Cardiac arrhythmias are common in horses and may have dramatic consequences such as collapse or sudden death. Sudden death occurs 10 times more frequently in horses compared to human athletes. Options for arrhythmia treatment are currently limited to drugs or electrical shocks with one major problem: a high recurrence rate. Transcatheter radiofrequency ablation is a minimally invasive procedure in which local heating through a thin tube in the heart is used to destroy tissue that causes the arrhythmia. This not only resolves the arrhythmia, it also drastically reduces recurrence risk. Radiofrequency ablation has been performed by our group in 8 horses but was associated with 2 deaths, 1 failure and 1 recurrence. Because important differences in myocardial properties and blood flow exist between horses and humans, the aim of this project is to develop a safe and efficient horse-adapted catheter ablation technique. The first objective is to evaluate normal and abnormal electrical activation of the equine heart in standing horses, by electrocardiography, intracardiac multiple catheter recording and 3D mapping, to identify the affected cardiac chamber. The second objective is to investigate the effect of ablation settings and blood flow on equine ablation lesion characteristics in vitro and in vivo. The ultimate goal is to apply a standardized approach for radiofrequency ablation in horses with atrial arrhythmias.