Enhanced calcium buffering increases atrial fibrillation burden: insights from a large-animal model

Abstract Background/Introduction Atrial fibrillation (AF) is the most common sustained arrhythmia globally, contributing to significant morbidity and mortality. Recent studies highlight the importance of calcium (Ca²+) handling disruptions in atrial myocytes as a key factor in AF initiation and persistence. Calcium buffering, a process stabilizing intracellular Ca²+ levels, has shown potential relevance in AF pathophysiology, yet its precise role remains insufficiently explored in large-animal models that closely mimic human physiology. Purpose This study aimed to investigate whether enhancing calcium buffering through levosimendan, a calcium-sensitizing agent, would increase AF burden, potentially uncovering a novel mechanism of arrhythmogenesis relevant to AF treatment. Methods Six adult Welsh Mountain sheep underwent a series of atrial pacing protocols combined with electrophysiological mapping, pre and post the administration of levosimendan. The atrial conduction velocity (CV) and effective refractory period (ERP) were assessed to evaluate conventional arrhythmic substrates, while AF burden was quantified by measuring total AF duration, episode frequency, and the longest AF episode duration. Statistical comparisons were made across groups using paired analyses, enabling direct evaluation of the effects of calcium buffering enhancement on arrhythmic activity. Data are shown as mean±SEM. Results Levosimendan significantly increased AF burden across multiple metrics. Total AF duration escalated more than fivefold, from 51.03±23.62s in controls to 301.8±114.8s with levosimendan (p=0.035). AF frequency similarly increased, rising from 5.33±1.5 episodes in controls to 9.17±1.9 episodes post-treatment (p=0.0028), while the longest single AF episode duration increased from 23.87±17.48s to 235.8±105.5s (p=0.031). Despite these marked changes in AF metrics, this could not be explained by a decrease in CV or ERP (156.7±9.19 ms vs. 161.7±7.49 ms), suggesting that the heightened AF susceptibility stems from intracellular Ca²+ cycling rather than traditional electrophysiological changes in conduction or refractoriness. Conclusion This study demonstrates that enhancing calcium buffering via levosimendan markedly increases AF burden, highlighting calcium handling as an independent and critical arrhythmogenic pathway in AF. By establishing a direct link between calcium buffering and AF susceptibility, our findings suggest that targeted modulation of calcium buffering may offer a novel therapeutic strategy for AF. Future research should focus on developing interventions that finely balance calcium buffering, with the potential to disrupt arrhythmogenic Ca²+ dynamics while preserving myocardial function, ultimately improving outcomes for AF patients.A: Conduction velocity B: ERP of atria. Atrial arrhythmia burden.