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Update on Atrial Fibrillation
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PATHOGENESIS OF ATRIAL FIBRILLATION
Atrial fibrillation is the most important and common pathological arrhythmia of the horse. Atrial fibrillation arises due to re-entry, occurring in the presence of a unidirectional conduction block. This creates circus waves, that providing there is a critical atrial mass, are electrophysiologically stable and thus tend to be readily sustained. The horse has an inherent predisposition for this rhythm disturbance for two reasons; high vagal tone creates electrophysiological heterogeneity necessary for initiation of the arrhythmia and the large size of the equine atria perpetuates atrial fibrillation. However, any circumstance that exacerbates these two predisposing features will further increase the risk of an individual animal to the development of atrial fibrillation. Specifically, atrial enlargement that may occur in association with mitral insufficiency or, less frequently, tricuspid insufficiency or congenital cardiac disease can lead to atrial fibrillation. Thus, atrial fibrillation is commonly found in horses in congestive cardiac failure. In this setting, atrial fibrillation is a consequence of the underlying cardiac disease and not a specific cause of clinical signs of congestive failure. Both primary and secondary myocardial disease can exacerbate atrial electrophysiological heterogeneity. Thus, paroxysmal atrial fibrillation is occasionally seen in horses following colic surgery and, electrolyte disturbances, and in particular hypokalaemia has been implicated in the pathogenesis of sustained and paroxysmal atrial fibrillation.
CLINICAL PRESENTATION AND DIAGNOSIS
Atrial fibrillation is most commonly identified as a clinical problem in horses engaged in vigorous athletic activities. Exercise-induced pulmonary haemorrhage may occur in conjunction with atrial fibrillation, presumably due to increases in pulmonary vascular pressure associated with impaired atrial emptying. In pleasure horses and breeding stock, it may frequently be an incidental finding. The condition is most often sustained but it can be paroxysmal and thus is generally considered to be the primary differential diagnosis in horses that develop arrhythmias during strenuous exercise. Paroxysmal atrial fibrillation will typically resolve spontaneously within around 24 hours in this situation. [...]
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