Atrial fibrillation (AF) is the most common arrhythmia (abnormal heart rhythm) affecting performance horses. A normal horse's heart beats in a steady, rhythmic pattern. In a horse with AF's heart, the upper chambers of the heart fibrillate (quiver or twitch quickly) and create an irregular rhythm. It's characterized by an irregularly irregular rhythm that can sound like a combination of premature beats and long pauses.

Causes of AF

Some horse breeds (such as the Standardbred, Warmblood, and Draft horse) have a genetic predisposition to AF. Any horses with existing structural heart disease are more at risk of developing recurrent or persistent AF. There is speculation that electrolyte abnormalities might be associated with the development of AF in horses.

Diagnosis and Evaluation of AF

AF is usually recognized by your veterinarian during auscultation (when listening to the horse's heart). The diagnosis is confirmed with an electrocardiography (ECG), which checks for problems with the electrical activity of the heart. It shows the heart's electrical activity as line tracings on paper. An echocardiogram may also be performed to identify any underlying structural heart disease, valvular regurgitation, and cardiac (atrial) enlargement.

AF Management strategies

Management strategies for horses with AF include no treatment, pharmacologic cardioversion (long-term anti-arrhythmic therapy) and transvenous electrical cardioversion (TVEC). Administration of anti-arrhythmic drugs (quinidine salts) has been the traditional method of management of AF in horses. It is effective in approximately 80% of horses but involves a range of dose-related and idiosyncratic toxic responses which vary from mild and benign to fatal. Indications for quinidine treatment in horses include lone AF, AF with mild LA enlargement, and comorbidities in which general anesthesia or TVEC are not options.

TVEC involves the application of electrical shock through the use of internal electrical cardioversion with catheter-mounted electrodes. It should only be performed by experienced operators using specialized equipment. TVEC can be used to treat lone AF, AF with mild LA enlargement, and horses either intolerant of or unresponsive to quinidine treatment or horses in which quinidine is contraindicated.

Recurrence of AF

Horses are more at risk of AF recurrence when they have an underlying cardiac disease, especially chronic valvular regurgitation with atrial enlargement.