Ostium-specific equine pulmonary vein myocardial sleeve gross anatomy and structural evidence of interatrial myocardial continuity with potential implications for ablation strategies.

BACKGROUND: Pulmonary vein (PV) myocardial sleeves (MSs) harbour an arrhythmogenic substrate for atrial fibrillation. However, equine-specific data on MS distribution and wall thickness are limited yet essential for developing ablation strategies. In humans, posterior interatrial connections can compromise right-sided PV isolation. OBJECTIVES: To study equine PV MS distribution, quantify ostial/branch diameters, antral lengths, characterise MS distal edge morphology, measure wall thickness and explore an interatrial myocardial continuity between the MSs of caudal branches draining through ostium III (PV III) and caudal vena cava (CaVC). STUDY DESIGN: Post-mortem, cross-sectional. METHODS: Twenty-three Warmblood hearts underwent left atrial and PV dissection. All four ostia (I-IV) were opened. From calibrated photographs, ostial and branch diameters, antral length, MS length and coverage, distal edge morphology (sharp/blurred), and myocardial thickness were assessed at standardised locations. In 6/23 hearts, a 3 × 1 cm dorsal interatrial septum (IAS) sample including PV III, and CaVC MSs was stained with Masson's trichrome and examined for myocardial continuity. RESULTS: Four principal ostia were present in 20/23 hearts and 5 in 3/23, with variable accessory vein openings. Ostia II and III were largest, I smallest. PV III showed full antral MS coverage with branch extensions, whereas MSs in other PVs were often confined to the antrum. Distal MS edges were often blurred at PV III. The MS thickness was largest at ostia II and III and smallest at ostium I. Histology showed that the dorsal interatrial myocardium was continuous with the PV III and CaVC MSs. MAIN LIMITATIONS: Postmortem study, Warmblood cohort, few samples, no electrophysiology. CONCLUSIONS: Equine PV MS anatomy is ostium-specific in extent, distal edge, and thickness. These data provide anatomical information relevant to ablation planning and to future catheter design. Morphological evidence suggests a dorsal interatrial myocardial continuity through the IAS that warrants electro-anatomical confirmation.