Page 728 - The Veterinary Care of the Horse

The treatment is successful in 82–88% of cases. Of these, 20–30% will have a recurrence of

the condition. Treatment is most successful in horses that have only just started to fibrillate.
VetBooks.ir In one study, 5% of treated horses died as a result of treatment.

An alternative anti-arrhythmic drug called amiodarone can be given intravenously; it has
a reported success rate of 60% in chronic cases of atrial fibrillation. Treatment may have to

be discontinued if side effects of hind limb weakness or severe diarrhoea occur.
The name given to the procedures that convert a cardiac arrhythmia back to normal sinus

rhythm is cardioversion. This can also be achieved by applying a controlled electric shock to
the atria at a particular time within the cardiac cycle. The electrodes are guided into the

jugular vein and positioned within the right atrium and the left pulmonary artery. With the
horse under general anaesthetic, electric shocks of increasing intensity are delivered at
intervals. This procedure, known as transvenous electrical cardioversion (TVEC), is reported

to have a success rate of 98% and can be used on horses that have not responded to
medication. Atrial fibrillation is not thought to increase the anaesthetic risk to the horse.

Owing to the small risk of serious side effects from treatment, horses used for light work

that have no symptoms may not be treated. Atrial fibrillation on its own is unlikely to cause
collapse during light exercise. However, if at all possible these horses should have an ECG

taken while exercising to rule out the possibility of additional problems.

Subsequent management

Following successful treatment, the horse is usually given a few days off work so the heart
can adjust to the normal rhythm. If the condition has been present for some time, a longer

period of up to 2 months off work is recommended.

The horse should be monitored regularly as the condition may recur. Most owners of
horses that have experienced atrial fibrillation purchase a stethoscope and listen to the heart
at regular intervals. The vet will explain how to distinguish between atrial fibrillation and a

normal rhythm.

Prognosis

The prognosis is good for horses that have no murmurs or other signs of underlying heart
disease. The earlier they are treated, the greater the chance of a successful outcome. If there
are other heart problems, the prognosis is poor.

Horses that do not respond to treatment and remain in atrial fibrillation may perform

successfully in less demanding activities. If no other heart conditions are present, they do not
necessarily develop them at a later date.