Morphological evidence of a potential arrhythmogenic substrate in the caudal and cranial vena cava in horses

Background Three-dimensional electro-anatomical mapping, previously performed in horses with atrial arrhythmias, has demonstrated the medial region of the caudal vena cava (CaVC), 1-8 cm caudal to the fossa ovalis, as an anatomical predilection site for atrial tachycardia associated with areas of slow conduction and conduction block. Slow conduction has also been recorded in the cranial vena cava (CrVC).ObjectivesTo investigate the morphological characteristics of the myocardial sleeves (MS) in the CaVC and CrVC, in order to identify a potential substrate of right sided atrial arrhythmias. Study design Cross sectional. Methods Post-mortem dissection of 37 hearts from adult warmblood horses without known cardiovascular disease. Macroscopic examination of the myocardial distribution, evaluated the MS area, length, width, and shape in the CaVC and the CrVC. At least 2 samples from each vena cava MS were histologically examined using Masson's trichrome staining. Results Myocardial sleeves into the medial CaVC and into the CrVC were observed in all horses and showed variations in distribution, shape, and size between horses. Their mean +/- standard deviation length from the limbus into the CaVC reached 5.7 +/- 1.0 cm (maximum 8.3 cm), and from the azygos vein into the CrVC 5.3 +/- 1.6 cm (maximum 8.6 cm). Myocardium-free islands were observed in the CaVC and CrVC in 30% and 6% of horses, respectively. Histologically, MS showed a non-uniform myocardial fibre arrangement, with presence of fibroadipose tissue, features known to result in slow conduction and pro-arrhythmia. Main limitations Study only included horses without history of atrial arrhythmia. Conclusions Myocardial sleeves are present in both CaVC and CrVC, showing anatomical variations between horses. Tissue characteristics known to favour re-entry were identified indicating that these venae cavae MS are a potential substrate for atrial tachyarrhythmias and a target for treatment by ablation.