Atrial Myopathy Underlying Atrial Fibrillation

Evaluation of left atrial strain imaging and integrated backscatter as predictors of recurrence in patients with paroxysmal, persistent, and long-standing persistent atrial fibrillation undergoing catheter ablation

BACKGROUND

Impaired left atrial (LA) strain predicts atrial fibrillation (AF) recurrence after catheter ablation (CA), but currently there is no cut-off to guide patient selection for CA. Integrated backscatter (IBS) is a promising tool for noninvasive quantification of myocardial fibrosis. The aim of this study was to compare LA strain and IBS between paroxysmal, persistent, and long-standing persistent AF and evaluate their association with AF recurrence after CA.

METHODS

Analysis of consecutive patients with symptomatic paroxysmal and persistent AF who underwent CA. LA phasic strain, strain rate and IBS were assessed by two-dimensional speckle-tracking at baseline.

RESULTS

We analyzed 78 patients, 31% with persistent AF (46% long-standing AF), 65% male, mean age 59 ± 14 years, who underwent CA and were followed-up for 12 months. AF recurrence occurred in 22 (28%) patients. LA phasic strain parameters were significantly impaired in patients with AF recurrence and were independent predictors of AF recurrence in a multivariable analysis. LA reservoir strain (LASr) < 18% predicted AF recurrence with 86% sensitivity and 71% specificity, with a higher predictive power compared to LA volume index (LAVI). LASr < 22% in paroxysmal AF and LASr < 12% in persistent AF correlated with AF recurrence. Increased IBS was a predictor of AF recurrence in patients with paroxysmal AF.

CONCLUSION

LA phasic strain parameters were predictors of AF recurrence after CA, independently of LAVI and AF subtype. LASr < 18% showed a higher predictive power compared to LAVI. Further studies are needed to investigate the role of IBS as a predictor of AF recurrence.

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.

Atrial secondary tricuspid regurgitation: pathophysiology, definition, diagnosis, and treatment

Atrial secondary tricuspid regurgitation (A-STR) is a distinct phenotype of secondary tricuspid regurgitation with predominant dilation of the right atrium and normal right and left ventricular function. Atrial secondary tricuspid regurgitation occurs most commonly in elderly women with atrial fibrillation and in heart failure with preserved ejection fraction in sinus rhythm. In A-STR, the main mechanism of leaflet malcoaptation is related to the presence of a significant dilation of the tricuspid annulus secondary to right atrial enlargement. In addition, there is an insufficient adaptive growth of tricuspid valve leaflets that become unable to cover the enlarged annular area. As opposed to the ventricular phenotype, in A-STR, the tricuspid valve leaflet tethering is typically trivial. The A-STR phenotype accounts for 10%-15% of clinically relevant tricuspid regurgitation and has better outcomes compared with the more prevalent ventricular phenotype. Recent data suggest that patients with A-STR may benefit from more aggressive rhythm control and timely valve interventions. However, little is mentioned in current guidelines on how to identify, evaluate, and manage these patients due to the lack of consistent evidence and variable definitions of this entity in recent investigations. This interdisciplinary expert opinion document focusing on A-STR is intended to help physicians understand this complex and rapidly evolving topic by reviewing its distinct pathophysiology, diagnosis, and multi-modality imaging characteristics. It first defines A-STR by proposing specific quantitative criteria for defining the atrial phenotype and for discriminating it from the ventricular phenotype, in order to facilitate standardization and consistency in research.

Evolving indications for transcatheter mitral edge-to-edge repair

Transcatheter mitral edge-to-edge repair (TEER) is an established procedure for managing mitral regurgitation (MR) in high-risk patients. It is effective in treating both primary and secondary MR, as reported in the surgical and interventional literature. Over time, TEER has gained popularity and achieved procedural success in various anatomies. The less invasive nature of TEER, along with its high safety profile and immediate haemodynamic improvement suggest potential benefits in high-risk populations who are not normally included in major trials. These patients, often deemed unsuitable for surgical intervention, are typically managed conservatively, despite accumulating evidence suggesting the potential of clinical improvement by reducing MR through TEER. Examples include post-myocardial infarction MR, patients with hypertrophic obstructive cardiomyopathy and patients experiencing recurrent MR after surgical intervention. This review discusses the utilisation of TEER beyond recognised indications, examining outcomes and limitations in diverse patient populations. Further studies are warranted to evaluate the benefits of TEER in clinical scenarios beyond the current indications.

Association of the interatrial block and left atrial fibrosis in the patients without history of atrial fibrillation

Presence of left atrial (LA) fibrosis reflects underlying atrial cardiomyopathy. Interatrial block (IAB) is associated with LA fibrosis in patients with atrial fibrillation (AF). The association of IAB and LA fibrosis in the patients without history of AF is unknown. We examined association of IAB and LA fibrosis in the patients without AF history. This is a retrospective analysis of 229 patients undergoing cardiac magnetic resonance imaging (CMR). LA fibrosis was reported from spatial extent of late gadolinium enhancement of CMR. IAB was measured from 12-lead electrocardiography using digital caliper. Of 229 patients undergoing CMR, prevalence of IAB was 50.2%. Patients with IAB were older (56.9±13.9 years vs. 45.9±19.2 years, p<0.001) and had higher prevalence of co-morbidities. Left ventricular ejection fraction was lower in IAB group. LA volume index (LAVI) was greater in IAB group (54.6±24.9 ml/m2 vs. 43.0±21.1 ml/m2, p<0.001). Patients with IAB had higher prevalence of LA fibrosis than those without IAB (70.4% vs. 21.2%; p<0.001). After multivariable analysis, only IAB and LAVI were independent factors that predict LA fibrosis. Prevalence of IAB in patients undergoing CMR was high. IAB was highly associated with LA fibrosis and larger LA size in patients without AF history.

Atrial Fibrillation: Rate Versus Rhythm Control

Description Atrial fibrillation (AF) remains the most common arrhythmia worldwide and is expected to affect approximately 12 million individuals in the United States alone by 2030. Thromboembolic events remain a feared complication of AF and should be treated and risk-stratified utilizing the CHA2DS2-VASc scoring system. Other complications of AF span a wide spectrum from impaired quality of life (QoL) to an increase in all-cause mortality. Rate control strategies consist of controlling the ventricular rate and have been shown to be a safe and effective strategy for asymptomatic AF patients. In patients who are plagued with symptoms leading to impaired QoL or a decrease in exercise capacity, rhythm control with antiarrhythmic drugs or catheter ablation may be suitable options. Mortality benefits when comparing rate versus rhythm control remain equivocal when comparing multiple studies over the past decade.

Atrial Cardiopathy: Redefining Stroke Risk Beyond Atrial Fibrillation

Atrial fibrillation (AF) and ischemic stroke are dual epidemics in society, both associated with poor clinical outcomes, patient disability, and significant healthcare expenditure. The conditions are interrelated and share complex causal pathways. Risk stratification algorithms such as the CHADS₂ and CHA₂DS₂-VASc score offer predictive value in stroke and systemic embolism risk in the AF population, however, have limitations. Recent evidence suggests that an intrinsically prothrombotic atrial substrate may precede and promote AF and lead to thromboembolic events independent of the arrhythmia, allowing for a window of intervention before arrhythmia detection and development of ischemic stroke. Initial work has found incremental value in addition of parameters of atrial cardiopathy to traditional stroke risk stratification algorithms, however, requires evaluation with dedicated prospective randomized studies before use in real-world clinical practice. In this narrative review, we explore current evidence and literature on the use of measures of atrial cardiopathy in stroke risk stratification and management.

Mechanisms of Sarcomere Protein Mutation-Induced Cardiomyopathies

PURPOSE OF REVIEW

The pace of identifying cardiomyopathy-associated mutations and advances in our understanding of sarcomere function that underlies many cardiomyopathies has been remarkable. Here, we aim to synthesize how these advances have led to the promising new treatments that are being developed to treat cardiomyopathies.

RECENT FINDINGS

The genomics era has identified and validated many genetic causes of hypertrophic and dilated cardiomyopathies. Recent advances in our mechanistic understanding of sarcomere pathophysiology include high-resolution molecular models of sarcomere components and the identification of the myosin super-relaxed state. The advances in our understanding of sarcomere function have yielded several therapeutic agents that are now in development and clinical use to correct contractile dysfunction-mediated cardiomyopathy. New genes linked to cardiomyopathy include targets with limited clinical evidence and require additional investigation. Large portions of cardiomyopathy with family history remain genetically undiagnosed and may be due to polygenic disease.

Fibroblast-specific inflammasome activation predisposes to atrial fibrillation

Background

Recent work has shown that the NLR-family-pyrin-domain-containing 3 (NLRP3) inflammasome is expressed in cardiomyocytes and when specifically activated causes atrial electrical remodeling and arrhythmogenicity. Whether the NLRP3-inflammasome system is functionally important in cardiac fibroblasts (FBs) remains controversial. In this study, we sought to uncover the potential contribution of FB NLRP3-inflammasome signaling to the control of cardiac function and arrhythmogenesis.

Methods

Digital-PCR was performed to determine the expression of NLRP3-pathway components in FBs isolated from human biopsy samples of AF and sinus rhythm patients. NLRP3-system protein expression was determined by immunoblotting in atria of canines with electrically maintained AF. Using the inducible, resident fibroblast (FB)-specific Tcf21-promoter-Cre system (Tcf21iCre as control), we established a FB-specific knockin (FB-KI) mouse model with FB-restricted expression of constitutively active NLRP3. Cardiac function and arrhythmia susceptibility in mice were assessed by echocardiography, programmed electrical stimulation, and optical mapping studies.

Results

NLRP3 and IL1B were upregulated in atrial FBs of patients with persistent AF. Protein levels of NLRP3, ASC, and pro-Interleukin-1β were increased in atrial FBs of a canine AF model. Compared with the control mice, FB-KI mice exhibited enlarged left atria (LA) and reduced LA contractility, a common determinant of AF. The FBs from FB-KI mice were more transdifferentiated, migratory, and proliferative compared to the FBs from control mice. FB-KI mice showed increased cardiac fibrosis, atrial gap junction remodeling, and reduced conduction velocity, along with increased AF susceptibility. These phenotypic changes were supported by single nuclei (sn)RNA-seq analysis, which revealed enhanced extracellular matrix remodeling, impaired communication among cardiomyocytes, and altered metabolic pathways across multiple cell types.

Conclusions

Our results show that the FB-restricted activation of the NLRP3-inflammasome system leads to fibrosis, atrial cardiomyopathy, and AF. Activation of NLRP3-inflammasome in resident FBs exhibits cell-autonomous function by increasing the activity of cardiac FBs, fibrosis, and connexin remodeling. This study establishes the NLRP3-inflammasome as a novel FB-signaling pathway contributing to AF pathogenesis.

The Role of Advanced Echocardiographic Parameters of the Left Atrial Function in the Incidence of Cryptogenic Ischemic Stroke: A Review

Stroke is one of the primary causes of morbidity and mortality worldwide, including a large proportion of cryptogenic strokes. Long-term electrocardiographic monitoring is beneficial in prospective studies for detecting atrial fibrillation in patients with cryptogenic stroke. This review aimed to evaluate the advanced echocardiographic parameters and their roles in assessing left atrial (LA) function in the incidence of cryptogenic and ischemic strokes. Main resources, including PubMed, Scopus, and ISI Web of Science databases, were evaluated for articles published in English from 2010 until May 2021. LA echocardiographic parameters such as LA strain and strain rate, isovolumetric relaxation time (IVRT), the mean left atrial volume index (LAVI), LA reservoir volume, systole strain rate (SSR) of left atrial appendage, and lack of LA function response to maximal exercise as measured by the LA ejection fraction during rest and exercise could be considered for assessing the risk of cryptogenic strokes and ischemic strokes. The results indicated that increased LA volumes and reduced LA strain rate were correlated with cryptogenic stroke. Advanced parameters of LA function, measured by speckle tracking echocardiography, such as strain and strain rate values in different parts of the cardiac cycle, in addition to standard measures of LA function such as LA ejection fraction and LAVI, will define an excellent understanding regarding LA myopathy and risk assessment of cryptogenic stroke, independent of considering conventional cardiovascular risk factors.

Left Atrial Cardiomyopathy - A Challenging Diagnosis

Left atrial cardiomyopathy (LACM) has been an ongoing focus of research for several years. There is evidence that LACM is responsible for atrial fibrillation and embolic strokes of undetermined sources. Therefore, the correct diagnosis of LACM is of clinical importance. Various techniques, including electrocardiography, echocardiography, cardiac magnetic resonance imaging, computed tomography, electroanatomic mapping, genetic testing, and biomarkers, can both identify and quantify structural, mechanical as well as electrical dysfunction in the atria. However, the question arises whether these techniques can reliably diagnose LACM. Because of its heterogeneity, clinical diagnosis is challenging. To date, there are no recommendations for standardized diagnosis of suspected LACM. However, standardization could help to classify LACM more precisely and derive therapeutic directions to improve individual patient management. In addition, uniform diagnostic criteria for LACM could be important for future studies. Combining several parameters and relating them seems beneficial to approach the diagnosis of LACM. This review provides an overview of the current evidence regarding the diagnosis of LACM, in which several potential parameters are discussed and, consequently, a proposal for a diagnostic algorithm is presented.

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.

From Left Atrial Dimension to Curved M-Mode Speckle-Tracking Images: Role of Echocardiography in Evaluating Patients with Atrial Fibrillation

Left atrial (LA) enlargement and dysfunction increase the risk of atrial fibrillation (AF). Traditional echocardiographic evaluation of the left atrium has been limited to dimensional and semi-quantification measurement of the atrial component of ventricular filling, with routine measurement of LA function not yet implemented. However, functional parameters, such as LA emptying fraction (LAEF), may be more sensitive markers for detecting AF-related changes than LA enlargement. Speckle-tracking echocardiography has proven to be a feasible and reproducible technology for the direct evaluation of LA function. The clinical application, advantages, and limitations of LA strain and strain rate need to be fully understood. Furthermore, the prognostic value and utility of this technique in making therapeutic decisions for patients with AF need further elucidation. Deep learning neural networks have been successfully adapted to specific tasks in echocardiographic image analysis, and fully automated measurements based on artificial intelligence could facilitate the clinical diagnostic use of LA speckle-tracking images for classification of AF ablation outcome. This review describes the fundamental concepts and a brief overview of the prognostic utility of LA size, LAEF, LA strain and strain rate analyses, and the clinical implications of the use of these measures.

Global Substrate Mapping and Targeted Ablation with Novel Gold-tip Catheter in De Novo Persistent AF

Results from catheter ablation for persistent AF are suboptimal, with no strategy other than pulmonary vein isolation showing clear benefit. Recently employed empirical strategies beyond pulmonary vein isolation involve widespread atrial ablation in all patients and do not take into account patient-specific differences in AF mechanisms or phenotype. Charge density mapping using the non-contact AcQMap system (Acutus Medical) allows visualisation of whole-chamber activation during AF and reveals localised patterns of complex activation thought to represent important mechanisms for AF maintenance that can be targeted with focal ablation. In this review, the authors outline the fundamentals of this technology, the initial data exploring the mechanistic role of activation patterns seen and the application to ablation of persistent AF.

Left atrial booster-pump function as a predictive parameter for atrial fibrillation in patients with severely dilated left atrium

BACKGROUND

Left atrial (LA) dimension ≥50 mm had approximately four times the risk of developing atrial fibrillation (AF). The aim of this study was to investigate whether the application of clinical and echocardiographic parameters could differentiate between the patients having severely dilated left atrium with and without AF.

METHODS

This retrospective cross-sectional study enrolled consecutive patients with LA dimension ≥50 mm and divided them into three groups: no AF (no-AF), paroxysmal AF (PAF) and non-paroxysmal AF (non-PAF) groups. For PAF and non-PAF groups, all patients underwent radiofrequency ablation, and the echocardiographic parameters were obtained on the next day after ablation.

RESULTS

Our study population comprised 160 patients, including 80, 53, and 27 patients in the non-AF, PAF and non-PAF groups, respectively. The no-AF group had a significantly higher body mass index (kg/m2) (29.31±6.27, 27.58±4.12 and 26.57±2.81, P=0.01), and a higher prevalence of diabetes mellitus (DM) [31 (38.80%), 13 (25.00%) and 4 (14.80%), P=0.01] and hypertension [67 (83.80%), 34 (65.40%), and 19 (70.40%), P=0.04], but a lower prevalence of rheumatic heart disease (RHD) [3 (3.80%), 6 (11.50%) and 5 (18.50%), P=0.02] and sick sinus syndrome [0 (0.00%), 6 (11.50%) and 4 (14.80%), P=0.045]. Echocardiographic studies showed that the non-AF group had significantly smaller LA minimal volume index (24.89±9.74, 34.06±19.38 and 42.83±17.44 mL/m2, P<0.01), higher LA emptying fraction (51.99%±13.97%, 38.40%±15.96% and 33.89%±10.73%, P<0.01), longitudinal strain (23.87%±7.72%, 17.11%±8.52% and 12.38%±4.28%, P<0.01) and strain rate than the AF groups. The multivariate analysis showed that the late diastolic component of LA strain rate was the only independent factor associated with the presence of AF (odds ratio, 21.69; 95% CI, 9.77-48.13, P<0.01).

CONCLUSIONS

LA function plays an important role in the absence of AF in patients with LA dimension ≥50 mm; the late diastolic component of LA strain rate was the only independent variable on multivariate analysis.

Left atrial function in male veteran endurance athletes with paroxysmal atrial fibrillation

Aims

To assess left atrial (LA) function in sinus rhythm in veteran athletes with a history of paroxysmal atrial fibrillation (AF) exposed to prolonged endurance exercise compared with veteran athletes without AF and controls with and without paroxysmal AF from a non-athletic population.

Methods and results

Three hundred and two male participants from four groups, veteran recreational skiers with paroxysmal AF (n = 62), veteran skiers without AF (n = 89), and controls from a non-athletic population with (n = 62) and without paroxysmal AF (n = 89) underwent an echocardiographic examination in sinus rhythm to evaluate LA anatomy and function. The skiers (mean age 70.8±6.7 years) reported an average exposure to regular endurance exercise for 40–50 years. LA maximum and minimum volumes were larger in skiers (P < 0.001). LA volumes differed within the athletic and non-athletic groups with larger volumes in the AF groups ( P < 0.001). We observed a considerable overlap in LA volumes among non-athletes with AF and athletes without AF. LA reservoir strain (33.6% ± 4.8% vs. 28.3% ± 6.7% P < 0.001) and contractile strain (18.3% ± 4.0% vs. 15.0% ± 5.2% P < 0.001) were lower in both AF groups regardless of athletic status. LA reservoir strain was superior to volumetric measurements at identifying participants with AF (area under the curve 0.740 ±0.041).

Conclusion

Male veteran athletes had significantly larger LA volumes than non-athletes. In contrast, LA strain values were similar in athletes and non-athletes with paroxysmal AF, and significantly lower than in subjects without AF.

P wave duration ≥150 ms predicts poor left atrial function and ablation outcomes in non-paroxysmal atrial fibrillation

BACKGROUND

It remains unknown whether P wave duration (PWD) ≥ 150 ms measured after extensive radiofrequency catheter ablation (RFCA) can identify non-paroxysmal atrial fibrillation (non-PAF) patients at increased risk of atrial tachyarrhythmia recurrence. We investigated the predicting power of PWD and its association with left atrial (LA) reverse remodeling in patients with non-PAF undergoing pulmonary vein isolation with LA linear ablation.

METHODS

We retrospectively evaluated 136 patients who underwent RFCA for drug-refractory non-PAF. Electroanatomic mapping was acquired during AF. Low-voltage area (LVA) was defined as an area with bipolar voltage ≤0.5 mV. Electrocardiography and echocardiography were performed during sinus rhythm 1 day and 3 months after RFCA. PWD was measured using amplified 12‑lead electrocardiography. Prolonged PWD was defined as maximum PWD ≥ 150 ms.

RESULTS

Over a mean follow-up duration of 48 ± 35 months, 28 patients experienced atrial tachyarrhythmia recurrence. PWD was positively correlated with LVA (r = 0.527, p < 0.001) and inversely correlated with LA emptying fraction (r = -0.399, p < 0.001). PWD was shortened and LA emptying fraction (LAEF) was increased in patients without atrial tachyarrhythmia recurrence during follow-up. Atrial tachyarrhythmia-free survival was significantly more likely in patients without a prolonged PWD (83.5% vs 60.7%, p = 0.002). Multivariate analysis showed that LAEF and PWD were independent predictors of atrial tachyarrhythmia recurrence.

CONCLUSIONS

PWD ≥ 150 ms measured after RFCA can identify patients with non-PAF at increased risk of atrial tachyarrhythmia recurrence. PWD is correlated with LVA and LAEF and reflects LA reverse remodeling.

Impact of the Pattern of Atrial Fibrillation on Stroke Risk and Mortality

Thromboembolism is the most serious complication of AF, and oral anticoagulation is the mainstay therapy. Current guidelines place all AF types together in terms of anticoagulation with the major determinants being associated comorbidities translated into risk marker. Among patients in large clinical trials, those with non-paroxysmal AF appear to be at higher risk of stroke than those with paroxysmal AF. Higher complexity of the AF pattern is also associated with higher risk of mortality. Moreover, continuous monitoring of AF through cardiac implantable devices provided us with the concept of ‘AF burden’. Usually, the larger the AF burden, the higher the risk of stroke; however, the relationship is not well characterised with respect to the threshold value above which the risk increases. The picture is more complex than it appears: AF and underlying disorders must act synergically respecting the magnitude of its own characteristics, which are the amount of time a patient stays in AF and the severity of associated comorbidities.

Left Atrial Myopathy in Atrial Fibrillation and Heart Failure: Clinical Implications, Mechanisms, and Therapeutic Targets

PURPOSE OF REVIEW

This review discusses the mechanisms, clinical implications, and treatments of left atrial (LA) myopathy in comorbid atrial fibrillation (AF) and heart failure (HF) across the spectrum of ejection fraction.

RECENT FINDINGS

AF and HF are highly comorbid conditions. Left atrial (LA) myopathy, characterized by impairments in LA structure, function, or electrical conduction, plays a fundamental role in the development of both AF and HF with preserved ejection fraction (AF-HFpEF) along with AF and HF with reduced ejection fraction (AF-HFrEF). While the nature of LA myopathy in AF-HFpEF is unique from that of AF-HFrEF, LA myopathy also leads to progression of both of these conditions. There may be a vulnerable cohort of AF-HF patients who have a disproportionate degree of LA myopathy compared with left ventricular (LV) dysfunction. Further investigations are required to identify therapies to improve LA function in this cohort.

Inducibility, but not stability, of atrial fibrillation is increased by NOX2 overexpression in mice

Aims

Gp91-containing NADPH oxidases (NOX2) are a significant source of myocardial superoxide production. An increase in NOX2 activity accompanies atrial fibrillation (AF) induction and electrical remodelling in animal models and predicts incident AF in humans; however, a direct causal role for NOX2 in AF has not been demonstrated. Accordingly, we investigated whether myocardial NOX2 overexpression in mice (NOX2-Tg) is sufficient to generate a favourable substrate for AF and further assessed the effects of atorvastatin, an inhibitor of NOX2, on atrial superoxide production and AF susceptibility.

Methods and results

NOX2-Tg mice showed a 2- to 2.5-fold higher atrial protein content of NOX2 compared with wild-type (WT) controls, which was associated with a significant (twofold) increase in NADPH-stimulated superoxide production (2-hydroxyethidium by HPLC) in left and right atrial tissue homogenates (P = 0.004 and P = 0.019, respectively). AF susceptibility assessed in vivo by transoesophageal atrial burst stimulation was modestly increased in NOX2-Tg compared with WT (probability of AF induction: 88% vs. 69%, respectively; P = 0.037), in the absence of significant alterations in AF duration, surface ECG parameters, and LV mass or function. Mechanistic studies did not support a role for NOX2 in promoting electrical or structural remodelling, as high-resolution optical mapping of atrial tissues showed no differences in action potential duration and conduction velocity between genotypes. In addition, we did not observe any genotype difference in markers of fibrosis and inflammation, including atrial collagen content and Col1a1, Il-1β, Il-6, and Mcp-1 mRNA. Similarly, NOX2 overexpression did not have consistent effects on RyR2 Ca²⁺ leak nor did it affect PKA or CaMKII-mediated RyR2 phosphorylation. Finally, treatment with atorvastatin significantly inhibited atrial superoxide production in NOX2-Tg but had no effect on AF induction in either genotype.

Conclusion

Together, these data indicate that while atrial NOX2 overexpression may contribute to atrial arrhythmogenesis, NOX2-derived superoxide production does not affect the electrical and structural properties of the atrial myocardium.