Atrial fibrillation in horses part 1: Pathophysiology.
Authors: Decloedt Annelies, Van Steenkiste Glenn, Vera Lisse, Buhl Rikke, van Loon Gunther
Journal: Veterinary journal (London, England : 1997)
Summary
# Editorial Summary: Atrial Fibrillation in Horses Part 1: Pathophysiology Atrial fibrillation represents the most frequently encountered clinically significant arrhythmia in equine practice, affecting up to 2.5% of the horse population, yet its underlying mechanisms remain poorly characterised in the species despite extensive investigation in experimental models and human cardiology. The arrhythmia develops through a two-factor process: electrical triggers (premature atrial depolarisations or localised reentry circuits) combined with a vulnerable atrial substrate altered by ion channel dysfunction, calcium-handling abnormalities, autonomic imbalance, and structural changes such as fibrosis or chamber enlargement. Whilst chronic atrial stretch from valvular regurgitation clearly predisposes horses to AF, a significant proportion develop idiopathic or "lone" AF with no detectable structural cardiac disease on routine examination—suggesting subclinical ion channel dysfunction or microstructural myocardial changes that current diagnostic techniques fail to identify. Once established, AF perpetuates itself through electrical remodelling (shortened atrial refractoriness favouring sustained reentry), contractile remodelling (reduced myocardial function), and structural remodelling (progressive fibrosis and atrial dilation), although reverse remodelling can occur after successful cardioversion over weeks to months. Clinically, horses with lone AF often remain asymptomatic at rest but develop exercise-related poor performance, respiratory distress, or occasionally exercise-induced pulmonary haemorrhage during work—distinguishing presentation severity from underlying haemodynamic disturbance and highlighting the importance of exercise-based diagnostic evaluation in performance horses.
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Practical Takeaways
- •Lone AF may present with absent clinical signs at rest but performance horses can show exercise intolerance, respiratory distress, weakness, or collapse during work—be alert to these signs even without cardiac murmurs
- •Diagnosis may require advanced imaging beyond routine techniques as many AF cases lack obvious structural heart disease, necessitating specialist referral
- •Recovery after successful cardioversion is prolonged (weeks to months); allow adequate rehabilitation time before returning to work
Key Findings
- •Atrial fibrillation is the most common clinically relevant arrhythmia in horses with prevalence up to 2.5%
- •AF results from interplay between electrical triggers and susceptible substrate involving ion channel alterations, Ca2+ handling changes, and structural abnormalities
- •Many horses present with 'lone AF' or idiopathic AF with no detectable underlying cardiac abnormalities on routine diagnostics
- •Electrical remodeling shortens atrial effective refractory period promoting reentry, while structural remodeling develops interstitial fibrosis and atrial enlargement with reverse remodeling potentially taking weeks to months after cardioversion