Cardiovasc Res 2026

HRS 2026

Background

Atrial size strongly influences atrial fibrillation (AF) development, maintenance, and therapeutic outcome, including pharmacological cardioversion. Yet, preclinical research lacks real-size models of human AF, which impedes therapeutic testing at a patient-relevant scale, encompassing intermediate and enlarged atrial sizes.

Objective

To develop a full human atrium–sized model of AF, establishing a new experimental scale for preclinical therapeutic testing to improve pharmacological cardioversion.

Methods

Monolayers of conditionally immortalized human atrial myocytes (hiAM) were created in three sizes: standard in vitro size (9.5 cm2) and young adult (55 cm2) and enlarged human atrium-sized (152 cm2). Optical voltage mapping visualized electrical activity during electrical pacing and burst pacing-induced re-entry. Arrhythmia complexity was quantified by the number of concurring circuits, which cores were tracked to assess their trajectories. Pharmacological testing included flecainide +/- ibutilide.

Results

Upon 1 Hz electrical pacing all monolayers showed uniform conduction and similar wavelength between sizes. Burst pacing-induced re-entrant circuits showed a size-dependent increase in arrhythmia complexity and circuit meandering, while dominant frequency declined with increasing size. Vehicle infusion (0.1% DMSO) had no effect on dominant frequency and arrhythmia complexity, and arrhythmias persisted in all models except for one in the small model. Yet, flecainide (3–10 μM) reduced dominant frequency and arrhythmia complexity in all models, but terminated re-entry less effectively at larger sizes. In contrast, combined treatment with 3 μM flecainide and 20 nM ibutilide enhanced re-entry termination in human atrium-sized models. Notably, this combination did terminate arrhythmias in the enlarged model—an effect not observed with 3 μM Flecainide alone—demonstrating synergistic anti-arrhythmic effects.

Conclusion

Using human atrium-sized in vitro modelling of AF, we demonstrate that size matters in the study of both arrhythmia dynamics and pharmacological responses, enabling identification of synergistic drugs for improved pharmacological cardioversion.