Atrial Fibrillation is the most common arrhythmia affecting the horse. Most horses with AF do not have a detectable cardiac disease and if of short (less than four months) duration a good prognosis with chemical conversion. This report contains information on the clinical signs, diagnosis, treatment and prognosis of atrial fibrillation in horses.

The primary symptom of AF is exercise intolerance. If there is underlying cardiac disease typically, the horse will present with a greater degree of exercise intolerance and there may be other signs present:

• nose bleeds
• difficulty breathing
• incoordination or weakness
• fainting
• ventral edema
• weight loss

In humans 15% of strokes are associated with afib) but this is not established in horses. On the other hand the mechanism of action for stroke in humans, pooling of blood in the atria, should be equally if not more applicable to horses do to their larger hearts and slower heart rates. In one large study the data suggests untreated cases may worsen over time.