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

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

Overview

• This study examines the structure of myocardial sleeves (thin cardiac muscle layers extending into veins) in Norwegian-Swedish Coldblooded Trotters (NSCTs), aiming to understand anatomical reasons behind their unusually low rates of atrial fibrillation (AF).

Background and Significance

• Myocardial Sleeves (MSs): Thin cardiac muscle extensions from atria to caval (vena cava) and pulmonary veins.
• Role in Arrhythmias: MSs are recognized as critical in triggering and sustaining atrial tachycardias, especially AF, a common and clinically important arrhythmia in horses.
• AF Prevalence: Standardbred racehorses exhibit AF rates around 1.3-2%, while NSCTs have markedly lower AF incidence despite similar physical demands.
• Research Question: Investigate whether anatomical differences in NSCTs’ MSs contribute to this lower AF prevalence.

Study Design and Methods

• Subjects: Hearts from 33 NSCT horses collected postmortem.
• Macro- and Microscopic Evaluation: Measurements and morphological analysis of atrial structures and myocardial sleeves in the cranial vena cava (CrVC) and caudal vena cava (CaVC).
• Fibrosis Quantification: Degree of interstitial fibrosis in MSs was measured to assess potential substrate for arrhythmia development.

Key Findings

• MS Presence: Found in both caval veins (CrVC and CaVC).
• Differences Between Vena Cavae:
  • CrVC showed clearer, more distinct borders of MSs.
  • CaVC contained myocardial islands uniquely and had significantly higher interstitial fibrosis than CrVC.
• Myocardial Fiber Orientation: MSs showed heterogeneous fiber arrangements—transverse, longitudinal, and oblique orientations—especially in CaVC.
• Comparison with Other Breeds: NSCT MS morphology largely resembles that of breeds with higher AF prevalence, suggesting anatomical potential for arrhythmia.

Interpretations and Implications

• The anatomical substrate for atrial arrhythmias appears present in NSCTs’ MSs, implying that anatomical structure alone may not explain their low AF rates.
• Other factors, possibly at the structural or electrical level, might protect NSCTs from developing AF despite having myocardial sleeves similar to other horses prone to this arrhythmia.
• Further studies could focus on these protective mechanisms, potentially contributing to better AF prevention or treatment strategies in horses and possibly humans.

Conclusion

• NSCTs exhibit myocardial sleeve morphology similar to breeds with higher AF prevalence, including fibrosis and muscle fiber variability.
• The breed’s lower AF rate suggests that factors beyond MS anatomical structure affect AF development, warranting further investigation into electrical or other protective properties.