Head of Equine Cardioteam Ghent and Equine Internal Medicine Ghent University Merelbeke, Oost-Vlaanderen, Belgium
Abstract:
Background: Accessory pathways (AP) are muscular bundles connecting atria with ventricles, bypassing the atrioventricular (AV) node-His-Purkinje system. Anterograde AP conduction results in ventricular pre-excitation. Retrograde conduction allows ventriculo-atrial (VA) conduction, thereby creating a potential re-entry circuit for orthodromic atrioventricular re-entry tachycardia (OAVRT).
Objectives: Describe clinical and electrocardiographic findings of horses with retrograde AP conduction, and in one horse, mapping and radiofrequency ablation results . Animals: Six horses with retrograde AP
Methods: Records from 6 horses were reviewed.
Results: The resting electrocardiogram showed P’ waves in the ST segment with a fixed RP’ coupling interval in all horses. In 5 horses, these P’ waves were conducted back to the ventricles along the AV node, resulting in OAVRT with a median heart rate of 94 [74-145] beats per minute (bpm). All horses also showed P’ waves blocked at the AV node, resulting in orthodromic atrioventricular re-entry bradycardia (OAVRB) due to sinus node resetting, with a median heart rate of 27 [21-37] bpm. In one horse, bi-directional AP conduction also resulted in ventricular pre-excitation. In one horse, 3D mapping identified a left atrial insertion of the AP by the shortest VA interval. Radiofrequency ablation without contact force was not successful in eliminating the AP.
Conclusions: Retrograde conducting AP resulted in OAVRT and OAVRB. Accessory pathways in these horses behaved differently compared to humans and small animals. Further research is necessary to elucidate AP behavior in horses, to evaluate the risk and effect on performance, and to assess the potential for treatment by ablation.
