Atrial fibrillation, commonly shortened to AFib or AF, is a heart rhythm disorder in which the upper chambers of the heart (the atria) fire electrical signals chaotically instead of in a coordinated rhythm. In cats, AFib is far less common than in dogs or humans, and when it does occur, it almost always signals serious underlying heart disease. Unlike in people, where AFib can sometimes show up in otherwise healthy hearts, the feline atria generally need to be significantly enlarged before they can sustain this arrhythmia, which means a cat diagnosed with AFib typically already has a cardiomyopathy or other structural cardiac problem driving the condition.
Why AFib Is Rare in Cats
A cat’s heart is small. That may sound obvious, but it is the single most important reason AFib behaves differently in cats than in larger species. The prevailing explanation for how AFib sustains itself involves multiple disorganized electrical wavelets circulating through atrial tissue simultaneously. For those wavelets to keep going without canceling each other out, they need a certain minimum mass of tissue to travel through. A normal-sized feline atrium simply does not provide enough real estate. Cats generally need severe atrial dilation before fibrillation can take hold, whereas humans can develop AFib even with normal-sized atria.1PubMed Central. Cardiac Abnormalities in Feline Hyperthyroidism This size requirement is why virtually every cat diagnosed with AFib has an identifiable structural heart problem.
In a study comparing cats with AFib to cats with other types of supraventricular tachycardia, every single cat with AFib had structural heart disease. Left atrial diameter was significantly larger in the AFib group, with a median of about 24 mm compared to roughly 19 mm in cats with other fast rhythms.2PubMed. Supraventricular tachycardia in 23 cats; comparison with 21 cats with atrial fibrillation (2004-2014) There are rare case reports of cats developing sustained AFib with a normal-sized left atrium at the time of diagnosis, but these are considered exceptions that challenge the traditional thinking rather than disprove it.3PubMed. A case of sustained atrial fibrillation in a cat with a normal sized left atrium at the time of diagnosis
The Underlying Heart Diseases That Lead to AFib
Hypertrophic cardiomyopathy, or HCM, is the most common heart disease in cats, and it is the condition most frequently lurking behind feline AFib. In HCM, the walls of the left ventricle thicken abnormally, which over time can cause the left atrium to stretch and enlarge as it struggles to push blood into a stiffened ventricle. That progressive atrial enlargement is what eventually sets the stage for fibrillation. Other forms of cardiomyopathy, including dilated and restrictive types, can produce the same kind of atrial stretching and carry the same AFib risk.3PubMed. A case of sustained atrial fibrillation in a cat with a normal sized left atrium at the time of diagnosis Arrhythmias of various kinds are a well-recognized complication of feline HCM, though not every cat with HCM develops them.4PubMed Central. The Unseen Side of Feline Hypertrophic Cardiomyopathy: Diagnostic and Prognostic Utility of Electrocardiography and Holter Monitoring
Hyperthyroidism, an overactive thyroid gland, is another condition that can affect the feline heart. In humans with hyperthyroidism, AFib is actually the most commonly reported rhythm disturbance and a major predictor of heart failure. In cats, though, hyperthyroidism-driven AFib is uncommon, again likely because the feline atria are simply too small for fibrillation to take hold without severe dilation. The thyroid excess can still cause other cardiac changes, including increased heart rate and ventricular thickening, but the jump to sustained AFib requires that the atria enlarge substantially.1PubMed Central. Cardiac Abnormalities in Feline Hyperthyroidism
Signs You Might Notice at Home
AFib in cats can be deceptively hard to spot. Cats are masters at hiding illness, and the signs of AFib often overlap with general signs of heart disease rather than producing a distinctive set of symptoms on their own. You might notice your cat breathing faster than usual, being less active, losing appetite, or seeming generally unwell. Some cats present in acute crisis with open-mouth breathing, which signals congestive heart failure with fluid buildup in or around the lungs.
One of the more telling physical findings, though one your veterinarian would detect rather than you, is an irregular heart rhythm combined with pulse deficits. A pulse deficit means the heart is beating but not every beat is strong enough to push a detectable pulse wave to the leg. In one reported case, a cat in AFib had a heart rate of about 190 beats per minute, but the palpable pulse rate at the femoral artery was estimated at roughly half that number.3PubMed. A case of sustained atrial fibrillation in a cat with a normal sized left atrium at the time of diagnosis That gap between heart rate and pulse rate is a classic red flag that the rhythm is not normal.
An especially dangerous presentation is arterial thromboembolism, sometimes called a “saddle thrombus” when it lodges at the point where the aorta branches to supply the hind legs. Blood that pools in enlarged, poorly contracting atria can form clots, and if one of those clots dislodges, it can travel downstream and block blood flow to the limbs. The result is sudden, severe hind-leg pain and paralysis. This is a veterinary emergency and one of the more devastating complications of feline heart disease in general, not just AFib specifically.
How Veterinarians Diagnose AFib
An electrocardiogram, or ECG, is the gold standard for identifying AFib. The hallmark pattern is an absence of the normal organized electrical waves from the atria, replaced by a chaotic, irregularly irregular rhythm. It looks distinctive enough that an experienced clinician can often recognize it at a glance, but the irregular rhythm alone is not enough. Other supraventricular arrhythmias can mimic certain features of AFib, so the ECG tracing needs careful interpretation.
Because AFib in cats nearly always accompanies structural heart disease, echocardiography (a cardiac ultrasound) is performed alongside the ECG. The ultrasound reveals the size and function of the heart chambers, the thickness of the walls, and whether there is fluid in the chest or around the heart. In cats with cardiomyopathy, the echocardiogram can also detect a phenomenon called spontaneous echocardiographic contrast, sometimes described informally as “smoke” swirling in the left atrium. This smoke-like appearance results from sluggish blood flow and signals an increased risk of clot formation. Research in cats with various cardiomyopathies found that measuring the blood flow velocity in the left atrial appendage, a small pouch off the main atrial chamber, was a strong predictor of this phenomenon, with low flow velocities increasing the odds of spontaneous contrast roughly thirtyfold.5Journal of Veterinary Internal Medicine. Assessment of Left Atrial Appendage Flow Velocity and its Relation to Spontaneous Echocardiographic Contrast in 89 Cats with Myocardial Disease
Standard in-clinic ECGs capture only a brief snapshot. Since arrhythmias can be intermittent, longer-term monitoring sometimes reveals problems a short recording would miss. Wireless body sensors designed for veterinary patients have been tested for 24-hour home ECG recording in dogs and cats, and the extended monitoring time allowed detection of additional arrhythmias in half of animals that had known rhythm disturbances.6PubMed Central. Wireless body sensor for electrocardiographic monitoring in dogs and cats This kind of ambulatory monitoring is not yet routine in general practice, but it is becoming more accessible through veterinary cardiology referral centers.
Treatment Goals and Rate Control
In human cardiology, there has been a long-running debate over whether it is better to restore and maintain a normal rhythm (rhythm control) or simply manage the heart rate while accepting that the rhythm will remain abnormal (rate control). In veterinary medicine, the practical answer leans heavily toward rate control. Electrical cardioversion, the use of a synchronized shock to reset the heart’s rhythm, is technically feasible in cats, but pharmacologic rate control is considered an effective and achievable strategy for most veterinary patients.7PubMed Central. Management of atrial fibrillation
Rate-control drugs work by slowing the speed at which electrical signals pass from the atria to the ventricles, bringing the overall heart rate down even though the atria themselves are still fibrillating. Common choices include diltiazem, a calcium channel blocker, and beta-blockers like atenolol. The goal is to reduce the ventricular rate enough that the heart has time to fill properly between beats, improving cardiac output and easing symptoms. Which drug a cardiologist reaches for depends on the individual cat’s underlying disease, blood pressure, and how well they tolerate the medication.
Rhythm control has not been abandoned entirely, though. A case series described three cats with AFib secondary to hypertrophic cardiomyopathy and left atrial enlargement who underwent electrical cardioversion after rate-control drugs failed. All three successfully converted to normal sinus rhythm and maintained it during follow-up, with improved clinical stability.8Veterinary Record Case Reports. Successful transthoracic electrical cardioversion in three cats with atrial fibrillation These cases are encouraging, but they represent a small number of cats treated at specialist centers. For most cats in general practice, rate control remains the default approach.
Preventing Blood Clots
Thromboembolism is one of the most feared complications for any cat with significant heart disease, and the risk is amplified by AFib because the chaotic atrial rhythm promotes blood stasis. Preventing clots is a key part of managing these cats, and the evidence base for the best anticlotting strategy has been slowly improving.
A systematic review of treatments for cardiogenic arterial thromboembolism in cats found that the combination of the antiplatelet drug clopidogrel and the anticoagulant rivaroxaban was the most effective protocol studied. None of the 14 cats receiving that combination experienced a clot event, and they had a median survival of 257 days. Another option, the injectable anticoagulant dalteparin, also performed well, with only one clot event among 23 treated cats, though median survival was shorter at 190 days.9PubMed Central. Treatment and Prevention of Cardiogenic Arterial Thromboembolism in the Cat: A Systematic Review These numbers come from relatively small groups, so they should be interpreted cautiously, but they give clinicians some evidence-based options to weigh.
Clopidogrel on its own has long been one of the more commonly prescribed antiplatelet drugs for cats with heart disease, and aspirin has also been used historically, though clopidogrel has largely replaced it in many practices. The addition of rivaroxaban, an oral anticoagulant originally developed for human medicine, represents a newer approach. If your cat is prescribed any of these medications, the veterinary team will likely monitor for signs of bleeding, the main downside of any drug that interferes with clotting.
Prognosis and What to Expect
A diagnosis of AFib in a cat is serious. In the largest published review of feline AFib cases, spanning 50 cats over more than two decades, the median survival time from diagnosis was 165 days. That said, there was enormous variation: some cats died within days, while about a third survived a year or longer.10PubMed. Atrial fibrillation in cats: 50 cases (1979-2002) A separate study found a median survival of 58 days in cats with AFib, though that study compared a smaller group of AFib cats with cats that had other supraventricular arrhythmias, and the AFib cats had more severe structural disease on average.2PubMed. Supraventricular tachycardia in 23 cats; comparison with 21 cats with atrial fibrillation (2004-2014)
What determines how long a cat survives has more to do with the severity of the underlying heart disease than with the AFib itself. A cat whose cardiomyopathy is caught before heart failure develops and who responds well to rate control and clot prevention has a better outlook than a cat that presents in acute crisis with congestive heart failure or thromboembolism. Cats that tolerate their medications, maintain a good appetite, and remain comfortable can do well for months to over a year. Early and consistent veterinary follow-up, including repeat echocardiograms and periodic bloodwork, helps the care team adjust treatment as the disease evolves.
Monitoring Your Cat at Home
One of the most practical things you can do for a cat with heart disease is track their resting or sleeping respiratory rate. This involves counting the number of breaths your cat takes in a 15-second window while they are relaxed or asleep, then multiplying by four. Research on healthy cats and cats with subclinical heart disease found that the average sleeping respiratory rate was consistently under 30 breaths per minute, typically hovering around 21 breaths per minute. Cats with severe left atrial enlargement sometimes exceeded 30 breaths per minute, and any cat repeatedly measuring above that threshold warrants further evaluation.11PubMed Central. Sleeping and resting respiratory rates in healthy adult cats and cats with subclinical heart disease
A sudden and sustained jump in sleeping respiratory rate is one of the earliest home-detectable signs that a cat with heart disease may be developing fluid buildup in the lungs. If you are managing a cat with AFib and cardiomyopathy, keeping a daily log and sharing it with your veterinarian gives them a real-time window into how the disease is progressing between clinic visits. Several smartphone apps designed for this purpose make the counting and logging easier.
The Emotional Side of Managing Feline Heart Disease
Caring for a cat with a chronic and potentially life-threatening condition takes a toll. A study measuring caregiver burden in pet owners found that owners of sick cats experienced roughly double the burden of owners of healthy cats, with significantly higher scores on a standardized scale adapted from human caregiving research. Interestingly, cat owners with ill pets reported somewhat lower burden than dog owners with ill dogs, possibly reflecting differences in the day-to-day care demands.12PubMed Central. Caregiver burden in cat owners: a cross-sectional observational study
The emotional weight of managing a heart condition like AFib can be compounded by the unpredictability of the disease. A cat might be doing well one day and develop a sudden thromboembolism or acute heart failure the next. If you are in this situation, being honest with your veterinary team about what feels manageable helps them tailor the treatment plan in a way that works for both you and the cat. Quality of life for the pet and sustainability for the caregiver are both legitimate considerations in these discussions, and good veterinary cardiologists factor them in routinely.
When AFib Looks Like Something Else
Not every fast, irregular heart rhythm in a cat is AFib. Other supraventricular tachycardias can produce rapid heart rates and overlapping symptoms, sometimes at even higher rates than AFib. In the comparative study cited earlier, cats with other supraventricular tachycardias had a median heart rate of 300 beats per minute, significantly faster than the median of 220 bpm in the AFib group.2PubMed. Supraventricular tachycardia in 23 cats; comparison with 21 cats with atrial fibrillation (2004-2014) Distinguishing between these arrhythmias matters because the prognosis and treatment can differ. Some supraventricular tachycardias occur in cats without structural heart disease, which immediately changes the clinical picture.
Stress-related sinus tachycardia is another common confounder. Cats in a veterinary clinic are often frightened, and their heart rate can spike to 240 bpm or more from adrenaline alone, without any underlying cardiac abnormality. A skilled clinician considers the whole picture: the ECG tracing’s specific pattern, the echocardiogram findings, whether the rhythm is truly irregular or just fast, and whether the cat calms down when given time to settle. Misdiagnosis in either direction carries consequences. Labeling a stressed healthy cat as having AFib leads to unnecessary treatment, while dismissing a genuine arrhythmia as stress could delay care for a cat in real danger.