Sessions

Atrial fibrillation (AF) is the most common clinically relevant arrhythmia in horses and may result in reduced performance and economic value. Recent technological advances have enabled investigation of the potential role of myocardial sleeves (MSs) in equine AF, a role that had previously been demonstrated in human patients. Myocardial sleeves extending into the venae cavae have also been identified as potential sites of ectopic impulse generation. Although increasing attention has been devoted to the anatomical and physiological characterization of these regions in horses, information on the detailed histological organization of caval MSs remains limited. The presence of cellular and tissue components potentially involved in impulse generation and conduction in these regions has not yet been characterized immunohistochemically.

In this study, anatomical, histological, and immunohistochemical examinations were performed on samples obtained from 21 warmblood horses slaughtered at an abattoir (13 mares, 6 geldings, and 2 stallions). Our aim was to characterize the myocardial sleeves of the cranial and caudal venae cavae (CrVC and CaVC) histologically and immunohistochemically, compare their morphological characteristics, investigate tissue components potentially involved in impulse generation and conduction, and determine the diameters of the two vessels.

The mean diameter of the CrVC was 43.5 ± 2.8 mm (n = 16), while that of the CaVC was 43.0 ± 4.0 mm (n = 21). Morphological examination revealed considerable variability in the appearance of the MSs, with a more sharply defined termination in the CrVC than in the CaVC.

In addition to hematoxylin and eosin staining, Mallory’s trichrome staining was used for histological examination. The markers selected for immunohistochemical analysis based on the human literature were connexin 40 (Cx40), connexin 43 (Cx43), connexin 45 (Cx45), hyperpolarization-activated cyclic nucleotide-gated potassium channel 4 (HCN4), choline acetyltransferase (ChAT), and tyrosine hydroxylase (TH). All investigated markers were detected within the MS region, either in the cardiomyocytes comprising the myocardial sleeves or in the surrounding connective tissue structures, in both the CrVC and CaVC. The presence of these markers suggests a potential role of the MS region in impulse generation and conduction, as well as in autonomic regulation; however, their abundance and distribution showed considerable inter-individual and inter-sample variability. Further characterization of these regions and their electrophysiological properties may contribute to a better understanding of the anatomical basis of atrial fibrillation and may facilitate the planning of transcatheter ablation treatments.