ISSN 2398-2977      

Heart: atrial fibrillation


Synonym(s): Atrial fibrillation


  • Most important arrhythmia affecting performance in athletic horses.
  • Paroxysmal atrial fibrillation = acute onset atrial fibrillation with spontaneous conversion to normal sinus rhythm within 24-48 h.
  • Lone atrial fibrillation = persistent atrial fibrillation in the absence of detectable underlying heart disease.
  • Cause: predisposed by large atrial size and high vagal tone.
  • Signs: usually poor performance during maximal exercise; may be no clinical signs (incidental finding).
  • Diagnosis: auscultation Heart: auscultation, electrocardiography (ECG) Cardiovascular: ECG (electrocardiography).
  • Treatment: in otherwise healthy horses, oral quinidine sulfate Quinidine or transvenous electrical cardioversion. Attempted cardioversion not recommended for horses with atrial fibrillation secondary to heart failure.
  • Prognosis: in otherwise healthy horses, good for life, although long-term survival of horses with persistent atrial fibrillation not known. Cardioversion advised for performance horses; some horses with persistent atrial fibrillation may be able to continue to be used at lower levels of work. In heart failure, prognosis is poor.



  • In most other species atrial fibrillation is associated with underlying cardiac disease Heart: disease - overview.
  • High vagal tone in horses shortens the refractory period to variable degrees in different cells in the atria → inhomogeneity of refractoriness → perpetuates dysrhythmia.
  • Atrial premature complex (APC) Heart: supraventricular premature complex → atrial fibrillation (AF).
  • Inhomogeneity of the refractory state in the atrial myocardium maintains AF → initiation of a circus movement (requires a large atrial mass and suitable refractory and excitable cells to be sustained).
  • In large horses the atria are large enough to perpetuate AF once it has been initiated.
  • These factors are enhanced in the presence of underlying cardiac pathology which results in atrial enlargement, eg mitral regurgitation Heart: mitral regurgitation or large ventricular septal defects Heart: ventricular septal defect.
  • Electrolyte imbalance, systemic, infectious or inflammatory disease can predispose to AF by causing APCs Heart: atrial premature complex.
  • Increased left atrial pressure during exercise increases the risk of exercise-induced pulmonary hemorrhage Lung: EIPH (exercise-induced pulmonary hemorrhage) and epistaxis.

Predisposing factors


  • Size of horse.


  • Electrolyte disturbance.
  • Strenuous exercise.
  • Standardbred breed.
  • Heart failure.


  • Horse predisposed to atrial fibrillation because of large atrial mass and high vagal tone.
  • Atrial contraction important during exercise therefore when absent will decrease maximal cardiac output and result in submaximal performance.
  • If horse performs at low exercise intensities the owners may not detect alterations in exercise tolerance.
    • In the absence of underlying heart disease and at rest, coordinated atrial contraction has little effect on ventricular filling, therefore many horses may show no clinical signs at rest.
    • During exercise with higher heart rates diastole is shorter and atrial contraction contributes significantly to ventricular filling.
    • Atrial fibrillation results in higher heart rates during all levels of exercise (maximal heart rate may exceed 300 beats/min cf normal horse maximal of 235-240 beats/min).
    • As a result of these factors, ventricular filling, stroke volume and hence maximal cardiac output are compromised, reducing athletic performance.


  • Clinical signs may appear immediately with the onset of atrial fibrillation, or it may not be detected for many months and only picked up as an incidental finding.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Verheyen T et al (2013) Ventricular response during lungeing exercise in horses with lone atrial fibrillation. Equine Vet J 45 (3), 309-314 PubMed.
  • Clercq de D et al (2007) Effects of an adapted intravenous amiodarone treatment protocol in horses with atrial fibrillation. Equine Vet J 39 (4), 344-349 PubMed.
  • De Clercq D, van Loon G, Baert K, Tavernier R, Croubels S, De Backer P & Deprez P (2006) Intravenous amiodarone treatment in horses with chronic atrial fibrillation. Vet J 172 (1), 129-134 PubMed.
  • McGurrin M K J, Physick-Sheard P W, Kenney D G, Kerr C & Hanna W J (2005) Transvenous electrical cardioversion of equine atrial fibrillation: technical considerations. J Vet Intern Med 19 (5), 695-702 PubMed.
  • McGurrin M K J, Physick-Sheard P W & Kenney D G (2005) How to perform transvenous electrical cardioversion in horses with atrial fibrillation. J Vet Cardiol 7 (2), 109-119 PubMed.
  • Risberg A I & McGuirk S M (2005) Successful conversion of equine atrial fibrillation using oral flecainide. J Vet Intern Med 20 (1), 207-209 PubMed.
  • van Loon G, De Clercq D, Tavernier R, Amory H & Deprez P (2005) Transient complete atrioventricular block following transvenous electrical cardioversion of atrial fibrillation in a horse. Vet J 170, 124-127 PubMed.
  • van Loon G, Blissitt K J, Keen J A & Young L E (2004) Use of intravenous flecainide in horses with naturally-occurring atrial fibrillation. Equine Vet J 36 (7), 609-614 PubMed.
  • Ohmura H, Nukada T, Mizuno Y, Yamaya Y, Nakayama T & Amada A (2000) Safe and efficacious dosage of flecainide acetate for treating equine atrial fibrillation. J Vet Med Sci 62, 711-715 PubMed.
  • Hirago A & Kubo K (1999) Two cases of paroxysmal atrial fibrillation during exercise in horses. Equine Vet Educ 11 (1), 6-10 VetMedResource.
  • Blissitt K (1999) Diagnosis and treatment of atrial fibrillation. Equine Vet Educ 11 (1), 11-19 VetMedResource.
  • Wijnberg I D, Van der Kolk J H, van Garderen & Binkhrst G J (1998) Atrial fibrillation associated with central nervous symptoms and colic in a horse - a case of equine cardiomyopathy. Vet Q 20 (2), 73-76 PubMed.
  • Collantos C (1995) Treating atrial fibrillation in horses. Comp Contin Educ 17 (2), 243-245 VetMedResource.
  • Reef V B, Reimer J M & Spencer P A (1995) Treatment of atrial fibrillation in horses - new perspectives. J Vet Intern Med 9, 57-67 PubMed.
  • Reef V B (1982) Factors affecting prognosis and conversion in equine atrial fibrillation.J Vet Intern Med 2, 1-6 PubMed.

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