In general, heart problems in horses are not as common as in small animals.  However, atrial fibrillation is an arrythmia that does occur in quite a few horses. The condition was recently reviewed by Dr. Kimberely McGurrin at the Ontario Veterinary College.  In atrial fibrillation, the atrial tissue depolarizes in a disorganized manner so that atrial contractions do not occur.  Consequently, the output of the heart is decreased and this can lead to a decrease in performance.  Fortunately, atrial fibrillation in the horse is usually not accompanied by structural heart disease as it is in humans.  Usually atrial fibrillation is a stable arrythmia in that once it develops it remains until treated.  Atrial fibrillation should be suspected anytime a horse has an irregular heart rate and has a sudden decrease in performance.

Diagnosis of the condition is confirmed by an electrocardiogram.  The most common treatment of the condition is with a drug called quinidine that is given orally or intravenously.  Quinidine is reported to be effective in 83 to 92% of the cases.  However, quinidine can be toxic and can cause signs from mild colic all the way to laminitis and even death has been reported.  Because of this, other treatments have been considered and one of the newest is converting the atrial fibrillation to normal rhythm by using an electric current.  The vets at the Ontario Veterinary College have had success placing catheters through the jugular vein and directly into the right atrium.  Currently, the success rate of electrical therapy has been excellent with 98% of the horses being successfully treated regardless of the length of time the condition has been present.