Effects of Cilazapril on atrial electrical, structural and functional remodeling in atrial fibrillation dogs

Pathophysiology and clinical relevance of atrial myopathy

Atrial myopathy is a condition that consists of electrical, structural, contractile, and autonomic remodeling of the atria and is the substrate for development of atrial fibrillation, the most common arrhythmia. Pathophysiologic mechanisms driving atrial myopathy are inflammation, oxidative stress, atrial stretch, and neurohormonal signals, e.g., angiotensin-II and aldosterone. These mechanisms initiate the structural and functional remodeling of the atrial myocardium. Novel therapeutic strategies are being developed that target the pathophysiologic mechanisms of atrial myopathy. In this review, we will discuss the pathophysiology of atrial myopathy, as well as diagnostic and therapeutic strategies.

Aging and atrial fibrillation: A vicious circle

Atrial fibrillation (AF) is the commonest sustained cardiac arrhythmia observed in clinical practice. Its prevalence increases dramatically with advancing age. This review article discusses the recent advances in studies investigating the relationship between aging and AF and the possible underlying mechanisms.

The Atrium in Atrial Fibrillation - A Clinical Review on How to Manage Atrial Fibrotic Substrates

Atrial fibrillation (AF) is the most common sustained arrhythmia in the population and is associated with a significant clinical and economic burden. Rigorous assessment of the presence and degree of an atrial arrhythmic substrate is essential for determining treatment options, predicting long-term success after catheter ablation, and as a substrate critical in the pathophysiology of atrial thrombogenesis. Catheter ablation of AF has developed into an essential rhythm-control strategy. Nowadays is one of the most common cardiac ablation procedures performed worldwide, with its success inversely related to the extent of atrial structural disease. Although atrial substrate evaluation remains complex, several diagnostic resources allow for a more comprehensive assessment and quantification of the extent of left atrial structural remodeling and the presence of atrial fibrosis. In this review, we summarize the current knowledge on the pathophysiology, etiology, and electrophysiological aspects of atrial substrates promoting the development of AF. We also describe the risk factors for its development and how to diagnose its presence using imaging, electrocardiograms, and electroanatomic voltage mapping. Finally, we discuss recent data regarding fibrosis biomarkers that could help diagnose atrial fibrotic substrates.

Arrhythmias in Chronic Kidney Disease

Arrhythmias cause disability and an increased risk of premature death in the general population but far more so in patients with renal failure. The association between the cardiac and renal systems is complex and derives in part from common causality of renal and myocardial injury from conditions including hypertension and diabetes. In many cases, there is a causal relationship, with renal dysfunction promoting arrhythmias and arrhythmias exacerbating renal dysfunction. In this review, the authors expand on the challenges faced by cardiologists in treating common and uncommon arrhythmias in patients with renal failure using pharmacological interventions, ablation and cardiac implantable device therapies. They explore the most important interactions between heart rhythm disorders and renal dysfunction while evaluating the ways in which the coexistence of renal dysfunction and cardiac arrhythmia influences the management of both.

Left atrial fibrosis in atrial fibrillation: Mechanisms, clinical evaluation and management

Atrial fibrillation (AF), the commonest arrhythmia, shows associations with various disease conditions. Mounting evidence indicates that atrial fibrosis is an important part of the arrhythmogenic substrate, with an essential function in the generation of conduction abnormalities that underlie the transition from paroxysmal to persistent AF, which in turn contributes to AF perpetuation. Left atrial (LA) fibrosis is considered a possible major factor and predictor in AF treatment. The present review provides insights into LA fibrosis’ association with AF. The information is focused on clinical aspects and mechanisms, clinical evaluating methods that evaluate fibrosis changes and examining possible options for the prevention of atrial fibrosis.

Atrial fibrosis underlying atrial fibrillation (Review)

Atrial fibrillation (AF) is one of the most common tachyarrhythmias observed in the clinic and is characterized by structural and electrical remodelling. Atrial fibrosis, an emblem of atrial structural remodelling, is a complex multifactorial and patient-specific process involved in the occurrence and maintenance of AF. Whilst there is already considerable knowledge regarding the association between AF and fibrosis, this process is extremely complex, involving intricate neurohumoral and cellular and molecular interactions, and it is not limited to the atrium. Current technological advances have made the non-invasive evaluation of fibrosis in the atria and ventricles possible, facilitating the selection of patient-specific ablation strategies and upstream treatment regimens. An improved understanding of the mechanisms and roles of fibrosis in the context of AF is of great clinical significance for the development of treatment strategies targeting the fibrous region. In the present review, a focus was placed on the atrial fibrosis underlying AF, outlining its role in the occurrence and perpetuation of AF, by reviewing recent evaluations and potential treatment strategies targeting areas of fibrosis, with the aim of providing a novel perspective on the management and prevention of AF.

Pathophysiology of atrial fibrillation and chronic kidney disease

Atrial fibrillation (AF) and chronic kidney disease (CKD) are closely related conditions with shared risk factors. The growing prevalence of both AF and CKD indicates that more patients will suffer from concurrent conditions. There are various complex interlinking mechanisms with important implications for the management of these patients. Furthermore, there is uncertainty regarding the use of oral anticoagulation (OAC) in AF and CKD that is reflected by a lack of consensus between international guidelines. Therefore, the importance of understanding the implications of co-existing AF and CKD should not be underestimated. In this review, we discuss the pathophysiology and association between AF and CKD, including the underlying mechanisms, risk of thrombo-embolic and bleeding complications, influence on stroke management, and evidence surrounding the use of OAC for stroke prevention.

Suppression of experimental atrial fibrillation in a canine model of rapid atrial pacing by the phosphodiesterase 3 inhibitor cilostazol

OBJECTIVE

Atrial fibrillation (AF) represents the most common arrhythmia encountered in cardiology department. The purpose of this study was designed to investigate whether cilostazol, an oral phosphodiesterase 3 inhibitor (PDE3) could have protective effects on atrial remodeling in a canine model of AF and explore the potential molecular mechanisms.

METHODS

Dogs were randomly assigned to Sham, Paced, Paced + cilostazol group, 7 dogs in each group. In Sham group, pacemaker was instrumented but without pacing. Rapid atrial pacing (RAP) at 600 or 500 bpm/min was maintained in Paced group and Paced + cilo group for 2 h or 2 weeks in acute or chronic experiment, respectively. The Paced + cilo group of dogs were pretreated with cilostazol orally (10 mg·kg^-1·d^-1, cilo) for 1 h or 2 days prior RAP induction and served as treatment group. Atrial effective refractory periods (AERP) at different basic cycle lengths (BCLs), inducibility, and duration time of AF were measured after pacing for 2 h. The blood sample, echocardiography, histopathology, inflammation and oxidative stress makers, protein and mRNA expression levels of matrix metalloproteinase-2 (MMP-2) and MMP-9 were detected after 2 weeks pacing in each group.

RESULTS

Significant changes in electrophysiological parameters were observed in the acute RAP canine model, the AERPs shortened with increased inducibility and duration of AF, which was attenuated by cilostazol (P < 0.05). The serum inflammation makers as interleukin-8 (IL-8) and toll like receptor 4 (TLR 4) levels and oxidative stress indicators like xanthine oxidative (XO) and reactive oxygen species (ROS) in the Paced group was significantly higher than that in Sham group (P < 0.01), and was significantly reduced by cilostazol treatment (P < 0.01). The level of mean platelet volume (MPV) which is one of the platelet indices was significantly elevated in Paced group (P < 0.01). While after cilostazol treated for 2 weeks, the level of MPV was obviously decreased than Paced group (P < 0.01). Pathology and echocardiography studies showed that cilostazol can also prevent RAP induced cardiac fibrosis and structural remodeling. The MPV level has close correlations with IL-8, TLR4, XO and ROS (all P < 0.01). MMP-2 and MMP-9 expression were significantly increased in Paced group (all P < 0.01), which can be attenuated by cilostazol.

CONCLUSIONS

Cilostazol may have protective effects on RAP-induced atrial remodeling by anti-inflammatory, anti-oxidative stress action and regulate the extracellular collagen matrix in a canine model. Moreover, MPV level is associated with inflammation and oxidative stress response of RAP, which might be an important predictors of new-onset and recurrent AF.

Effects of febuxostat on atrial remodeling in a rabbit model of atrial fibrillation induced by rapid atrial pacing

Background

Febuxostat, a novel nonpurine selective inhibitor of xanthine oxidase (XO), may be used in the prevention and management of atrial fibrillation (AF). The purpose of this study was to evaluate the effects of febuxostat on atrial remodeling in a rabbit model of AF induced by rapid atrial pacing (RAP) and the mechanisms by which it acts.

Methods

Twenty-four rabbits were randomly divided into four groups: sham-operated group (Group S), RAP group (Group P), RAP with 5 mg/kg per day febuxostat group (Group LFP), and RAP with 10 mg/kg per day febuxostat group (Group HFP). All rabbits except those in Group S were subjected to RAP at 600 beats/min for four weeks. The effects of febuxostat on atrial electrical and structural remodeling, markers of inflammation and oxidative stress, and signaling pathways involved in the left atrium were examined.

Results

Shortened atrial effective refractory period (AERP), increased AF inducibility, decreased mRNA levels of Cav1.2 and Kv4.3, and left atrial enlargement and dysfunction were observed in Group P, and these changes were suppressed in the groups treated with febuxostat. Prominent atrial fibrosis was observed in Group P, as were increased levels of TGF-β1, Collagen I, and α-SMA and decreased levels of Smad7 and eNOS. Treatment with febuxostat attenuated these differences. Changes in inflammatory and oxidative stress markers induced by RAP were consistent with the protective effects of febuxostat.

Conclusions

This study is the first to find that febuxostat can inhibit atrial electrical and structural remodeling of AF by suppressing XO and inhibiting the TGF-β1/Smad signaling pathway.

Metoprolol prevents chronic obstructive sleep apnea-induced atrial fibrillation by inhibiting structural, sympathetic nervous and metabolic remodeling of the atria

Chronic obstructive sleep apnea (OSA) may promote the development of atrial fibrillation (AF) by inducing atrial electrical and structural remodeling as well as autonomic nerve hyperinnervation. Here, we investigated the roles of metoprolol in regulation of atrial remodeling induced by chronic OSA. A canine model of chronic OSA was established by stopping the ventilator and closing the airway for 4 h/day every other day for 12 weeks, while metoprolol (5 mg·kg-1·day-1) was continuously administered. Using that model, we observed that increases in sympathetic sprouting and atrial structural remodeling were sharply inhibited by metoprolol. Moreover, metoprolol dramatically inhibited the impairment of atrial energy metabolism by activating the Sirt1-AMPK pathway. In vitro, metoprolol significantly activated the Sirt1-AMPK pathway in intermittent hypoxic and isoproterenol-treated HL-1 cells, and the effect was abolished by the coadministration of EX-527, an inhibitor of Sirt1 activation. In summary, metoprolol protects against chronic OSA-induced atrial remodeling. Our results suggest a new and feasible treatment strategy for AF induced by OSA.

Linking atrial fibrillation with non-alcoholic fatty liver disease: potential common therapeutic targets

Non-alcoholic fatty liver disease (NAFLD) and atrial fibrillation (AF) are common chronic non-infectious diseases with rising incidences. NAFLD is an independent risk factor for the onset of AF, after adjusting potentially related factors. The pathogenesis of these diseases share several mechanisms including reduced adiponectin level, insulin resistance, and renin angiotensin aldosterone system (RAAS) activation, in addition to activation of common disease pathways that promote inflammation, oxidative stress, and fibrosis. Furthermore, statins and RAAS blockers exert therapeutic effects concurrently on NAFLD and AF. The common pathogenesis of NAFLD and AF may serve as a potential therapeutic target in the future.

Cardiac Magnetic Resonance-Measured Left Atrial Volume and Function and Incident Atrial Fibrillation: Results From MESA (Multi-Ethnic Study of Atherosclerosis)

BACKGROUND

Early detection of structural changes in left atrium (LA) before atrial fibrillation (AF) development could be helpful in identification of those at higher risk for AF. Using cardiac magnetic resonance imaging, we examined the association of LA volume and function, and incident AF in a multiethnic population free of clinical cardiovascular diseases.

METHODS AND RESULTS

In a case-cohort study embedded in MESA (Multi-Ethnic Study of Atherosclerosis), baseline LA size and function assessed by cardiac magnetic resonance feature-tracking were compared between 197 participants with incident AF and 322 participants randomly selected from the whole MESA cohort. Participants were followed up for 8 years. Incident AF cases had a larger LA volume and decreased passive, active, and total LA emptying fractions and peak global LA longitudinal strain (peak LA strain) at baseline. In multivariable analysis, elevated LA maximum volume index (hazard ratio, 1.38 per SD; 95% confidence interval, 1.01-1.89) and decreased peak LA strain (hazard ratio, 0.68 per SD; 95% confidence interval, 0.48-0.96), and passive and total LA emptying fractions (hazard ratio for passive LA emptying fractions, 0.55 per SD; 95% confidence interval, 0.40-0.75 and hazard ratio for active LA emptying fractions, 0.70 per SD; 95% confidence interval, 0.52-0.95), but not active LA emptying fraction, were associated with incident AF.

CONCLUSIONS

Elevated LA volumes and decreased passive and total LA emptying fractions were independently associated with incident AF in an asymptomatic multiethnic population. Including LA functional variables along with other risk factors of AF may help to better risk stratify individuals at risk of AF development.

Interatrial block in the modern era

Interatrial block (IAB; P-wave duration ≥ 110 ms), which represents a delay in the conduction between the atria, is a pandemic conduction abnormality that is frequently underappreciated in clinical practice. Despite its comprehensive documentation in the medical literature, it has still not received adequate attention and also not adequately described and discussed in most cardiology textbooks. IAB can be of varying degrees and classified based on the degree of P-duration and its morphology. It can transform into a higher degree block and can also manifest transiently. IAB may be a preceding or causative risk factor for various atrial arrhythmias (esp. atrial fibrillation) and also be associated with various other clinical abnormalities ranging from left atrial dilation and thromboembolism including embolic stroke and mesenteric ischemia. IAB certainly deserves more attention and prospective studies are needed to formulate a standard consensus regarding appropriate management strategies.

Atrial fibrillation from the pathologist's perspective

Atrial fibrillation (AF), the most common sustained cardiac arrhythmia encountered in clinical practice, is associated with increased morbidity and mortality. Electrophysiologically, it is characterized by a high rate of asynchronous atrial cell depolarization causing a loss of atrial contractile function and irregular ventricular rates. For a long time, AF was considered as a pure functional disorder without any structural background. Only in recent years, have new mapping and imaging techniques identified atrial locations, which are very often involved in the initiation and maintenance of this supraventricular arrhythmia (i.e. the distal portion of the pulmonary veins and the surrounding atrial myocardium). Morphological analysis of these myocardial sites has demonstrated significant structural remodeling as well as paved the way for further knowledge of AF natural history, pathogenesis, and treatment. This architectural myocardial disarrangement is induced by the arrhythmia itself and the very frequently associated cardiovascular disorders. At the same time, the structural remodeling is also capable of sustaining AF, thereby creating a sort of pathogenetic vicious circle. This review focuses on current understanding about the structural and genetic bases of AF with reference to their classification, pathogenesis, and clinical implications.

Role of Inflammation in Initiation and Perpetuation of Atrial Fibrillation: A Systematic Review of the Published Data

Inflammation has emerged as being strongly associated with AF initiation and perpetuation, including being implicated as a possible causal factor. Its role needs further elucidation to assist with the optimal prevention and treatment of AF using an individualized strategy. In the present review article the current published data linking inflammation to AF is summarized.

MicroRNA-101 inhibited postinfarct cardiac fibrosis and improved left ventricular compliance via the FBJ osteosarcoma oncogene/transforming growth factor-β1 pathway

BACKGROUND

Cardiac interstitial fibrosis is a major cause of the deteriorated performance of the heart in patients with chronic myocardial infarction. MicroRNAs (miRs) have recently been proven to be a novel class of regulators of cardiovascular diseases, including those associated with cardiac fibrosis. This study aimed to explore the role of miR-101 in cardiac fibrosis and the underlying mechanisms.

METHODS AND RESULTS

Four weeks after coronary artery ligation of rats, the expression of miR-101a and miR-101b (miR-101a/b) in the peri-infarct area was decreased. Treatment of cultured rat neonatal cardiac fibroblasts with angiotensin II also suppressed the expression of miR-101a/b. Forced expression of miR-101a/b suppressed the proliferation and collagen production in rat neonatal cardiac fibroblasts, as revealed by cell counting, MTT assay, and quantitative reverse transcription-polymerase chain reaction. The effect was abrogated by cotransfection with AMO-101a/b, the antisense inhibitors of miR-101a/b. c-Fos was found to be a target of miR-101a because overexpression of miR-101a decreased the protein and mRNA levels of c-Fos and its downstream protein transforming growth factor-β1. Silencing c-Fos by siRNA mimicked the antifibrotic action of miR-101a, whereas forced expression of c-Fos protein canceled the effect of miR-101a in cultured cardiac fibroblasts. Strikingly, echocardiography and hemodynamic measurements indicated remarkable improvement of the cardiac performance 4 weeks after adenovirus-mediated overexpression of miR-101a in rats with chronic myocardial infarction. Furthermore, the interstitial fibrosis was alleviated and the expression of c-Fos and transforming growth factor-β1 was inhibited.

CONCLUSION

Overexpression of miR-101a can mitigate interstitial fibrosis and the deterioration of cardiac performance in postinfarct rats, indicating the therapeutic potential of miR-101a for cardiac disease associated with fibrosis.

Inhibition of the renin-angiotensin system for prevention of atrial fibrillation

Atrial fibrillation (AF) is a source of considerable morbidity and mortality. There has been compelling evidence supporting the role of renin-angiotensin system (RAS) in the genesis and perpetuation of AF through atrial remodeling, and experimental studies have validated the utilization of RAS inhibition for AF prevention. This article reviews clinical trials on the use of angiotensin-converting enzyme inhibitors (ACEIs) and angiotensin receptor blockers (ARBs) for the prevention of AF. Results have been variable, depending on the clinical background of treated patients. ACEIs and ARBs appear beneficial for primary prevention of AF in patients with heart failure, whereas they are not equally effective in hypertensive patients with normal left ventricular function. Furthermore, the use of ACEIs or ARBs for secondary prevention of AF has been found beneficial only after electrical cardioversion. Additional data are needed to establish the potential clinical role of renin-angiotensin inhibition for prevention of AF.

Interatrial block: is it time for more attention?

Interatrial block (IAB) is defined as delayed conduction between the right and left atrium, which results in prolonged P-wave duration (> or =110 milliseconds). Interatrial block can be partial or advanced (much less common), depending on the severity of the conduction abnormality. Several studies have reported that the prevalence of IAB is more than 40% in hospital inpatients. Despite this, IAB remains largely underdiagnosed and commonly ignored. Although more investigations are needed to identify the cause of IAB, coronary artery disease and conditions related to cardiovascular disease, such as hypertension or diabetes mellitus, have been described as potential risk factors for developing IAB. Interatrial block has strong associations with multiple medical conditions including atrial fibrillation, myocardial ischemia, left atrial enlargement, and systemic emboli. Treatment modalities for IAB to preclude its consequences include pacing and medical management, in which angiotensin-converting enzyme inhibitors and angiotensin receptor blockers have given promising results. However, more interest, attention, and research for IAB is required to explore this uncertain issue thoroughly.

Effect of Angiotensin converting enzyme inhibitors and Angiotensin receptor blockers on patients following ablation of atrial fibrillation

Background and Objectives

It is known that angiotensin converting enzyme inhibitors and angiotensin II type 1 receptor blockers (ACEIs and ARBs, respectively) are effective in preventing atrial fibrillation (AF) in high-risk patients. However, it is not known whether ACEIs and ARBs are effective in preventing the recurrence of AF after catheter ablation.

Subjects and Methods

One hundred fifty-two patients (mean age, 57±10 years; M : F=94 : 58) who underwent catheter ablation due to drug-refractory paroxysmal (mean age, 57±10 years; M : F=58 : 43) or persistent AF (mean age, 56±10 years; M : F=36 : 15) were enrolled. We compared the recurrence rates between the groups with and without ACEIs or ARBs use in paroxysmal and persistent AF. The mean duration of follow-up was 18±14 months.

Results

The overall recurrence rate after ablation therapy was 26% (n=39). The recurrence rate was significantly decreased in the patients with persistent AF with the use of ACEIs or ARBs (12.1% vs. 61.1%, p<0.01), but this difference was not observed in the patients with paroxysmal AF (24.2% vs. 22.9%, p=0.87). In patients with persistent AF with and without recurrence, the size of the left atrium (44.2±8.4 mm vs. 44.3±5.8 mm, respectively, p=0.45) and the ejection fraction (62±6.5% vs. 61.5±6.2%, respectively, p=0.28) were not significantly different. In multivariate analysis, the use of ACEIs or ARBs was independently associated with recurrence after adjusting for the size of the left atrium and the ejection fraction {odds ratio (OR)=0.078, 95% confidence interval (CI)=0.02-0.35, p<0.01}.

Conclusion

ACEIs and ARBs were shown to be effective in preventing AF recurrence after catheter ablation in patients with persistent AF.

Probucol attenuates atrial structural remodeling in prolonged pacing-induced atrial fibrillation in dogs

AIMS

Oxidative stress has recently been implicated in atrial fibrillation (AF); however, the mechanisms remain unclear. Herein, we hypothesize that probucol can attenuate atrial structure remodeling.

METHODS

Twenty dogs were randomly divided into sham-operated, control, and probucol-treated groups. We identified apoptosis and histopathological changes in the atria. Oxidative stress was measured by lipid peroxidation and echocardiographic examinations were performed.

RESULTS

Atrial apoptosis indexes were dramatically decreased in the probucol-treated group compared to the control group. Relative to the control group, the percentage of myolysis was dramatically decreased in the probucol-treated group (p < 0.01). There was less lipid peroxidation in the probucol-treated group than the control group. Atrial function was dramatically elevated in the probucol-treated group.

CONCLUSIONS

The results of this study indicate that the antioxidant probucol suppresses atrial structural remodeling and may act as a new therapy for AF.

Atrial fibrosis: mechanisms and clinical relevance in atrial fibrillation

Atrial fibrillation (AF) is the most common arrhythmia in the clinical setting, and traditional pharmacological approaches have proved to have important weaknesses. Structural remodeling has been observed in both clinical and experimental AF paradigms, and is an important feature of the AF substrate, producing fibrosis that alters atrial tissue composition and function. The precise mechanisms underlying atrial fibrosis are not fully elucidated, but recent experimental studies and clinical investigations have provided valuable insights. A variety of signaling systems, particularly involving angiotensin II and related mediators, seem to be centrally involved in the promotion of fibrosis. This paper reviews the current understanding of how atrial fibrosis creates a substrate for AF, summarizes what is known about the mechanisms underlying fibrosis and its progression, and highlights emerging therapeutic approaches aimed at attenuating structural remodeling to prevent AF.

Structural and functional remodeling of the left atrium: clinical and therapeutic implications for atrial fibrillation

Left atrial (LA) structural and functional remodeling reflects a spectrum of pathophysiological changes that have occurred in response to specific stressors. These changes include alterations at the levels of ionic channels, cellular energy balance, neurohormonal expression, inflammatory response, and physiologic adaptations. There is convincing evidence demonstrating an important pathophysiological association between LA remodeling and atrial fibrillation (AF). Measures that will prevent, attenuate, or halt these processes of LA remodeling may have a major public health impact with respect to the epidemic of AF. In this review, we describe the mechanisms involved in LA remodeling and highlight the existing and potential therapeutic options for its reversal, and implications for AF development.

Electrical atrial fibrillation induction affects the characteristics of induced arrhythmia

Several methods are being used to induce atrial fibrillation (AF) in experimental investigations, which may affect the electrophysiologic parameters of the induced arrhythmia. The aim of our study was the investigation of temporal characteristics of AF during and after electrical induction. Direct current and high-frequency stimulation was used for induction in bipolar biatrial, right and left atrial appendage configurations in 6 dogs. Atrial and ventricular electrical activity was recorded near the bundle of His. Seven statistical parameters were calculated to analyze the temporal characteristics of electrical activity of both chambers. The induction method affected 5 atrial and no ventricular electrophysiologic parameters during stimulation, and the effect disappeared after ceasing induction, during the induced transient or persistent AF. Electrical stimulation affects the properties of the induced arrhythmia during the induction; thus, the investigation of AF is recommended only after ceasing the induction to avoid bias.

Atrial fibrosis and the mechanisms of atrial fibrillation

Atrial fibrillation (AF) is commonly associated with congestive heart failure (CHF), and CHF has been shown to be associated with atrial structural remodeling resulting in fibrosis. Atrial interstitial fibrosis has been seen in patients with CHF and in animal models of pacing-induced heart failure. With atrial fibrosis, conduction abnormalities result in increased AF vulnerability. The mechanism of AF associated with CHF is under debate, as both focal and reentrant mechanisms have been observed in animal models of CHF. However, recent studies using frequency-domain analysis have shown that the AF within this model is characterized by discrete, stable, high-frequency areas. The precise signaling processes involved in the development of atrial fibrosis are unknown. Angiotensin appears to play a role, as inhibition of angiotensin-converting enzyme (or angiotensin-receptor blocker) blunts atrial fibrosis in animal models of heart failure and decreases the incidence of AF in patients with heart failure. Transforming growth factor-beta (TGF-beta) also appears to play an important role. Mouse models that overexpress TGF-beta1 have profound atrial fibrosis and AF (with normal ventricles). Heart failure in canine models also produces increases in atrial TGF-beta1 expression, and inhibition of this expression prevents atrial fibrosis and the development of a substrate for AF. Atrial fibrosis appears to play a role in the development of a vulnerable substrate for AF, especially in the setting of CHF.