Atrial fibrillation in heart failure: steady progress but still a long way to go

Atrial fibrillation promotion in a rat model of heart failure induced by left ventricle radiofrequency ablation

Background

Atrial fibrillation (AF) frequently coexists with congestive heart failure (CHF). The increased susceptibility to AF in CHF has been attributed to a variety of structural and electrophysiological changes in the atria, particularly dilation and interstitial fibrosis. We evaluated atrial remodeling and AF vulnerability in a rat model of CHF induced by left ventricle (LV) radiofrequency (RF) ablation.

Methods

Wistar rats were divided into 3 groups: RF-induced CHF (Ab, n = 36), CHF animals treated with spironolactone (AbSpi, n = 20) and sham controls (Sham, n = 29). After 12 weeks, animals underwent echocardiographic and electrophysiological evaluation and were sacrificed for histological (atrial fibrosis) and Western blotting (TGF-β1, collagen I/III, connexin 43 and Ca_V1.2) analysis.

Results

Mild LV dysfunction and marked atrial enlargement were noted in both ablated groups. AF inducibility (episodes ≥2 s) increased in the Ab group compared to sham animals (31/36, 86%; vs. 15/29, 52%; p = 0.005), but did not differ from the AbSpi group (16/20, 80%; p = NS). Sustained AF (>30 s) was also more frequent in the Ab group compared to shams (56% vs. 28%; p = 0.04). Spironolactone reduced atrial fibrosis (p < 0.01) as well as TGF-β1 (p < 0.01) and collagen I/III (p < 0.01) expression but did not affect connexin 43 and Ca_V1.2 expression.

Conclusions

Rats with RF-induced CHF exhibit pronounced atrial structural remodeling and enhanced AF vulnerability. This model may be useful for studying AF substrate in CHF.

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.

Atrial fibrillation complicated by heart failure induces distinct remodeling of calcium cycling proteins

Atrial fibrillation (AF) and heart failure (HF) are two of the most common cardiovascular diseases. They often coexist and account for significant morbidity and mortality. Alterations in cellular Ca²⁺ homeostasis play a critical role in AF initiation and maintenance. This study was designed to specifically elucidate AF-associated remodeling of atrial Ca²⁺ cycling in the presence of mild HF. AF was induced in domestic pigs by atrial burst pacing. The animals underwent electrophysiologic and echocardiographic examinations. Ca²⁺ handling proteins were analyzed in right atrial tissue obtained from pigs with AF (day 7; n = 5) and compared to sinus rhythm (SR) controls (n = 5). During AF, animals exhibited reduction of left ventricular ejection fraction (from 73% to 58%) and prolonged atrial refractory periods. AF and HF were associated with suppression of protein kinase A (PKA)_RII (-62%) and Ca²⁺-calmodulin-dependent kinase II (CaMKII) δ by 37%, without changes in CaMKIIδ autophosphorylation. We further detected downregulation of L-type calcium channel (LTCC) subunit α₂ (-75%), sarcoplasmic reticulum Ca²⁺-ATPase (Serca) 2a (-29%), phosphorylated phospholamban (Ser16, -92%; Thr17, -70%), and phospho-ryanodine receptor 2 (RyR2) (Ser2808, -62%). Na⁺-Ca²⁺ exchanger (NCX) levels were upregulated (+473%), whereas expression of Ser2814-phosphorylated RyR2 and LTCCα_1c subunits was not significantly altered. In conclusion, AF produced distinct arrhythmogenic remodeling of Ca²⁺ handling in the presence of tachycardia-induced mild HF that is different from AF without structural alterations. The changes may provide a starting point for personalized approaches to AF treatment.