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What is it?

Atrial fibrillation is a type of heart arrhythmia, which means the hearth rhythm is irregular.

Time Window

Please inform the vet within the first hour if you suspect this problem, the sooner it is diagnosed, the more effective the treatment.


  • Subtle to dramatically poor performance

  • Collapse

  • Horse feeling flat during exercise

  • Exercise Induced Pulmonary Haemorrhage can be a consequence (you will notice the horse bleeding from both nostrils)

  • Racehorses often 'cut out' whilst training or during a race

What should I do?

Get off your horse and do not attempt to ride it until your vet has seen it.

What will the vet do?

  • Listen to the horse’s heart. This is often diagnostic.

  • Perform an ECG (electrocardiogram) to confirm the heart issue (see pictures). We use portable ECG machines, diagnosing this condition at the yard.

  • Bloodwork can indicate other underlying conditions.

  • Perform an endoscopy to establish if there were any bleeds from the horse’s lungs.

  • The condition can resolve spontaneously. Persistent A-fibs need a referral to a specialist internal medicine facility for specific treatment.

  • Complicated cases need ECG performed during exercise, overnight and ultrasound scan examination of the heart.


Atrial fibrillation (AF) is the most common heart rhythm dysfunction in horses. It most frequently occurs in performance horses.

In each heartbeat cycle, the top half of the heart (left and right atria) pumps blood into the corresponding left and right ventricles or bottom part of the heart, they then contract to pump blood around the body (left ventricle) or to the lungs (right ventricle).

Atrial fibrillation is an uncoordinated contraction of the heart’s atria which then cannot contribute to ventricular filling; basically, the heart is pumping erratically and cannot move blood properly around the body.

Horses who develop atrial fibrillation do not necessarily have an underlying heart disease causing this issue.

When the horse’s heart is normal, the condition is termed 'lone AF'. Sometimes concurrent heart issues are present (enlargement, murmurs etc); this complicates treatment and outcome.

What treatment options do I have?

There are 2 ways to treat atrial fibrillation, either with drugs or by applying electrical current to the heart. Both of these methods aim to reset the heart's electrical conduction.

Both methods require the horse to be hospitalised for close monitoring during and after the treatment.

Both treatments are reported to have similar recurrence rates (one is not considered better than the other).

If your horse performs at a high level in its discipline and does not revert back to a normal rhythm within 24-48 hours of developing AF, it should undergo cardio-conversion treatment.

What is my horse’s prognosis?

This condition carries a good prognosis for return to full performance. Once the heart is restored to its regular rate and rhythm, and if no other pathology is present, the horse can go back to normal training.

There is a low chance for recurrence, more so in warmbloods, or horses that have had a longer duration of the condition.

After treatment

Ideally, a horse should be rested until all electrical and mechanical functions of the heart are back to normal. If AF was of very short duration, the horse can return to work fairly quickly after treatment.


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