Bit & Bridle

Equine Atrial Fibrillation

Other Names

  • A-Fib
  • Equine Atrial Fibrillation
  • Cardiac Arrhythmia (general term)

What is Atrial Fibrillation in Horses?

Atrial fibrillation (AF) is the most common clinically significant cardiac arrhythmia in horses. It occurs when the electrical signals that normally coordinate the beating of the heart’s upper chambers (the atria) become chaotic and disorganised. Instead of contracting in a coordinated, rhythmic fashion, the atria quiver rapidly and irregularly — a process known as fibrillation. The ventricles, which pump blood around the body, then receive erratic electrical impulses, resulting in an irregular and often abnormally fast heart rate.

In horses, the large atrial mass and naturally low resting heart rate (28–44 beats per minute) create conditions that make the species particularly susceptible to AF. The condition may be well tolerated at rest but becomes clinically significant during exercise, when the heart cannot meet the increased circulatory demands of the working horse. Performance horses — Thoroughbreds, sport horses, and eventers — are most commonly affected, though AF can occur in any breed.

AF may be classified as:

  • Lone AF: No underlying structural heart disease identified; often seen in large, athletic horses. Has the best prognosis for conversion to normal sinus rhythm.
  • Secondary AF: Associated with underlying cardiac or systemic disease. Treatment of the primary condition is essential.
  • Paroxysmal AF: Intermittent episodes that resolve spontaneously, often associated with exercise.
  • Persistent AF: Does not resolve without treatment; most common form presented to clinicians.

Causes

Several factors can trigger or predispose a horse to atrial fibrillation:

  • Structural heart disease: Valvular regurgitation (particularly mitral or tricuspid valve disease) causes chronic atrial enlargement and stretch, increasing susceptibility to AF.
  • Electrolyte imbalances: Low blood potassium (hypokalaemia) — caused by heavy sweating, furosemide (Lasix) administration, or prolonged diarrhoea — is a significant trigger. Magnesium deficiency may also play a role.
  • Autonomic tone: The large vagal tone in athletic horses predisposes them to AF, particularly during recovery from intense exercise.
  • Underlying systemic disease: Severe respiratory disease affecting the right heart, gastrointestinal conditions such as colic, and endotoxaemia have all been associated with AF.
  • Myocarditis: Inflammatory disease of the heart muscle can disrupt normal conduction pathways.
  • Idiopathic / lone AF: In many athletic horses, no underlying cause is identified. Ion channel dysfunction or subclinical structural changes may be responsible.

Symptoms and Signs

The clinical signs of AF vary considerably depending on whether the horse is at rest or being exercised:

  • At rest: Many horses with AF show no obvious signs at rest. An irregularly irregular heartbeat may be detectable on auscultation with a stethoscope — characterised by an absence of the normal 4th heart sound (atrial kick).
  • Exercise intolerance: The most common presenting complaint. The horse tires earlier than expected, labours at speeds it previously handled comfortably, or stops unexpectedly mid-exercise.
  • Poor performance: Subtle declines in competition results before the condition is recognised.
  • Respiratory distress after exercise: Laboured breathing, prolonged recovery, or epistaxis (nosebleed) during or after work.
  • Collapse or sudden weakness: In severe cases, particularly those with concurrent cardiac disease, syncope or collapse can occur — though this is relatively uncommon in lone AF.
  • Irregular pulse: Felt clearly at the facial or submandibular arteries.

Diagnosis

AF is diagnosed via electrocardiography (ECG). The hallmarks on ECG are:

  • Absence of P waves (which represent coordinated atrial contraction)
  • Irregular baseline fibrillation waves (f waves)
  • Irregularly spaced QRS complexes (representing ventricular beats)

A resting ECG is usually sufficient to diagnose persistent AF. Exercise ECG (using a telemetric system such as a Televet or similar) may be required to detect paroxysmal AF that only manifests during or immediately after exertion.

Echocardiography (cardiac ultrasound) is strongly recommended in all AF cases to assess atrial size, identify structural disease, and guide prognosis. Horses with severe atrial dilation have a lower likelihood of successful cardioversion and a higher risk of recurrence. Blood work including electrolyte panels should be run to identify contributing factors.

Treatment

The goal of treatment is to restore normal sinus rhythm (cardioversion). Options include:

  • Quinidine sulphate (oral): The traditional treatment for AF in horses, used for over six decades. Administered via nasogastric tube, it is effective in around 80–90% of cases of lone AF. Requires careful monitoring for side effects including ataxia, nasal oedema, urticaria, colic, and — most critically — ventricular arrhythmias. Treatment is carried out in a clinic setting with continuous ECG monitoring.
  • Electrical cardioversion (transvenous electrical cardioversion, TVEC): A more modern technique where a controlled electrical shock is delivered directly to the heart via catheters passed through the jugular vein to the right atrium and pulmonary artery. Requires general anaesthesia. Highly effective (success rate around 95%) and well tolerated. Preferred for horses with side effects or resistance to quinidine, and for horses in which speed of recovery is important.
  • Flecainide (intravenous): Used in some cases, particularly in Europe, when other options are unsuitable. Requires careful dose management.
  • Treat underlying cause: Where AF is secondary to electrolyte imbalances or systemic disease, correcting the primary problem (e.g., electrolyte supplementation) may allow spontaneous conversion.

Horses with structural heart disease causing AF, or those with marked atrial dilation, have lower conversion rates and higher recurrence. These cases may require long-term management rather than cure.

Prevention

  • Ensure adequate electrolyte balance — particularly potassium and magnesium — in horses in hard work or those sweating heavily.
  • Avoid excessive use of loop diuretics (furosemide) without electrolyte supplementation.
  • Regular cardiac auscultation as part of routine health checks, especially in performance horses — early detection of irregular rhythms allows prompt investigation.
  • Manage underlying conditions (respiratory disease, colic, valvular disease) promptly to reduce secondary cardiac risk.
  • Allow adequate recovery time after intense exercise before resuming work following illness.

Prognosis

The prognosis for horses with lone AF treated promptly is generally good. The majority (80–90%) convert to normal sinus rhythm with appropriate treatment and can return to full athletic performance. Recurrence rates are approximately 20–30% over a horse’s lifetime, and a second episode should be investigated thoroughly with echocardiography.

Prognosis is more guarded in horses with:

  • Significant structural heart disease or marked atrial enlargement
  • Long-standing AF (duration over 4 months significantly reduces conversion success)
  • Recurrent episodes requiring multiple cardioversions
  • Underlying diseases that cannot be resolved

Horses that cannot be converted to sinus rhythm may be able to continue in light, non-competitive work under veterinary guidance, but should not be used for high-performance competition.

When to Call the Vet

Contact your vet promptly if your horse shows:

  • Unexplained exercise intolerance or sudden performance decline
  • An obviously irregular heartbeat detected during grooming or post-exercise
  • Epistaxis (nosebleed) during or after exercise
  • Laboured breathing that does not resolve normally after work
  • Any episode of collapse, weakness, or stumbling during exercise

AF is not usually a life-threatening emergency in the short term, but prompt diagnosis and treatment significantly improve the odds of successful cardioversion. Delaying treatment increases atrial remodelling and reduces the chance of returning to normal rhythm. Do not continue exercising a horse suspected of having AF — rest the horse until assessed by a vet.

Frequently Asked Questions

Can a horse with atrial fibrillation still be ridden?

In most cases, a horse with untreated AF should not be ridden, particularly in strenuous work. At rest, many horses appear normal, but under exercise the heart cannot respond appropriately to circulatory demands, increasing the risk of collapse or cardiovascular emergency. Once successfully treated and returned to normal sinus rhythm, most horses can resume their previous level of work — your vet will advise on a safe return-to-work protocol after treatment.

How common is atrial fibrillation in horses?

AF is the most common clinically relevant cardiac arrhythmia in horses. It is particularly prevalent in large-bodied, highly athletic breeds — Thoroughbreds, Warmbloods, and Draught horses — due to their large atrial mass. Studies suggest it affects around 0.5–2.5% of sport horses, with higher rates seen in endurance and eventing disciplines.

Is equine atrial fibrillation the same as in humans?

The underlying mechanism is similar — chaotic electrical activity in the atria — but there are important differences. Horses tolerate AF considerably better at rest than humans do, owing to their physiology. The large vagal tone and atrial mass of horses make them uniquely susceptible, and treatment protocols differ significantly from those used in human medicine. Quinidine, rarely used in human cardiology today, remains a first-line option in equine medicine.

What is TVEC and is it available in the UK?

Transvenous electrical cardioversion (TVEC) is a procedure where a controlled electrical shock is delivered to the heart under general anaesthesia via catheters. It has a high success rate and is considered very safe in horses. TVEC is available at several specialist equine hospitals and referral centres in the UK. Your vet can refer to a specialist cardiac centre — the British Equine Veterinary Association (BEVA) can help locate specialist equine cardiologists.