When to see a vet urgently
The following signs alongside an abnormal rhythm require immediate contact with an emergency vet:
- laboured or open-mouth breathing at rest;
- collapse, loss of consciousness or sudden severe weakness;
- sudden limb pain, coldness or paralysis;
- pale or bluish mucous membranes with rapidly deteriorating condition.
Keep your cat calm and avoid unnecessary handling. Do not delay leaving to count its pulse. You can find a vet near you.
Overview
Normally, the heart contracts in an organised sequence: the atria push blood into the ventricles, which pump it into the blood vessels. With atrial fibrillation, contractions become disorganised and very frequent, up to hundreds per minute, and the atria effectively stop functioning. This reduces pumping efficiency, causes blood to pool and increases clot risk.
The atrial electrical rate differs from the heartbeat detected at a paw or chest. The atrioventricular node allows only some chaotic impulses through to the ventricles. Their rhythm is therefore irregular and usually fast, but not every atrial impulse produces a pumping beat. A high heart rate alone cannot identify atrial fibrillation.
The intervals between successive beats vary, giving the ventricles different filling times. Not every contraction produces an equally strong pulse wave. A vet may therefore detect a pulse deficit: more beats are heard at the heart than pulses felt in an artery. You need not, and should not, try to confirm this at home before seeking care.
Causes
- Hypertrophic cardiomyopathy, HCM, the most common cause in cats.
- Dilated cardiomyopathy, uncommon in cats and more common in dogs.
- Marked atrial enlargement with chronic heart failure.
- Consequences of myocarditis or other cardiac inflammation.
- Heart tumours, rarely.
The rhythm's name alone does not identify the type of cardiomyopathy. Cats with restrictive or other severe cardiomyopathy can also have atrial fibrillation. Investigation assesses atrial size and overall cardiac function. Atrial fibrillation differs from isolated extra beats and other fast atrial rhythms, although listening findings may raise similar suspicions.
Risk factors
- Middle-aged and elderly cats.
- Cats with diagnosed cardiomyopathy.
- Breed susceptibility to HCM, including Maine Coons, Ragdolls and British Shorthairs.
- Previous myocarditis or chronic infections.
Evidence for the contribution of each listed factor specifically to feline atrial fibrillation is limited. Age, breed or a previous infection alone does not establish the rhythm disturbance. In practice, an identified abnormal rhythm or signs compatible with heart disease are the main reasons for further investigation. Non-pedigree and younger cats' symptoms need examination too.
Typical symptoms
- A rapid heartbeat with an irregular pulse.
- Breathlessness and fast breathing.
- Reduced exercise tolerance and lethargy.
- Fainting or sudden weak episodes.
- Pale or bluish mucous membranes.
- Coughing and noisy breathing if fluid accumulates in the lungs.
- Possible sudden arterial blockage causing hindleg paralysis.
Coughing is not a typical feline heart failure sign. Lower airway disease often causes it, so do not automatically attribute a cough to diagnosed atrial fibrillation. Breathing difficulty may involve fluid within or around the lungs, but another respiratory disease is also possible. The vet assesses which signs relate to the rhythm and which to underlying disease.
For fainting, describe duration, any exertion or preceding event, and recovery speed. A safely obtained video may help. Not every fall or episode is caused by an arrhythmia, and investigation may need assessment of both the heart and other body systems.
Complications and consequences
- Chronic heart failure.
- Arterial thromboembolism, blocking major arteries, particularly the iliac arteries, and paralysing limbs.
- Sudden death.
- Progressive wasting from poor circulation.
Atrial enlargement and slow blood flow contribute to clot formation. The rhythm disturbance is therefore not the only risk factor, and slowing the heartbeat does not automatically eliminate clot risk. Arterial blockage may affect one limb or somewhere other than the hindlegs. Sudden new limb pain or weakness needs emergency assessment even if your cat already receives medication.
Diagnosis at the vet
- Listening to the heart for an irregular, rapid rhythm.
- An ECG showing the characteristic pattern.
- Heart ultrasound to identify cardiomyopathy and atrial enlargement.
- Chest X-rays to assess enlargement and lung fluid.
- Blood tests to assess other diseases.
ECG and examination of heart structure
An ECG looks for a completely irregular ventricular rhythm and absent normal P waves representing atrial activation. This distinguishes atrial fibrillation from many other rapid rhythms. Ultrasound shows whether the atria are enlarged, how ventricular walls move and which heart disease fits the findings. ECG and ultrasound complement each other.
Electrical activity differs from contraction strength. An ECG alone therefore does not show how well the heart pumps; ultrasound assessment of structure and function is also needed.
X-rays assess the heart's outline and fluid within the lungs or chest cavity. Blood tests provide information about kidney function and other illnesses affecting treatment, for example. Not every symptom can be explained by the arrhythmia, so tests follow the cat's condition. Stabilising a severely breathless patient may come before extensive investigations.
If abnormal rhythm or weakness occurs intermittently, prolonged ECG recording may provide more information than a brief appointment measurement. A Holter monitor records rhythm during ordinary daily life. Your diary of eating, rest, movement and symptoms helps link the recording to events. It can also assess medication effectiveness.
Treatment and follow-up
Treatment goals
The main aim in feline atrial fibrillation is usually to slow an excessively rapid ventricular response. Vet-selected medicines affect conduction of electrical impulses from the atria to the ventricles. The aim is to improve filling and reduce workload. Successful treatment does not require every cat to return to normal sinus rhythm.
Drug choice depends on concurrent heart failure, contraction strength and other medication. Some rate-slowing medicines can weaken contraction, so do not use or combine them yourself. A human atrial fibrillation prescription is not a suitable home trial for a cat. The vet also sets an individual heart-rate target and how its effect will be checked.
Underlying heart disease is treated at the same time. Fluid-related heart failure needs targeted treatment. Clot risk is assessed separately, and some cats need medication to reduce clotting. These decisions are not based on the rhythm's name alone. Hospitalisation may be needed if breathing or circulation is unstable.
What should you monitor at home?
Watch breathing while your cat sleeps or rests without purring. Record the rate and notice increased effort, restlessness or difficulty finding a comfortable resting position too. Changes from its previous baseline help the treating vet. Do not stay at home monitoring open-mouth breathing or obvious respiratory difficulty.
Monitor appetite, weight, muscle condition and successful medication administration. Adequate nutrition matters in heart disease; severe food restriction is not a general treatment. Diet and any sodium reduction are tailored to disease stage. Report treats and food given with medicines too, as they affect the overall diet.
Check-ups assess rhythm and cardiac function as well as general condition. An outwardly calm cat may still need medication review based on ECG findings. Conversely, one fast heart rate at the practice does not alone show home treatment has failed. Do not change doses from measurements without previously agreed instructions.
Prevention
- Early identification and treatment of cardiomyopathies.
- Monitoring breeds susceptible to heart disease.
- Reducing stress and exertion in cats with heart disease.
- Regular heart examinations for cats older than 6–7 years.
There is no reliable home method to prevent atrial fibrillation. The age mentioned is not a universal cardiac examination timetable either. Monitoring follows identified findings and risks, and symptomatic cats are examined at any age.
Important for cat owners
Atrial fibrillation in a cat always indicates serious heart disease. Remember that breathlessness, weakness or sudden hindleg paralysis require immediate veterinary attention. Appropriate treatment can slow progression, reduce clot risk and improve quality of life, but prognosis remains guarded.
This article provides general information and does not replace veterinary care. If you notice signs of illness in your cat, contact a vet.
Frequently asked questions
Is a fast heart rate always atrial fibrillation?
No. Stress or another illness can increase a cat's rate, for example, and several rapid arrhythmias exist. Atrial fibrillation is confirmed by ECG, examining both beat-interval variation and atrial and ventricular electrical activity.
Has treatment failed if atrial fibrillation continues?
Not necessarily. Treatment often aims to control ventricular rate and manage underlying disease. Response is assessed through symptoms, ECG and cardiac function. Rhythm may remain irregular even when medication reduces workload.
Does good rate control remove clot risk?
Not by itself. Atrial size, blood flow and other heart disease also affect risk. Possible clot-preventive treatment is therefore assessed separately and must not be stopped yourself when symptoms improve.
This article is for general information only and is not a guide to treatment. If you notice the first signs of illness, contact your vet.
Paw4care editorial team
Sources
- Heart Disease: Conduction Abnormalities in Dogs and Cats, MSD Veterinary Manual, accessed 2026-11-05.
- Atrial Fibrillation in Cats, VCA Animal Hospitals, accessed 2026-11-05.
- ACVIM consensus statement guidelines for the classification, diagnosis, and management of cardiomyopathies in cats, American College of Veterinary Internal Medicine / ABVP, accessed 2026-11-05.
- Heart Disease Diagnosis, Tufts University Cummings School of Veterinary Medicine, accessed 2026-11-05.
- Holter Monitor in Cats, VCA Animal Hospitals, accessed 2026-11-05.
- Arterial Thromboembolism in Dogs and Cats, Merck Veterinary Manual, accessed 2026-11-05.
- Coughing in Cats, VCA, accessed 2026-11-05.
- Home Breathing Rate Evaluation, VCA Animal Hospitals, accessed 2026-11-05.
- Heart Disease – Nutrition, Tufts University Cummings School of Veterinary Medicine, accessed 2026-11-05.
- Fainting, Tufts University Cummings School of Veterinary Medicine, accessed 2026-11-05.