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Home / Equine Research Bank / J Vet Intern Med / Orthodromic Atrioventricular Reentry Bradycardia and Tachyca...
Journal of veterinary internal medicine2025; 39(4); e70175; doi: 10.1111/jvim.70175

Orthodromic Atrioventricular Reentry Bradycardia and Tachycardia Caused by an Accessory Pathway in Horses.

Abstract: Accessory pathways (APs) are muscular bundles directly connecting the atria and ventricles, bypassing the atrioventricular (AV) node-His-Purkinje system. Anterograde conduction along the AP results in ventricular preexcitation. A retrograde conducting AP allows ventriculo-atrial (VA) conduction, creating a reentry circuit that mediates orthodromic atrioventricular reentry tachycardia (OAVRT). This condition is well described in humans and small animals, but has not been reported previously in horses. Objective: Describe clinical and electrocardiographic findings of horses with retrograde AP conduction and reporte mapping and radiofrequency ablation results in one horse. Methods: Six horses with retrograde AP conduction. Methods: Records from six horses with retrograde AP conduction were reviewed. Results: Resting ECGs showed P' waves in the ST segment with fixed coupling intervals to the preceding QRS complexes. In five of six horses, P' waves were conducted back to the ventricles along the AV node, resulting in OAVRT. All horses had episodes where P' waves were blocked at the AV node, which resulted in orthodromic atrioventricular reentry bradycardia (OAVRB). In one horse, the AP conducted bidirectionally resulting in ventricular preexcitation. An electrophysiological study in one horse identified a left-atrial AP insertion by the shortest VA interval. Radiofrequency ablation using an impedance-based mapping system could not eliminate the AP. Conclusions: Retrograde conducting APs in horses caused OAVRT and OAVRB. APs in horses behaved differently compared to those in humans and dogs. Further research is necessary to elucidate AP behavior, evaluate risk and effect on performance, and assess treatment by ablation.
© 2025 The Author(s). Journal of Veterinary Internal Medicine published by Wiley Periodicals LLC on behalf of American College of Veterinary Internal Medicine.
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Publication Date: 2025-07-17 PubMed ID: 40671630PubMed Central: PMC12268229DOI: 10.1111/jvim.70175Google Scholar: Lookup
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Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

The research article discusses a study conducted on horses that have an accessory pathway (AP), which leads to types of abnormal heartbeats, namely orthodromic atrioventricular reentry tachycardia (OAVRT) and orthodromic atrioventricular reentry bradycardia (OAVRB).

Explanation of Scientific Terms

  • Accessory Pathways (APs): These are extra bundles of muscle that connect the upper and lower chambers of the heart. They skip the atrioventricular node-His-Purkinje system which is the normal path for electrical signals in the heart. This can cause ventricular preexcitation where the ventricles contract sooner than they should.
  • Orthodromic Atrioventricular Reentry Tachycardia (OAVRT) and Orthodromic Atrioventricular Reentry Bradycardia (OAVRB): These conditions arise due to a circuit forming in the heart due to retrograde (or backward) conduction of electrical signals via the AP. Tachycardia refers to a fast heartbeat while bradycardia denotes a slow heartbeat.
  • Electrophysiological Study: This is a test that records the electrical activity and pathways of the heart.
  • Radiofrequency Ablation: It’s a procedure that uses radiofrequency energy to destroy a small area of heart tissue causing abnormal heart rhythm.

Study Findings

  • The study focused on six horses with APs that were retrograde, which is unusual compared to reported cases in humans and dogs.
  • The resting Electrocardiographs (ECGs) of these horses showed abnormal heart rhythm patterns. Specifically, P’ waves (which represent the electrically triggered contraction of the atria) occurred in the ST segment (the recovery phase of the heartbeat) with fixed intervals following the preceding QRS complex (which corresponds to the depolarization of the ventricles).
  • In five of the six horses, these P’ waves led to OAVRT as they were conducted back to the ventricles via the AV node.
  • All horses experienced episodes of OAVRB when these P’ waves were blocked at the AV node.
  • In one horse, the AP allowed bidirectional conduction leading to ventricular preexcitation, or early contraction of the ventricles.
  • An electrophysiological study was conducted on one horse which lead to the identification of a left-atrial AP insertion point. A radiofrequency ablation procedure was attempted on this horse, but it was not successful in eliminating the AP.

Conclusions and Future Implications

  • APs in horses can lead to abnormal heart rhythms, namely OAVRT and OAVRB, and these appear to be different from those observed in humans and dogs.
  • Further research is needed to explore the behavior of the APs, evaluate the impact the abnormal heart rhythms may have on the performance and health of the horses, and also to assess the potential of treating these conditions with ablation.

Cite This Article

APA
Buschmann E, Easton-Jones C, Van Steenkiste G, De Wilde H, Roberts V, Durando M, Decloedt A, Marr C, van Loon G. (2025). Orthodromic Atrioventricular Reentry Bradycardia and Tachycardia Caused by an Accessory Pathway in Horses. J Vet Intern Med, 39(4), e70175. https://doi.org/10.1111/jvim.70175

Publication

Journal of veterinary internal medicine
ISSN: 1939-1676
NlmUniqueID: 8708660
Country: United States
Language: English
Volume: 39
Issue: 4
Pages: e70175
PII: e70175

Researcher Affiliations

Buschmann, Eva
  • Equine Cardioteam Ghent, Department of Internal Medicine, Reproduction and Population Medicine, Faculty of Veterinary Medicine, Ghent University, Ghent, Belgium.
Easton-Jones, Charlotte
  • Rossdales Equine Hospital and Diagnostic Centre, Suffolk, UK.
Van Steenkiste, Glenn
  • Equine Cardioteam Ghent, Department of Internal Medicine, Reproduction and Population Medicine, Faculty of Veterinary Medicine, Ghent University, Ghent, Belgium.
De Wilde, Hans
  • Department of Cardiology, UZ Gent, Ghent University, Ghent, Belgium.
Roberts, Veronica
  • Bristol Veterinary School, University of Bristol, Bristol, UK.
Durando, Mary
  • Rood and Riddle Equine Hospital, Wellington, Florida, USA.
Decloedt, Annelies
  • Equine Cardioteam Ghent, Department of Internal Medicine, Reproduction and Population Medicine, Faculty of Veterinary Medicine, Ghent University, Ghent, Belgium.
Marr, Celia
  • Rossdales Equine Hospital and Diagnostic Centre, Suffolk, UK.
van Loon, Gunther
  • Equine Cardioteam Ghent, Department of Internal Medicine, Reproduction and Population Medicine, Faculty of Veterinary Medicine, Ghent University, Ghent, Belgium.

MeSH Terms

  • Animals
  • Horses
  • Horse Diseases / etiology
  • Horse Diseases / physiopathology
  • Horse Diseases / surgery
  • Electrocardiography / veterinary
  • Male
  • Female
  • Bradycardia / veterinary
  • Bradycardia / etiology
  • Accessory Atrioventricular Bundle / veterinary
  • Accessory Atrioventricular Bundle / complications
  • Tachycardia, Atrioventricular Nodal Reentry / veterinary
  • Tachycardia, Atrioventricular Nodal Reentry / etiology
  • Tachycardia / veterinary
  • Tachycardia / etiology

Grant Funding

  • 01B05818 / Bijzonder Onderzoeksfonds UGent
  • 1SE9122N / Fonds Wetenschappelijk Onderzoek

Conflict of Interest Statement

The authors declare no conflicts of interest.

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