Morphological investigation of myocardial sleeves with potential relevance to atrial fibrillation in Norwegian-Swedish Coldblooded Trotters

Myocardial sleeves (MSs) are thin layers of cardiac muscle that extend from the atria into the caval and pulmonary veins. They are recognised as important structures in both initiating and maintaining atrial tachycardia, and especially atrial fibrillation (AF), which is the most clinically important cardiac arrhythmia in horses with a reported prevalence of 1.3-2% in Standardbred racehorses. In contrast, the Norwegian-Swedish Coldblooded Trotter (NSCT) has a much lower prevalence of AF despite experiencing similar physical demands. The purpose of this study was to describe the macro-, and microscopic features of the caval MSs in NSCTs including quantifying the amount of fibrosis to investigate whether this breed has anatomical properties that might protect against the development of AF. The hearts of 33 horses were collected during postmortem examination and measurements of atrial structures and MSs were performed. MSs were present in both caval veins with the cranial vena cava (CrVC) displaying a clearer border compared to the caudal vena cava (CaVC). Myocardial islands were present only in the CaVC, and there was significantly more interstitial fibrosis in the MS of the CaVC than in the CrVC. Varying orientation of myocardium with a mix of transverse, longitudinal, and oblique myofibres was evident, especially in the CaVC. The caval MSs in NSCTs displayed many similarities with other horse breeds with a higher reported prevalence of AF. The findings indicate that NSCTs have an anatomical substrate that might allow development and maintenance of atrial arrhythmias. Other structural or electrical properties might be protective in this breed.