Atrial Fibrillation in Horses

The atria are the upper chambers of the heart. These receive the blood from the body and push it into the ventricles to be dispersed to the next steps of the cardiac cycle. Irregular rhythms, as seen in atrial fibrillation, cause improper blood flow and, in serious cases, cardiac failure or stroke.

Atrial fibrillation is uncommon in horses overall, but is the most common cardiac arrhythmia diagnosed in this species. Horses showing symptoms of atrial fibrillation require prompt veterinary evaluation to begin treatment, as treating the condition promptly allows for better outcomes. Most horses have an excellent prognosis for quality of life, regardless of treatment, however atrial fibrillation can have significant impacts on athletic performance.

Many cases of atrial fibrillation show no symptoms, and the condition is only diagnosed when a veterinarian performs a routine physical examination.

In very severe cases, atrial fibrillation may lead to weakness or collapse.

Atrial fibrillation occurs when the electrical signaling in the heart is disrupted, causing the atria (the top of the heart) to contract rapidly.

Unlike many other heart arrhythmias, horses can develop atrial fibrillation without having underlying heart disease.

Administration of diuretics is also associated.

Underlying heart disease can also predispose horses to developing atrial fibrillation.

Exercise intolerance is the primary symptom, and is most obvious during periods of intense exercise.

Atrial fibrillation is easily diagnosed by a physical examination. Further diagnostics are necessary to determine if there are any underlying heart conditions causing the irregular heart beat. Diagnostics may include:

• Bloodwork
• Electrocardiogram (ECG), including during exercise
• Ultrasound of the heart (echocardiogram)
• Monitoring of the heart rhythm continuously over 24 hours

Many horses in atrial fibrillation do not require treatment, as long as they are able to meet the requirements of their athletic careers. It may be recommended that these horses perform an exercise test to determine whether the horse is safe to ride, as horses in atrial fibrillation are at risk of developing serious arrhythmias that may lead to collapse or sudden death.

If pursued, treatment aims at restoring the normal rhythm of the atria. The main medication used is quinidine. Several doses must be given over short periods of time through nasogastric intubation, so horses must be hospitalized for the duration of treatment. Other treatments that may be used include:

• Medications to manage congestive heart failure
• Supplementation of potassium
• Electrical heart stimulation under general anesthesia

Horses treated within 4 months of atrial fibrillation starting are more likely to respond to treatment. After 4 months, the chances of returning to a normal heartbeat rhythm are lower, and more aggressive treatment protocols may be required.

Most cases of atrial fibrillation respond quickly to treatment, although recurrence is likely to occur within the first year after treatment. Horses that respond well to treatment are able to return to their previous level of performance, and have an excellent long-term prognosis. Horses that do not respond to treatment, and do not have underlying heart disease, have an excellent prognosis for low level performance and a good quality of life. Horses with underlying heart disease have a guarded to grave prognosis, and may only respond to treatment for a short time before euthanasia is recommended.

There are no methods for preventing atrial fibrillation in horses. Routine examination by a veterinarian can help identify atrial fibrillation quickly, which is associated with a better prognosis.

Atrial fibrillation is not contagious.

Atrial fibrillation is uncommon in horses overall, but is the most common cardiac arrhythmia in horses.

• Quinidine
• Electrical conversion
• Management of heart failure