. Pharmacologic rate and rhythm control remain limited by variable efficacy, intolerance and adverse reactions. Catheter ablation for AF has resulted in a paradigm shift with evidence indicating superiority over medical therapy. While its therapeutic success is high for paroxysmal AF, it remains suboptimal in persistent AF. A better mechanistic understanding of AF as well as innovations in ablation technology may improve patient outcomes in the future. Refractory cases may benefit from atrioventricular junction ablation and biventricular pacing. The value of risk factor modification, especially with regard to obesity, sleep apnea, hypertension and diabetes, cannot be emphasized enough. Close interdisciplinary collaboration between HF specialists and electrophysiologists is an essential component of good long-term outcomes in this challenging population.

Atrial Electrophysiological Remodeling and Fibrillation in Heart Failure

Heart failure (HF) causes complex, chronic changes in atrial structure and function, which can cause substantial electrophysiological remodeling and predispose the individual to atrial fibrillation (AF). Pharmacological treatments for preventing AF in patients with HF are limited. Improved understanding of the atrial electrical and ionic/molecular mechanisms that promote AF in these patients could lead to the identification of novel therapeutic targets. Animal models of HF have identified numerous changes in atrial ion currents, intracellular calcium handling, action potential waveform and conduction, as well as expression and signaling of associated proteins. These studies have shown that the pattern of electrophysiological remodeling likely depends on the duration of HF, the underlying cardiac pathology, and the species studied. In atrial myocytes and tissues obtained from patients with HF or left ventricular systolic dysfunction, the data on changes in ion currents and action potentials are largely equivocal, probably owing mainly to difficulties in controlling for the confounding influences of multiple variables, such as patient’s age, sex, disease history, and drug treatments, as well as the technical challenges in obtaining such data. In this review, we provide a summary and comparison of the main animal and human electrophysiological studies to date, with the aim of highlighting the consistencies in some of the remodeling patterns, as well as identifying areas of contention and gaps in the knowledge, which warrant further investigation.

The Stress-Response MAP Kinase Signaling in Cardiac Arrhythmias

Stress-response kinases, the mitogen-activated protein kinases (MAPKs) are activated in response to the challenge of a myriad of stressors. c-Jun N-terminal kinase (JNK), extracellular signal-regulated kinases (ERKs), and p38 MAPKs are the predominant members of the MAPK family in the heart. Extensive studies have revealed critical roles of activated MAPKs in the processes of cardiac injury and heart failure and many other cardiovascular diseases. Recently, emerging evidence suggests that MAPKs also promote the development of cardiac arrhythmias. Thus, understanding the functional impact of MAPKs in the heart could shed new light on the development of novel therapeutic approaches to improve cardiac function and prevent arrhythmia development in the patients. This review will summarize the recent findings on the role of MAPKs in cardiac remodeling and arrhythmia development and point to the critical need of future studies to further elucidate the fundamental mechanisms of MAPK activation and arrhythmia development in the heart.

SR calcium handling dysfunction, stress-response signaling pathways, and atrial fibrillation

Atrial fibrillation (AF) is the most common sustained arrhythmia. It is associated with a markedly increased risk of premature death due to embolic stroke and also complicates co-existing cardiovascular diseases such as heart failure. The prevalence of AF increases dramatically with age, and aging has been shown to be an independent risk of AF. Due to an aging population in the world, a growing body of AF patients are suffering a diminished quality of life and causing an associated economic burden. However, effective pharmacologic treatments and prevention strategies are lacking due to a poor understanding of the molecular and electrophysiologic mechanisms of AF in the failing and/or aged heart. Recent studies suggest that altered atrial calcium handling contributes to the onset and maintenance of AF. Here we review the role of stress-response kinases and calcium handling dysfunction in AF genesis in the aged and failing heart.

Calcium-activated potassium current modulates ventricular repolarization in chronic heart failure

The role of I_KCa in cardiac repolarization remains controversial and varies across species. The relevance of the current as a therapeutic target is therefore undefined. We examined the cellular electrophysiologic effects of I_KCa blockade in controls, chronic heart failure (HF) and HF with sustained atrial fibrillation. We used perforated patch action potential recordings to maintain intrinsic calcium cycling. The I_KCa blocker (apamin 100 nM) was used to examine the role of the current in atrial and ventricular myocytes. A canine tachypacing induced model of HF (1 and 4 months, n = 5 per group) was used, and compared to a group of 4 month HF with 6 weeks of superimposed atrial fibrillation (n = 7). A group of age-matched canine controls were used (n = 8). Human atrial and ventricular myocytes were isolated from explanted end-stage failing hearts which were obtained from transplant recipients, and studied in parallel. Atrial myocyte action potentials were unchanged by I_KCa blockade in all of the groups studied. I_KCa blockade did not affect ventricular myocyte repolarization in controls. HF caused prolongation of ventricular myocyte action potential repolarization. I_KCa blockade caused further prolongation of ventricular repolarization in HF and also caused repolarization instability and early afterdepolarizations. SK2 and SK3 expression in the atria and SK3 in the ventricle were increased in canine heart failure. We conclude that during HF, I_KCa blockade in ventricular myocytes results in cellular arrhythmias. Furthermore, our data suggest an important role for I_KCa in the maintenance of ventricular repolarization stability during chronic heart failure. Our findings suggest that novel antiarrhythmic therapies should have safety and efficacy evaluated in both atria and ventricles.

Cellular and molecular electrophysiology of atrial fibrillation initiation, maintenance, and progression

Atrial fibrillation (AF) is the most common clinically relevant arrhythmia and is associated with increased morbidity and mortality. The incidence of AF is expected to continue to rise with the aging of the population. AF is generally considered to be a progressive condition, occurring first in a paroxysmal form, then in persistent, and then long-standing persistent (chronic or permanent) forms. However, not all patients go through every phase, and the time spent in each can vary widely. Research over the past decades has identified a multitude of pathophysiological processes contributing to the initiation, maintenance, and progression of AF. However, many aspects of AF pathophysiology remain incompletely understood. In this review, we discuss the cellular and molecular electrophysiology of AF initiation, maintenance, and progression, predominantly based on recent data obtained in human tissue and animal models. The central role of Ca(2+)-handling abnormalities in both focal ectopic activity and AF substrate progression is discussed, along with the underlying molecular basis. We also deal with the ionic determinants that govern AF initiation and maintenance, as well as the structural remodeling that stabilizes AF-maintaining re-entrant mechanisms and finally makes the arrhythmia refractory to therapy. In addition, we highlight important gaps in our current understanding, particularly with respect to the translation of these concepts to the clinical setting. Ultimately, a comprehensive understanding of AF pathophysiology is expected to foster the development of improved pharmacological and nonpharmacological therapeutic approaches and to greatly improve clinical management.

Differential effects of the peroxynitrite donor, SIN-1, on atrial and ventricular myocyte electrophysiology

Oxidative stress has been implicated in the pathogenesis of heart failure and atrial fibrillation and can result in increased peroxynitrite production in the myocardium. Atrial and ventricular canine cardiac myocytes were superfused with 3-morpholinosydnonimine-N-ethylcarbamide (SIN-1), a peroxynitrite donor, to evaluate the acute electrophysiologic effects of peroxynitrite. Perforated whole-cell patch clamp techniques were used to record action potentials. SIN-1 (200 µM) increased the action potential duration (APD) in atrial and ventricular myocytes; however, in the atria, APD prolongation was rate independent, whereas in the ventricle APD, prolongation was rate dependent. In addition to prolongation of the action potential, beat-to-beat variability of repolarization was significantly increased in ventricular but not in atrial myocytes. We examined the contribution of intracellular calcium cycling to the effects of SIN-1 by treating myocytes with the SERCA blocker, thapsigargin (5-10 µM). Inhibition of calcium cycling prevented APD prolongation in the atrial and ventricular myocytes, and prevented the SIN-1-induced increase in ventricular beat-to-beat APD variability. Collectively, these data demonstrate that peroxynitrite affects atrial and ventricular electrophysiology differentially. A detailed understanding of oxidative modulation of electrophysiology in specific chambers is critical to optimize therapeutic approaches for cardiac diseases.

Pro-arrhythmogenic effects of atrial fibrillation-induced electrical remodelling: insights from the three-dimensional virtual human atria

Chronic atrial fibrillation (AF) is associated with structural and electrical remodelling in the atria, which are associated with a high recurrence of AF. Through biophysically detailed computer modelling, this study investigated mechanisms by which AF-induced electrical remodelling promotes and perpetuates AF. A family of Courtemanche–Ramirez–Nattel variant models of human atrial cell action potentials (APs), taking into account of intrinsic atrial electrophysiological properties, was modified to incorporate various experimental data sets on AF-induced changes of major ionic channel currents (I_CaL, I_Kur, I_to, I_K1, I_Ks, I_NaCa) and on intracellular Ca²⁺ handling. The single cell models for control and AF-remodelled conditions were incorporated into multicellular three-dimensional (3D) atrial tissue models. Effects of the AF-induced electrical remodelling were quantified as the changes of AP profile, AP duration (APD) and its dispersion across the atria, and the vulnerability of atrial tissue to the initiation of re-entry. The dynamic behaviour of re-entrant excitation waves in the 3D models was characterised. In our simulations, AF-induced electrical remodelling abbreviated atrial APD non-uniformly across the atria; this resulted in relatively short APDs co-existing with marked regional differences in the APD at junctions of the crista terminalis/pectinate muscle, pulmonary veins/left atrium. As a result, the measured tissue vulnerability to re-entry initiation at these tissue junctions was increased. The AF-induced electrical remodelling also stabilized and accelerated re-entrant excitation waves, leading to rapid and sustained re-entry. Under the AF-remodelled condition, re-entrant scroll waves in the 3D model degenerated into persistent and erratic wavelets, leading to fibrillation. In conclusion, realistic 3D atrial tissue models indicate that AF-induced electrical remodelling produces regionally heterogeneous and shortened APD; these respectively facilitate initiation and maintenance of re-entrant excitation waves.

Chronic myocardial infarction promotes atrial action potential alternans, afterdepolarizations, and fibrillation

Aims

Atrial fibrillation (AF) is increased in patients with heart failure resulting from myocardial infarction (MI). We aimed to determine the effects of chronic ventricular MI in rabbits on the susceptibility to AF, and underlying atrial electrophysiological and Ca²⁺-handling mechanisms.

Methods and results

In Langendorff-perfused rabbit hearts, under β-adrenergic stimulation with isoproterenol (ISO; 1 µM), 8 weeks MI decreased AF threshold, indicating increased AF susceptibility. This was associated with increased atrial action potential duration (APD)-alternans at 90% repolarization, by 147%, and no significant change in the mean APD or atrial global conduction velocity (CV; n = 6–13 non-MI hearts, 5–12 MI). In atrial isolated myocytes, also under β-stimulation, L-type Ca²⁺ current (I_CaL) density and intracellular Ca²⁺-transient amplitude were decreased by MI, by 35 and 41%, respectively, and the frequency of spontaneous depolarizations (SDs) was substantially increased. MI increased atrial myocyte size and capacity, and markedly decreased transverse-tubule density. In non-MI hearts perfused with ISO, the I_CaL-blocker nifedipine, at a concentration (0.02 µM) causing an equivalent I_CaL reduction (35%) to that from the MI, did not affect AF susceptibility, and decreased APD.

Conclusion

Chronic MI in rabbits remodels atrial structure, electrophysiology, and intracellular Ca²⁺ handling. Increased susceptibility to AF by MI, under β-adrenergic stimulation, may result from associated production of atrial APD alternans and SDs, since steady-state APD and global CV were unchanged under these conditions, and may be unrelated to the associated reduction in whole-cell I_CaL. Future studies may clarify potential contributions of local conduction changes, and cellular and subcellular mechanisms of alternans, to the increased AF susceptibility.

New directions in antiarrhythmic drug therapy for atrial fibrillation

Atrial fibrillation (AF) is the most prevalent cardiac arrhythmia and has a significant impact on morbidity and mortality. Current antiarrhythmic drugs for AF suffer from limited safety and efficacy, probably because they were not designed based on specific pathological mechanisms. Recent research has provided important insights into the mechanisms contributing to AF and highlighted several potential novel antiarrhythmic strategies. In this review, we highlight the main pathological mechanisms of AF, discuss traditional and novel aspects of atrial antiarrhythmic drugs in relation to these pathological mechanisms, and present potential novel therapeutic approaches including structure-based modulation of atrial-specific cardiac ion channels, restoring abnormal Ca2+ handling in AF and targeting atrial remodeling.

Transient outward K+ current reduction prolongs action potentials and promotes afterdepolarisations: a dynamic-clamp study in human and rabbit cardiac atrial myocytes

Human atrial transient outward K(+) current (I(TO)) is decreased in a variety of cardiac pathologies, but how I(TO) reduction alters action potentials (APs) and arrhythmia mechanisms is poorly understood, owing to non-selectivity of I(TO) blockers. The aim of this study was to investigate effects of selective I(TO) changes on AP shape and duration (APD), and on afterdepolarisations or abnormal automaticity with β-adrenergic-stimulation, using the dynamic-clamp technique in atrial cells. Human and rabbit atrial cells were isolated by enzymatic dissociation, and electrical activity recorded by whole-cell-patch clamp (35-37°C). Dynamic-clamp-simulated I(TO) reduction or block slowed AP phase 1 and elevated the plateau, significantly prolonging APD, in both species. In human atrial cells, I(TO) block (100% I(TO) subtraction) increased APD(50) by 31%, APD(90) by 17%, and APD(-61 mV) (reflecting cellular effective refractory period) by 22% (P < 0.05 for each). Interrupting I(TO) block at various time points during repolarisation revealed that the APD(90) increase resulted mainly from plateau-elevation, rather than from phase 1-slowing or any residual I(TO). In rabbit atrial cells, partial I(TO) block (∼40% I(TO) subtraction) reversibly increased the incidence of cellular arrhythmic depolarisations (CADs; afterdepolarisations and/or abnormal automaticity) in the presence of the β-agonist isoproterenol (0.1 μm; ISO), from 0% to 64% (P < 0.05). ISO-induced CADs were significantly suppressed by dynamic-clamp increase in I(TO) (∼40% I(TO) addition). ISO+I(TO) decrease-induced CADs were abolished by β(1)-antagonism with atenolol at therapeutic concentration (1 μm). Atrial cell action potential changes from selective I(TO) modulation, shown for the first time using dynamic-clamp, have the potential to influence reentrant and non-reentrant arrhythmia mechanisms, with implications for both the development and treatment of atrial fibrillation.

Altered Excitation-Contraction Coupling in Human Chronic Atrial Fibrillation

This review focuses on the (mal)adaptive processes in atrial excitation-contraction coupling occurring in patients with chronic atrial fibrillation. Cellular remodeling includes shortening of the atrial action potential duration and effective refractory period, depressed intracellular Ca²⁺ transient, and reduced myocyte contractility. Here we summarize the current knowledge of the ionic bases underlying these changes. Understanding the molecular mechanisms of excitation-contraction-coupling remodeling in the fibrillating human atria is important to identify new potential targets for AF therapy.

Low atrial fibrillatory rate is associated with poor outcome in patients with mild to moderate heart failure

BACKGROUND

Atrial fibrillatory rate (AFR) is a measure of atrial remodeling caused by atrial fibrillation (AF), and its acceleration negatively affects outcome of interventions for persistent AF. However, the prognostic value of AFR in patients with congestive heart failure (CHF) has not been studied. We sought to evaluate whether AFR can predict outcome in patients with mild to moderate (New York Health Association II-III) CHF.

METHODS AND RESULTS

High-resolution 20-minute long Holter ECGs obtained from 169 CHF patients with AF at enrollment were analyzed. AFR was estimated using spatiotemporal QRST cancellation and time-frequency analysis. The patients were followed for a median of 44 months, with primary end point defined as total mortality and secondary end points as sudden death and heart failure death. Atrial signal quality was sufficient for AFR estimation in 142 patients (mean age 69±11 years, 101 male). Of those, 48 patients died during follow-up, including 18 because of CHF progression. Mean AFR was 390±60 fpm and decreased with age (r=-0.3, P<0.001). Patients with AFR ≤371 fpm (lower tertile) had 44% 3-year mortality as compared with 26% with higher AFR. Lower AFR was an independent predictor of all cause mortality (HR=1.99, 95% CI=1.09-3.63, P=0.025) and CHF death (HR=3.74, 95% CI=1.38-10.14, P=0.010) after adjustment for significant clinical covariates in multivariable Cox analysis.

CONCLUSIONS

In CHF patients with AF, reduced AFR assessed using noninvasive approach is associated with increased risk of death because of heart failure progression, and may be considered a predictor of outcome.

Tetrahydrobiopterin depletion and NOS2 uncoupling contribute to heart failure-induced alterations in atrial electrophysiology

AIMS

Heart failure is a common antecedent to atrial fibrillation; both heart failure and atrial fibrillation are associated with increased myocardial oxidative stress. Chronic canine heart failure reduces atrial action potential duration and atrial refractoriness. We hypothesized that inducible nitric oxide synthase 2 (NOS2) contributes to atrial oxidative stress and electrophysiologic alterations.

METHODS AND RESULTS

A 16-week canine tachypacing model of heart failure was used (n= 21). At 10 weeks, dogs were randomized to either placebo (n = 12) or active treatment (n = 9) with NOS cofactor, tetrahydrobiopterin (BH(4), 50 mg), and NOS substrate (L-arginine, 3 g) twice daily for 6 weeks. A group of matched controls (n = 7) was used for comparison. Heart failure increased atrial NOS2 and reduced atrial BH(4), while L-arginine was unchanged. Treatment reduced inducible atrial fibrillation and normalized the heart failure-induced shortening of the left atrial myocyte action potential duration. Treatment increased atrial [BH(4)] while [L-arginine] was unchanged. Treatment did not improve left ventricular function or dimensions. Heart failure-induced reductions in atrial [BH(4)] resulted in NOS uncoupling, as measured by NO and superoxide anion (O(2)(·-)) production, while BH(4) and L-arginine treatment normalized NO and O(2)(·-). Heart failure resulted in left atrial oxidative stress, which was attenuated by BH(4) and L-arginine treatment.

CONCLUSION

Chronic non-ischaemic heart failure results in atrial oxidative stress and electrophysiologic abnormalities by depletion of BH(4) and uncoupling of NOS2. Modulation of NOS2 activity by repletion of BH(4) may be a safe and effective approach to reduce the frequency of atrial arrhythmias during heart failure.

Atrial remodeling in an ovine model of anthracycline-induced nonischemic cardiomyopathy: remodeling of the same sort

INTRODUCTION

All preclinical studies of atrial remodeling in heart failure (HF) have been confined to a single model of rapid ventricular pacing. To evaluate whether the atrial changes were specific to the model or represented an end result of HF, this study aimed to characterize atrial remodeling in an ovine model of doxorubicin-induced cardiomyopathy.

METHODS AND RESULTS

Fourteen sheep, 7 with cardiomyopathy induced by repeated intracoronary doxorubicin infusions and 7 controls, were studied. The development of HF was monitored by cardiac imaging and hemodynamic parameters. Open chest electrophysiological study was performed using custom-made 128-electrode epicardial plaque assessing effective refractory period (ERP) and conduction velocity. Atrial tissues were harvested for structural analysis. The HF group had demonstrable moderate global HF (left ventricular ejection fraction [LVEF]: 37.1 vs 46.4%; P = 0.003) and showed the following compared to controls: left atrial dilatation (P = 0.02) and dysfunction (P = 0.005); longer P-wave duration (P < 0.05); higher ERP at all cycle lengths (P ≤ 0.002) and locations (P < 0.001); slower conduction velocity (P < 0.001); increased conduction heterogeneity index (P < 0.001); increased atrial fibrosis (right atrial [RA]: 5.9 ± 2.6 vs 2.8 ± 0.9%; P < 0.0001, left atrial [LA]: 3.7 ± 2.2 vs 2.4 ± 1.1%; P = 0.002), and longer induced atrial fibrillation (AF) episodes (16 ± 22 vs 2 ± 3 seconds; P = 0.04).

CONCLUSION

In this model of HF, there was significant atrial remodeling characterized by atrial enlargement/dysfunction, increased fibrosis, slowed/heterogeneous conduction, and increased refractoriness associated with more sustained AF. These findings appear the "same sort" to previous models of HF implicating a final common substrate leading to the development of AF in HF.

Rhythm control strategies and the role of antiarrhythmic drugs in the management of atrial fibrillation: focus on clinical outcomes

Atrial fibrillation (AF) is a common disorder that significantly impacts the lives of affected patients. The restoration of sinus rhythm may prevent AF progression and reduce the occurrence of negative sequelae; however, available antiarrhythmic drugs (AADs) have largely failed to demonstrate significant benefit relative to rate control with respect to morbidity and mortality outcomes. The review commentary will address current knowledge regarding the pathologic mechanisms of AF, current trials that investigate rate and rhythm strategies, and future therapies that may change treatment approaches based on preliminary evidence suggesting a more favorable safety profile. The observed outcomes are likely a reflection of the limited efficacy plus poor safety and tolerability of available AADS. However, data from patients who attained and maintained sinus rhythm in a number of clinical studies demonstrate that the achievement of normal sinus rhythm can indeed reduce AF-associated morbidity and mortality. Furthermore, the results of trials designed to assess specific morbidity and mortality outcomes such as cardiovascular death hospitalization suggest that the development of safer AF therapies, whether pharmacologic or nonpharmacologic, can potentially improve clinical outcomes.

Pathophysiological mechanisms of atrial fibrillation: a translational appraisal

Atrial fibrillation (AF) is an arrhythmia that can occur as the result of numerous different pathophysiological processes in the atria. Some aspects of the morphological and electrophysiological alterations promoting AF have been studied extensively in animal models. Atrial tachycardia or AF itself shortens atrial refractoriness and causes loss of atrial contractility. Aging, neurohumoral activation, and chronic atrial stretch due to structural heart disease activate a variety of signaling pathways leading to histological changes in the atria including myocyte hypertrophy, fibroblast proliferation, and complex alterations of the extracellular matrix including tissue fibrosis. These changes in electrical, contractile, and structural properties of the atria have been called "atrial remodeling." The resulting electrophysiological substrate is characterized by shortening of atrial refractoriness and reentrant wavelength or by local conduction heterogeneities caused by disruption of electrical interconnections between muscle bundles. Under these conditions, ectopic activity originating from the pulmonary veins or other sites is more likely to occur and to trigger longer episodes of AF. Many of these alterations also occur in patients with or at risk for AF, although the direct demonstration of these mechanisms is sometimes challenging. The diversity of etiological factors and electrophysiological mechanisms promoting AF in humans hampers the development of more effective therapy of AF. This review aims to give a translational overview on the biological basis of atrial remodeling and the proarrhythmic mechanisms involved in the fibrillation process. We pay attention to translation of pathophysiological insights gained from in vitro experiments and animal models to patients. Also, suggestions for future research objectives and therapeutical implications are discussed.

Mechanisms of termination and prevention of atrial fibrillation by drug therapy

Atrial fibrillation (AF) is a disorder of the rhythm of electrical activation of the cardiac atria. It is the most common cardiac arrhythmia, has multiple aetiologies, and increases the risk of death from stroke. Pharmacological therapy is the mainstay of treatment for AF, but currently available anti-arrhythmic drugs have limited efficacy and safety. An improved understanding of how anti-arrhythmic drugs affect the electrophysiological mechanisms of AF initiation and maintenance, in the setting of the different cardiac diseases that predispose to AF, is therefore required. A variety of animal models of AF has been developed, to represent and control the pathophysiological causes and risk factors of AF, and to permit the measurement of detailed and invasive parameters relating to the associated electrophysiological mechanisms of AF. The purpose of this review is to examine, consolidate and compare available relevant data on in-vivo electrophysiological mechanisms of AF suppression by currently approved and investigational anti-arrhythmic drugs in such models. These include the Vaughan Williams class I-IV drugs, namely Na(+) channel blockers, β-adrenoceptor antagonists, action potential prolonging drugs, and Ca(2+) channel blockers; the "upstream therapies", e.g., angiotensin converting enzyme inhibitors, statins and fish oils; and a variety of investigational drugs such as "atrial-selective" multiple ion channel blockers, gap junction-enhancers, and intracellular Ca(2+)-handling modulators. It is hoped that this will help to clarify the main electrophysiological mechanisms of action of different and related drug types in different disease settings, and the likely clinical significance and potential future exploitation of such mechanisms.

Alterations of atrial Ca(2+) handling as cause and consequence of atrial fibrillation

Atrial fibrillation (AF) is the most prevalent sustained arrhythmia. As the most important risk factor for embolic stroke, AF is associated with a high morbidity and mortality. Despite decades of research, successful (pharmacological and interventional) 'ablation' of the arrhythmia remains challenging. AF is characterized by a diverse aetiology, including heart failure, hypertension, and valvular disease. Based on this understanding, new treatment strategies that are specifically tailored to the underlying pathophysiology of a certain 'type' of AF are being developed. One important aspect of AF pathophysiology is altered intracellular Ca(2+) handling. Due to the increase in the atrial activation rate and the subsequent initial [Ca(2+)](i) overload, AF induces 'remodelling' of intracellular Ca(2+) handling. Current research focuses on unravelling the contribution of altered intracellular Ca(2+) handling to different types of AF. More specifically, changes in intracellular Ca(2+) homeostasis preceding the onset of AF, in conditions which predispose to AF (e.g. heart failure), appear to be different from changes in Ca(2+) handling developing after the onset of AF. Here we review and critique altered intracellular Ca(2+) handling and its contribution to three specific aspects of AF pathophysiology, (i) excitation-transcription coupling and Ca(2+)-dependent signalling pathways, (ii) atrial contractile dysfunction, and (iii) arrhythmogenicity.

Selective molecular potassium channel blockade prevents atrial fibrillation

BACKGROUND

Safety and efficacy limit currently available atrial fibrillation (AF) therapies. We hypothesized that atrial gene transfer would allow focal manipulation of atrial electrophysiology and, by eliminating reentry, would prevent AF.

METHODS AND RESULTS

In a porcine AF model, we compared control animals to animals receiving adenovirus that encoded KCNH2-G628S, a dominant negative mutant of the I(Kr) potassium channel alpha-subunit (G628S animals). After epicardial atrial gene transfer and pacemaker implantation for burst atrial pacing, animals were evaluated daily for cardiac rhythm. Electrophysiological and molecular studies were performed at baseline and when animals were euthanized on either postoperative day 7 or 21. By day 10, none of the control animals and all of the G628S animals were in sinus rhythm. After day 10, the percentage of G628S animals in sinus rhythm gradually declined until all animals were in AF by day 21. The relative risk of AF throughout the study was 0.44 (95% confidence interval 0.33 to 0.59, P<0.01) among the G628S group versus controls. Atrial monophasic action potential was considerably longer in G628S animals than in controls at day 7, and KCNH2 protein levels were 61% higher in the G628S group than in control animals (P<0.01). Loss of gene expression at day 21 correlated with loss of action potential prolongation and therapeutic efficacy.

CONCLUSIONS

Gene therapy with KCNH2-G628S eliminated AF by prolonging atrial action potential duration. The effect duration correlated with transgene expression.

Cardiac adrenergic control and atrial fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia, and it causes substantial mortality. The autonomic nervous system, and particularly the adrenergic/cholinergic balance, has a profound influence on the occurrence of AF. Adrenergic stimulation from catecholamines can cause AF in patients. In human atrium, catecholamines can affect each of the electrophysiological mechanisms of AF initiation and/or maintenance. Catecholamines may produce membrane potential oscillations characteristic of afterdepolarisations, by increasing Ca(2+) current, [Ca(2+)](i) and consequent Na(+)-Ca(2+) exchange, and may also enhance automaticity. Catecholamines might affect reentry, by altering excitability or conduction, rather than action potential terminal repolarisation or refractory period. However, which arrhythmia mechanisms predominate is unclear, and likely depends on cardiac pathology and adrenergic tone. Heart failure (HF), a major cause of AF, causes adrenergic activation and adaptational changes, remodelling, of atrial electrophysiology, Ca(2+) homeostasis, and adrenergic responses. Chronic AF also remodels these, but differently to HF. Myocardial infarction and AF cause neural remodelling that also may promote AF. beta-Adrenoceptor antagonists (beta-blockers) are used in the treatment of AF, mainly to control the ventricular rate, by slowing atrioventricular conduction. beta-Blockers also reduce the incidence of AF, particularly in HF or after cardiac surgery, when adrenergic tone is high. Furthermore, the chronic treatment of patients with beta-blockers remodels the atria, with a potentially antiarrhythmic increase in the refractory period. Therefore, the suppression of AF by beta-blocker treatment may involve an attenuation of arrhythmic activity that is caused by increased [Ca(2+)](i), coupled with effects of adaptation to the treatment. An improved understanding of the involvement of the adrenergic system and its control in basic mechanisms of AF under differing cardiac pathologies might lead to better treatments.

Chronic heart failure and the substrate for atrial fibrillation

AIMS

We sought to define the underlying mechanisms for atrial fibrillation (AF) during chronic heart failure (HF).

METHODS AND RESULTS

Preliminary studies showed that 4 months of HF resulted in irreversible systolic dysfunction (n = 9) and a substrate for sustained inducible AF (>3 months, n = 3). We used a chronic (4-month) canine model of tachypacing-induced HF (n = 10) to assess atrial electrophysiological remodelling, relative to controls (n = 5). Left ventricular fractional shortening was reduced from 37.2 +/- 0.83 to 13.44 +/- 2.63% (P < 0.05). Left atrial (LA) contractility (fractional area change) was reduced from 34.9 +/- 7.9 to 27.9 +/- 4.23% (P < 0.05). Action potential durations (APDs) at 50 and 90% repolarization were shortened by approximately 60 and 40%, respectively, during HF (P < 0.05). HF-induced atrial remodelling included increased fibrosis, increased I(to), and decreased I(K1), I(Kur), and I(Ks) (P < 0.05). HF induced increases in LA Kv channel interacting protein 2 (P < 0.05), no change in Kv4.3, Kv1.5, or Kir2.3, and reduced Kir2.1 (P < 0.05). When I(Ca-L) was elicited by action potential (AP) clamp, HF APs reduced the integral of I(Ca) in control myocytes, with a larger reduction in HF myocytes (P < 0.05). I(CaL) measured with standard voltage clamp was unchanged by HF. Incubation of myocytes with N-acetylcysteine (a glutathione precursor) attenuated HF-induced electrophysiological alterations. LA angiotensin-1 receptor expression was increased in HF.

CONCLUSION

Chronic HF causes alterations in ion channel expression and ion currents, resulting in attenuation of the APD and atrial contractility and a substrate for persistent AF.