Applied cardiac anatomy to improve diagnosis and treatment of equine supraventricular arrhythmias

Supraventricular arrhythmias, particularly atrial tachycardia and atrial fibrillation, are clinically important in horses, compromising athletic performance and posing a potential risk to both horse and rider. Medical or electrical cardioversion can resolve these arrhythmias, however, the recurrence rate is high due to persistence of the underlying arrhythmogenic substrate. While advanced three-dimensional electro-anatomical mapping (3D EAM) technologies and catheter ablation techniques have revolutionised diagnostic and therapeutic capabilities by identifying and targeting the arrhythmogenic substrate, an optimal application would benefit from a comprehensive understanding of equine cardiac anatomy which remains incomplete in several critical areas. The foundational step of placing a reference catheter in the coronary sinus is essential for 3D EAM procedures, yet this step is often complicated by anatomical obstacles within this area. Myocardial extensions in the coronary sinus and great cardiac vein, and the spatial relationship with the neighbouring myocardium have been previously undefined, potentially compromising accurate electrophysiological signal recording and interpretation. Three-dimensional EAM identifies the caudal vena cava as the favoured site for macro-reentry atrial tachycardia in horses, however, the anatomical and histological characterisation of this region, specifically the presence and nature of arrhythmogenic myocardial sleeves, is completely unknown. Furthermore, left atrial access via transseptal puncture is preferred over the retrograde atrial approach which requires carotid artery catheterisation and is more technically challenging. For a safe transseptal puncture technique, precise knowledge of the equine interatrial septum, its wall thickness, composition, or morphological variations is needed. Given that pulmonary veins are the primary targets of atrial fibrillation ablation in humans and pulmonary vein myocardial sleeves exhibit pro-arrhythmogenic characteristics in horses, this region warrants investigation as an ablation target. However, the distribution of the myocardial sleeves and their length into the different branches as well as the wall thickness at the pulmonary vein to left atrial junction has not yet been quantified, which is important for planning effective transmural ablation lesions tailored to the thick equine heart. The general aim of this thesis is to provide anatomical and histological characterisation of these key atrial structures relevant to supraventricular arrhythmias, electrophysiological studies and catheter ablation in horses. By filling critical knowledge gaps, this work aims to provide foundational data necessary to improve diagnostic confidence, enable safer left atrial access, and support the development of ablation strategies tailored specifically to the equine heart. Chapter 3 characterises the coronary sinus and great cardiac vein to guide catheter placement, electrogram interpretation, and explore potential arrhythmogenic features. Multiple valves, including the Thebesian valve at the coronary sinus ostium and additional valves within the great cardiac vein, together with the nearby vein of Marshall and middle cardiac vein opening near the coronary sinus ostium, are identified and likely account for catheterisation difficulties, failure to enter the great cardiac vein, and occasional catheter malpositioning. This study provides the first morphological description of a right atrial myocardial sleeve extending several centimetres into the great cardiac vein. Histological analysis confirms that this myocardial sleeve forms muscular connections with the adjacent left atrial myocardium, analogous to the "inferior interatrial connection" in humans. A key finding is that the coronary sinus and great cardiac vein spatial relationship within the coronary groove changes along its course. Proximally, it is located further away from the left atrial myocardium (median 10 mm) than from the left ventricular myocardium (median 2 mm), indicating that signals recorded here likely originate from the right atrial myocardial sleeves and left ventricular myocardium. A similar relationship is observed in the distal part of the coronary groove, with the great cardiac vein being located further from the left atrial myocardium (median 32 mm) and closer to the left ventricular myocardium (median 5 mm). Conversely, in the middle of the coronary groove, the great cardiac vein lies closest to the left atrial myocardium (median, 2 mm), identifying it as the optimal location for recording left atrial signals and pacing. In relation to the mitral annulus, the great cardiac vein exhibits similar dynamic spatial relationships as with the left ventricular myocardium, which has important implications for potential feasibility of ablation procedures targeting accessory pathways via the great cardiac vein. This could be attempted at specific locations but might still be complicated by the by presence of adipose tissue between the vein and the mitral annulus. Three-dimensional EAM in horses with atrial tachycardia shows that macro reentry circuits characterised by an area of slow conduction around a line of conduction block typically originate near the caudal vena cava to right atrial junction. In Chapter 4 the caudal and cranial vena cava are examined, and myocardial sleeves are reported for the first time in the wall of both veins. Histological analysis reveals that these myocardial sleeves possess pro-arrhythmic features, including non-uniform myocardial fibre arrangement and significant fibroadipose interruptions, which create a substrate for slow conduction. A major finding is the identification of myocardium-free islands surrounded by myocardium in both veins, mainly the caudal vena cava, providing a direct structural basis for the lines of conduction block and macro-reentrant circuits observed during 3D EAM in clinical atrial tachycardia cases. These findings establish the presence of a structural substrate for atrial arrhythmias in the venae cavae. Chapter 5 characterises the morphology of the adult equine interatrial septum, the gateway for left atrial access through transseptal puncture. The study reveals significant morphological variations and major differences from human anatomy. Most notably, a right-sided septal pouch is consistently present in all horses, whereas no patent foramen ovale was identified in this study. Histological analysis shows that the septum wall consisted of three layers, including a central muscle layer containing cardiomyocytes in complex orientations interspersed with fibroadipose tissue, which is a potential arrhythmogenic substrate. Wall composition also varies, with the highest proportion of connective tissue (78%) found at the level of the right-sided septal pouch, suggesting that this location may be more difficult to puncture than caudal to the limbus. Chapter 6 focuses on the anatomy of the pulmonary veins and associated myocardial sleeves to facilitate pulmonary vein isolation using ablation strategies. Among the four principal pulmonary vein ostia, the myocardial sleeve length, distribution, and thickness vary significantly by location. Pulmonary vein III consistently exhibits the most extensive myocardial sleeve, showing full antral coverage with extension into its branches in all studied horses. The myocardial wall at the veno-atrial junction is the thickest at ostia II and III and the thinnest at the measured locations around ostium I. These findings demonstrate that ablation strategies must be "ostium-tailored," prioritising, complex substrates of ostium III while applying cautious energy delivery to the thin-walled ostium I. Furthermore, the study histologically confirms the hypothesis of a dorsal interatrial connection: the interatrial septum myocardium is continuous with both caudal vena cava and the pulmonary vein III myocardial sleeves. This interatrial connection could potentially undermine the durability of ostium III isolation procedures. Together, these chapters deliver an equine-specific anatomical reference for the atria that links measurements to clinical practice. For coronary sinus catheterisation, the findings recommend anticipating the coronary sinus and great cardiac vein valves, the vein of Marshall, and the middle cardiac vein at entry. Using a J-tip guidewire would allow following the path of least resistance within the largest vessel. Additionally, for left atrial timing reference and pacing, the mid great cardiac vein location should be favoured. For ablation, the caudal vena cava myocardial sleeve should be considered a potential structural substrate for atrial tachycardia, and pulmonary vein isolation, should prioritise lesion design around ostia III, where the myocardial sleeve is most extensive. The identification of a dorsal interatrial connection encourages operators to verify the true electrical isolation of the pulmonary vein III and to consider complementary lines if remaining conduction is suspected. This work is limited by being based on post-mortem material from adult Warmblood horses with predefined histology subsets and without immunohistochemistry or in vivo electrophysiological testing. Nevertheless, the consistency of gross anatomical patterns across specimens, quantitative myocardial sleeve characterisation, and alignment with clinical observations from 3D EAM make the dataset immediately useful for procedural planning and interpretation. Anatomical data also provide reference values that can inform the design of equine-specific ablation catheters. In conclusion, this dissertation establishes comprehensive anatomical and histological foundations that were previously lacking in equine cardiac electrophysiology. It identifies the anatomical challenges for coronary sinus and great cardiac vein catheterisation and suggests the mid-great cardiac vein as an anatomically ideal location for recordi