Fib, short for atrial fibrillation (often written as “AFib”), is a heart rhythm disorder in which the upper chambers of the heart fire off rapid, chaotic electrical signals instead of beating in a steady pattern. In cats, atrial fibrillation is rare compared to dogs, and every cat diagnosed with it has severe underlying heart disease. The condition causes an irregular, usually fast heartbeat (above 160 beats per minute) that reduces the heart’s ability to pump blood efficiently.
How Atrial Fibrillation Works in Cats
A healthy cat’s heart follows a coordinated electrical rhythm: the upper chambers (atria) contract first, pushing blood into the lower chambers (ventricles), which then pump it to the body. In atrial fibrillation, the atria fire more than 600 to 700 disorganized electrical impulses per minute. The ventricles can’t keep up with that pace, so they beat irregularly and often too fast.
This irregular rhythm means the ventricles don’t fill with the same amount of blood each beat. Some heartbeats push out a normal volume, while others barely move blood at all. You or your vet might notice this as “pulse deficits,” where some heartbeats are too weak to produce a detectable pulse at the wrist or leg.
Why It Happens: Underlying Heart Disease
Unlike in humans, where atrial fibrillation sometimes develops on its own, cats with AFib always have a serious structural heart problem driving it. A study of 50 cats with atrial fibrillation published in JAVMA found that every single cat had visible abnormalities on echocardiography. The most common conditions were restrictive cardiomyopathy (38% of cases) and left ventricular hypertrophy, which includes hypertrophic cardiomyopathy or HCM (36%). Dilated cardiomyopathy accounted for another 12%. The remaining cases involved rarer conditions like valve defects and right-sided heart disease.
In practical terms, this means AFib in a cat is never an isolated finding. It signals that the heart has already undergone significant structural changes, and the fibrillation is a complication of that damage rather than the root problem.
Signs You Might Notice
Cats are notoriously good at hiding illness, and the signs of atrial fibrillation overlap heavily with general heart disease. You might observe rapid or labored breathing, lethargy, reduced appetite, or reluctance to exercise. Some cats develop fluid buildup in the chest or abdomen as the heart loses pumping efficiency, which can cause visible belly swelling or open-mouth breathing.
Because cats don’t pant or show exertion the way dogs do, AFib often goes undetected until a vet listens to the chest and picks up the irregular rhythm during a routine exam or an appointment for another concern entirely.
How It’s Diagnosed
A vet who suspects AFib will confirm it with an electrocardiogram (ECG). The hallmark pattern is an irregular spacing between heartbeats with no visible P waves, the small blips that represent normal atrial contraction. Instead, the baseline of the ECG looks wavy or almost flat, reflecting the chaotic atrial activity. The ventricular complexes themselves look normal in shape, just unevenly spaced.
Because atrial fibrillation always points to underlying disease, the next step is typically an echocardiogram (cardiac ultrasound) to identify which structural problem is present and how severely the heart is affected. Blood work and chest X-rays help assess whether fluid has accumulated in the lungs or other areas.
Treatment and Outlook
Managing AFib in cats focuses on two goals: slowing the heart rate to a more sustainable pace and treating the underlying heart disease responsible for the rhythm disturbance. Rate control helps the ventricles fill more completely with each beat, improving blood flow to the body. The specific approach depends on which heart condition is involved and how advanced it is.
Converting a cat’s heart back to a normal rhythm is rarely the primary target, because the structural disease that triggered AFib tends to make the arrhythmia recur. Instead, treatment aims to keep the cat comfortable and manage symptoms like fluid buildup. Cats with AFib caused by advanced heart disease generally have a guarded prognosis, though some respond well enough to treatment to maintain a reasonable quality of life for months.
AFib vs. Other Conditions With Similar Abbreviations
If you searched “fib in cats,” it’s worth noting two other conditions with similar-sounding names that sometimes come up in feline health discussions.
FIP (Feline Infectious Peritonitis)
FIP is a viral disease, not a heart condition. It develops when a common, usually harmless intestinal coronavirus mutates inside a cat’s body. This happens in roughly 10% of cats carrying the virus. The mutated form infects white blood cells, spreading throughout the body and triggering intense inflammation, particularly around blood vessels in the abdomen, kidneys, or brain. FIP has a “wet” form (fluid accumulation in the belly or chest) and a “dry” form (which tends to cause neurological symptoms like seizures or uncoordinated movement). An antiviral treatment has transformed outcomes dramatically: survival rates now sit around 85%, and early diagnosis with aggressive treatment can push mortality as low as 5%.
Fibrosarcoma
Feline fibrosarcoma is a type of cancer that most commonly appears as a firm lump under the skin at a previous injection site. Owners usually discover it while petting their cat. Veterinary guidelines suggest having any lump evaluated if it persists longer than three months, grows larger than two centimeters, or increases in size over one month. To reduce fibrosarcoma risk, current vaccination guidelines recommend giving injections as far down the limbs as possible, and minimizing the frequency of vaccination where appropriate.