Atrial fibrillation is a disorder of theheart, which causes its regular rhythm to become interrupted and go out of synch. A potentially serious condition, atrial fibrillation is most often seen in large, athletic horses with big hearts, but can also be caused by a heart defect.

The healthy heart

The heart is divided into two parts: the left side, which supplies the muscles and vital organs with oxygenated blood, and the right side, which must pump blood to the lungs to replenish them with oxygen. Each side has a muscular pumping chamber, the ventricle, and a collecting chamber or atrium.

The heart is instructed to beat by an electrical impulse that originates high in the wall of the right atrium. This impulse travels quickly through the walls of both collecting chambers (atria) and causes them to contract, pushing any remaining blood into the pumps.

The electrical impulse is then delayed for almost 0.5sec before it reaches the pumping chambers, enabling all the blood to arrive in the ventricles before they contract and ensuring maximum efficiency.

This electrical activity can be recorded as an electrocardiogram (ECG), which is a map of the journey of the electrical impulse through the heart.

Heart rhythm

In a healthy state, the electrical impulses fire off regularly from the right atrium, providing a constant rhythm. As a horse becomes fitter, the rate of firing is damped down because his larger, more muscular heart does not need to beat so frequently.

In atrial fibrillation, the electrical activity of the collecting chambers is completely chaotic. As a result, instead of beating regularly in a co-ordinated pumping motion, the atria beat out of synch.

Meanwhile, the ventricles are being bombarded by electrical “nonsense”. There is no longer a regular heart rhythm with the odd missed beat – instead, there is a chaotic rhythm, which the vet can recognise by taking a pulse or listening with a stethoscope.

During atrial fibrillation there is no useful contraction of the atria, so there is no effective turbo charge to fill either ventricle. Instead, the pumps have to fill under gravity. This is not as disastrous as it sounds,because 70% of filling does occur passively under gravity. At rest and during slow paces, the heart can easily make up for this inefficiency by beating a little faster every minute.

Symptoms and treatment

Affected horses usually appear normal at rest. However, at all levels of exercise,the heart must beat faster to make up for reduced efficiency. In horses, which work well below maximum cardiac capacity, the rider may be unaware of the problem.

It is in the disciplines which require all of the heart’s capacity that atrial fibrillation becomes obvious such as hunters, eventers, racehorses and point-to-pointers.

When atrial fibrillation is affecting performance, the condition may be treated with quinidine. Treatment is effective and normal rhythmis restored in eight out of nine horses. Unfortunately, the drug must be administered repeatedly by a stomach tube and can have unpleasant side effects, including,in a handful of cases, death.

As a result, when the chaotic rhythm is not affecting your horse’s performance, your vet may decide to leave well alone.

Unfortunately, the problem can recur, and for every eight horses returned to normal rhythm, one will remain in atrial fibrillation. When treatment fails, horses can continue successfully in a less demanding sporting discipline, so failure may still not mark the end of their working life.

Treatment is always delayed for at least 48hr after the onset of the abnormal rhythm. In a number of horses,the heart willrevert to normal of its own accord, so the vet will always give it the opportunity to do this by itself.

Self-correcting atrial fibrillation is common in smaller flat racing Thoroughbreds. In these horses, by the time the horse has walked back from the racetrack to his stable, the heart rhythm may have returned to normal.