Atrial fibrillation and sudden death risk in racehorses

An irregularly irregular rhythm sounds like tennis balls in a dryer. [1] Atrial fibrillation is the most common pathologic arrhythmia in horses, and the lecture identifies it as an arrhythmia associated with poor performance. [1]

How is the diagnosis confirmed?

Atrial fibrillation is sometimes found incidentally, during a routine checkup or a walk through the barn. [1] Sometimes you are asked to listen after an episode of poor performance. [1] An ECG is necessary to confirm the diagnosis even when clinical suspicion is already high, and an echocardiogram is almost always recommended as well. [1] In the illustrated tracing, RR intervals are irregular, with an underlying fibrillation-type potential on the baseline. [1]

Many horses convert spontaneously, typically within 24 to 48 hours of onset, and these episodes can be difficult to identify. [1] If a horse performs poorly at an event but its heart is normal when examined the following day, it may be unclear whether atrial fibrillation was present during the event. [1]

Check the heart rate, whether every P wave has a QRS complex, and whether every QRS complex has a P wave. [1] Also check whether the QRS complexes look alike, whether the P waves look alike, and whether the RR interval is regular. [1]

What does echocardiography assess?

Echocardiography is used to rule out primary structural heart disease. [1] An enlarged left atrium carries a worse prognosis for conversion and successful management of atrial fibrillation. [1]

Which horses should stop or change work?

Racehorses in atrial fibrillation should not race. [1] Horses with atrial fibrillation that are not treated should not be exercised to heart rates over 220 beats per minute, a point the lecturer says the thoroughbred sudden-death study reinforced. [1]

Athletic horses that have not returned to sinus rhythm within 48 hours require treatment if they are to continue as athletes. [1]
Event horses with atrial fibrillation need treatment or another purpose, including horses competing at lower levels. [1]
Some lower-level horses doing trail riding or lower-level dressage can be ridden by an informed rider, with heart rates kept below maximal intensity. [1]

How is atrial fibrillation treated, and what follows?

The two main treatments are quinidine and transvenous electrocardioversion, or TVEC. [1] Quinidine sulfate has long been the mainstay of treatment. [1] It works pretty well, but its significant adverse effects include more serious arrhythmias and colic. [1] TVEC uses a timed shock to restore normal sinus rhythm, and it requires anesthesia. [1]

Following TVEC or quinidine treatment, left ventricular function normalizes relatively quickly. [1] After treatment, they need a 24-hour ECG and an echocardiogram to check that cardiac function is returning. [1] When atrial fibrillation has been of short duration, return to training may be possible within about one to two weeks of cardioversion, depending on the specialist's recommendation. [1] For atrial fibrillation of longer or unknown duration, recommendations are often closer to six weeks. [1]

The longer abnormal waveforms circulate in the atria, the more cardiac remodeling occurs. [1] After the horse returns to sinus rhythm, it is easier to return to the pathways that are still present. [1]

Related lectures

On-demand lecture

Equine Cardiology Essentials

References

Equine Cardiology Essentials (lecture recording, May 30 2026) Vet On It lecture transcriptLecture source retained by Vet On It.