Veterinary session

Atrial fibrillation is the most common arrhythmia in horses causing poor performance. The role of pulmonary vein triggers in the pathogenesis of atrial fibrillation has been identified in horses. The aim of the study was to describe the morphological properties of left atrial pulmonary vein ostia in healthy horses which is essential for the development of ablation methods. Eighty-three equine hearts were dissected, tissue samples for histology were taken from the four main pulmonary vein ostia. These samples contained tissue from the myocardial sleeve. S100 and tyrosine hydroxylase (TH) were used to detect adrenergic and non-adrenergic fibres, and connexin 40, 43 and 45 antibodies were used to detect elements of the conduction system. Canine heart tissue was used as a positive control. The mean number of pulmonary vein ostia was 4.5 (4 veins: 46 horses, 5 veins: 31 horses, 6 veins: 6 horses). Diameters (mean±SD) of the main ostia were as follows: vein I: 20.2±7.0 mm, vein II: 32.7±7.1 mm, vein III: 33.4±5.9 mm, vein IV: 18.1±4.5 mm. Diameters of supernumerary vein ostia varied between 3.0 and 28.0 mm (mean±SD: 11,5±5.5 mm). Early branching was found in 26 horses (31.3%) and 30 veins (vein I: 14, vein II: 9, vein III: 5, vein IV: 2). Histology confirmed the presence of a muscle sleeve composed of myocardial tissue in each pulmonary vein. S100 and TH positivity was detected in each vein, and it confirmed the presence of adrenergic and non-adrenergic nerve fibres within the myocardial sleeve. Connexin 40, 43 and 45 positivity was also found in each vein indicating the presence of conducting tissue. Future ablation techniques should consider that conductive tissue is present in the entire myocardial sleeve in all pulmonary vein ostia.