Cardiovascular - Atrial Fibrillation

Horses have enormous hearts in comparison to other species, and with training, they become even larger.

Intrinsic neurologic (vagal) input to the horse's heart also ensures that the horse has a low resting heart rate (32-44 beats per minute). 

This is known as having high vagal tone.

Both the size of the horses heart and the high vagal tone contribute to the development of atrial fibrillation.

 In horses, atrial fibrillation is usually benign, meaning that there is no underlying cardiac disease (this is different from cats, dogs, and humans).

 Occasionally, horses may develop atrial fibrillation because of problems such as leaky heart valves.

With atrial fibrillation, the electrical signals that ordinarily pass from the atria, the first set of filling and pumping chambers of the heart, to the ventricles, the second set of chambers, becomes disorganized. This causes the atria to beat in a very erratic fashion.

Because the ventricles still beat normally despite the erratic signals sent to them, the heart is able to pump adequate amount of blood at rest.

 But the extra little blood pumped in by the atria, while insignificant during rest, becomes important for the horse to perform strenuous exercise.

 For this reason, atrial fibrillation may go undetected for a very long time in horses that do light work, such as pleasure horses, trail horses, and show hunters.

 In a race horse, A fib would be picked up very soon.