Atrial Fibrillation in Dogs and Cats: Signs, Treatment and Outlook

By Dr Duncan Houston

Hearing that your dog or cat has atrial fibrillation can be frightening, especially if the problem was found during an otherwise routine examination. The diagnosis describes an abnormal electrical rhythm, but it does not tell you on its own how sick your animal is.

The decisions that matter are whether the heart is beating too quickly, whether underlying heart disease is present, and whether breathing or circulation is being affected.

Quick Answer

Atrial fibrillation, often called AF, is an irregular heart rhythm caused by disorganised electrical activity in the heart's upper chambers. It commonly accompanies structural heart disease, although some large or giant dogs develop AF without detectable structural changes; in cats, it usually signals substantial underlying heart disease. An ECG confirms the rhythm, and treatment often aims to control the heart rate while managing the underlying condition. Breathing difficulty, collapse, or sudden painful loss of leg movement requires emergency veterinary care.

What happens to the heart during atrial fibrillation?

The upper heart chambers, called atria, normally contract in a coordinated way to help fill the lower pumping chambers, called ventricles. During AF, the atria lose that coordinated contraction and the ventricles beat irregularly, often too fast.

Some beats move very little blood, so the pulse felt in a leg can underestimate the actual heart rate. This is why feeling a pulse at home cannot reliably assess AF control. A rapid rhythm can reduce pumping efficiency and add to the strain of existing heart disease. VCA's explanation of atrial fibrillation in dogs describes these changes and the signs owners may notice.

Why does atrial fibrillation affect dogs and cats differently?

Dogs with underlying heart disease

Important associated conditions include dilated cardiomyopathy, which weakens the heart muscle, and advanced degenerative mitral valve disease. Enlargement of the atria makes AF more likely. Weakness, reduced stamina, fainting, or breathing changes may reflect the rhythm disturbance, heart failure, or both.

Dogs with apparently lone atrial fibrillation

Some large or giant dogs have AF without another heart abnormality being detected. This is sometimes called primary or lone AF. If a cardiologist confirms an acceptable heart rate and no important structural disease, monitoring without rate-control medication may be appropriate.

A reassuring first scan does not mean follow-up is unnecessary. A study of Irish wolfhounds with initially subclinical AF found that some later developed dilated cardiomyopathy. That finding supports continued surveillance, but does not provide a prognosis for every breed. See the study of disease progression in Irish wolfhounds with AF.

Cats with atrial fibrillation

AF is less common in cats and usually occurs with substantial atrial enlargement and cardiomyopathy. Hypertrophic cardiomyopathy is one possible underlying disease, but restrictive and other forms also occur. A clinical study of 50 cats with AF documented several different heart muscle diseases, so AF should not automatically be labelled as HCM.

Clot risk is especially important in cats with advanced heart disease. The risk depends on the condition of the heart and blood flow, not simply the presence of an irregular rhythm.

How do vets confirm the diagnosis?

• Examination: Your vet listens to the rhythm, compares heartbeats with the pulse, and assesses breathing and circulation.
• Electrocardiogram, or ECG: This records electrical activity. AF produces an irregularly irregular rhythm without the normal atrial signals called P waves. Listening alone cannot confirm it.
• Echocardiogram: An ultrasound checks chamber size, valves, muscle function, and evidence of underlying disease.
• Chest imaging and blood tests: These are selected to investigate suspected heart failure, other illness, and medication safety. A distressed animal may need stabilisation before extensive testing.
• Holter monitoring: A portable ECG records the rhythm during everyday activity, commonly for 24 hours, to measure rate control more accurately than a brief clinic reading.

The ECG criteria were used in the feline AF study; the ORCA II canine study shows why repeated Holter assessment can matter even when structural heart disease is not apparent.

How urgent are the signs?

What you notice	Why it matters	What to do
An irregular rhythm has been found, but your animal is comfortable and behaving normally	AF can be present without obvious illness; the rate and heart structure still need assessment	Arrange the recommended ECG and heart assessment. Ask for a definite appointment and monitoring plan
New tiredness, reduced appetite, poorer exercise tolerance, or a sustained rise in sleeping breathing rate	These may indicate worsening disease or a medication problem	Contact your vet the same day. Escalate immediately if breathing becomes difficult or weakness is marked
Laboured breathing, collapse, blue or grey gums, or a cat suddenly unable to use a painful leg	Possible heart failure, inadequate circulation, or an arterial blood clot	Go to an emergency veterinary service immediately

What else can cause similar signs?

An uneven heartbeat is not always AF. Other abnormal rhythms and, in dogs, normal rhythm variation with breathing can sound irregular. An ECG distinguishes them. Coughing, breathlessness, and poor stamina also have causes outside the heart, including airway or lung disease.

In cats with thickened heart muscle, vets also assess for high blood pressure and an overactive thyroid because these can produce changes resembling HCM. Cornell's information on feline hypertrophic cardiomyopathy explains why these conditions need to be considered. Do not assume that every new symptom in an animal with AF is caused by the same heart problem.

How is atrial fibrillation treated?

Control the rate and treat the underlying disease

For many dogs, treatment aims to slow the ventricular rate enough to improve circulation and comfort. The rhythm can remain irregular despite successful treatment. Diltiazem and digoxin are commonly used, sometimes together; other drugs may suit particular patients. Heart failure requires its own treatment as well. Drug selection depends on heart function, blood pressure, concurrent illness, and tolerance.

Monitoring is part of treatment. Digoxin has a narrow safety margin, so blood-level testing may be needed. Keep the prescribed formulation and schedule consistent, and report vomiting, poor appetite, or new weakness promptly. VCA outlines canine treatment and medication monitoring.

In cats, a cardiologist may consider drugs such as diltiazem or selected beta blockers when AF causes an excessively fast ventricular response. The evidence is more limited than in dogs, and feline treatment must be tailored to the underlying cardiomyopathy. The ACVIM feline cardiomyopathy consensus provides this species-specific guidance.

Why there is no single home heart-rate target

Research in dogs has associated better survival with a 24-hour Holter average of 125 beats per minute or less. That is an average over an entire day, not a requirement for every heartbeat or a number to apply to cats. A short reading at home or in a stressful clinic visit is not interchangeable with that measurement. Your vet sets the target and adjusts treatment using appropriate recordings. See the prospective ORCA rate-control study.

Can a normal rhythm be restored?

Electrical cardioversion can restore a normal rhythm in selected dogs at a specialist centre. Suitability depends on the heart disease and how long AF has been present; the rhythm may relapse afterwards. Research on electrical cardioversion in dogs documented successful conversion, with variable duration of the restored rhythm.

Does my animal need medication to prevent clots?

This requires a separate risk assessment. In cats, marked left atrial enlargement, poor atrial function, abnormal blood flow, or a previous clot can lead a vet to recommend clot-prevention treatment. Clopidogrel is an antiplatelet drug, not an anticoagulant; additional medicines may be considered for some high-risk cats. It does not eliminate clot risk. The ACVIM recommendations on feline clot prevention base decisions on these clinical findings. Do not start aspirin or a human blood thinner yourself.

What is the prognosis?

There is no reliable life-expectancy figure for every dog or cat with AF. The underlying diagnosis, heart failure, clot complications, rate control, and response to treatment all matter. A clinically well dog with no detectable structural disease has a different outlook from a cat with advanced cardiomyopathy and breathing difficulty.

Canine studies support pursuing effective rate control, but their survival figures describe particular groups of dogs. They cannot promise that reaching one heart-rate number will add a fixed number of years. Feline evidence is smaller, and a study comparing AF with another fast rhythm in cats found that heart failure was associated with shorter survival. Ask your vet what the findings mean for your animal's comfort and likely course.

When is this an emergency?

Seek emergency veterinary care now for:

• Breathing with obvious abdominal effort, an extended neck, or an inability to settle comfortably.
• Open-mouth breathing in a cat, particularly at rest.
• Collapse, severe weakness, or blue or grey gums.
• Sudden severe leg pain, cold paws, or loss of movement, particularly in a cat's back legs.

Sudden painful leg dysfunction can indicate a blood clot blocking an artery. Cornell describes the breathing and clot complications of feline heart disease. Call ahead, keep handling gentle, and transport your animal with minimal exertion. Do not delay departure to count heartbeats or force food, water, or tablets into a distressed animal.

What should you do at home?

1. Get a written plan. Confirm each medicine, the next review date, activity limits, and who to call out of hours.
2. Track breathing during sleep. If your vet recommends monitoring, count chest rises for a full minute while your animal is fully asleep. Record the result and use the threshold your vet provides. A consistent rise from baseline warrants a same-day call; any breathing effort warrants emergency assessment.
3. Record useful changes. Note appetite, energy, fainting episodes, and difficulty giving medication. A short video of an episode may help if recording does not delay care.
4. Keep activity comfortable. Avoid strenuous exercise while a newly diagnosed rhythm is being assessed. Ask what level of walking or play is suitable once treatment is established.
5. Attend monitoring appointments. Feeling better does not prove the heart rate is controlled throughout the day. After any prescription change, confirm when repeat recordings or blood tests are due.

Common mistakes to avoid

• Assuming a comfortable-looking animal cannot have important heart disease.
• Using a pulse count or consumer device to diagnose AF or change medication.
• Applying a dog's Holter target to a cat.
• Stopping treatment because the animal seems brighter, or doubling a missed dose without advice.
• Waiting for a routine appointment after collapse, breathing difficulty, or sudden painful leg weakness.

Can atrial fibrillation be prevented?

There is no dependable home method that prevents all AF. Routine examinations and recommended follow-up for known heart disease can identify changes that need attention. Once AF is diagnosed, consistent treatment and monitoring help your vet respond to deterioration. Food changes, supplements, and exercise alone cannot reliably correct an established abnormal rhythm.

Frequently asked questions

Can a dog have atrial fibrillation without heart disease?

Yes. Some large or giant dogs have no detectable structural disease at diagnosis. They still need assessment of their heart rate and follow-up because heart disease may become apparent later.

Is atrial fibrillation the same as heart failure?

No. AF is an electrical rhythm problem. Heart failure is a consequence of impaired heart function that can cause fluid accumulation and breathing problems. An animal may have either condition or both.

Can atrial fibrillation go away on its own?

Do not rely on spontaneous recovery. Persistent AF commonly needs ongoing management. A specialist can assess whether an attempt to restore normal rhythm is appropriate.

Can my dog still go for walks?

Often, comfortable activity remains possible, but the amount depends on rate control, symptoms, and underlying disease. Get an individual exercise plan, and stop activity if your dog becomes weak or breathless.

The next decision that matters

A diagnosis of AF needs a plan for both the rhythm and the rest of the heart. If your animal is comfortable, arrange the recommended investigations and monitoring. If breathing, circulation, or leg movement suddenly changes, seek urgent care rather than waiting for the next review.

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For help understanding the questions to discuss with your veterinary team, ASK A VET™ is a supportive starting point. Keep your own vet's treatment plan close at hand, and contact an emergency clinic directly for urgent signs.