The most common questions cat owners ask about feline heart disease now have evidence-based answers: roughly one in seven apparently healthy cats already has hypertrophic cardiomyopathy, the first drug for it won conditional FDA approval in March 2025, and a free at-home breathing count remains the best early warning of heart failure. Hypertrophic cardiomyopathy — HCM — is a thickening of the left ventricle's muscular wall that stiffens the heart and makes it a poor pump, and it is the disease behind most feline cardiac emergencies. This page answers the questions that send owners searching at 11 p.m.: what a murmur means, what the staging letters on a cardiology report stand for, whether the new weekly tablet is worth asking about, how clots are prevented after one has already happened, and why a single abnormal scan is not a life sentence.
Table of Contents
- How common is heart disease in cats, and does a murmur mean my cat has it?
- What do stages B1, B2 and C on a cardiology report mean?
- Is there finally a drug for subclinical HCM?
- After a clot, what actually prevents the next one?
- The 30-breath rule, and how to use it
- Genetic tests, thyroid tests, and why one scan is not a diagnosis
- Frequently Asked Questions
How common is heart disease in cats, and does a murmur mean my cat has it?
The CatScan study screened 780 apparently healthy, non-purebred cats in UK rehoming centres with echocardiography and found hypertrophic cardiomyopathy in 14.7% of them, reported by Payne and colleagues in the *Journal of Veterinary Cardiology* in 2015. These were ordinary cats with no clinical signs. Prevalence rose with age and was higher in males. A murmur is a much weaker clue than most owners assume.
In a 2022 *Veterinary Sciences* study of 856 auscultated cats, murmurs were present in 40.8% of apparently healthy cats, and 70.4% of those murmurs were functional — noise from turbulent flow, not structural disease. The positive predictive value of a murmur for HCM was only 17.9% to 42.6%. The reverse matters more. Many cats with HCM have no murmur at all, so a clean stethoscope exam is not clearance. Echocardiography is the only test that answers the question, which is why "he sounds fine" and "his heart is fine" are different statements.
What do stages B1, B2 and C on a cardiology report mean?
The 2020 ACVIM consensus statement, published by Luis Fuentes and colleagues in *JVIM*, classifies feline cardiomyopathy by phenotype and then stages it. Those letters drive every treatment decision your vet makes, so they are worth reading off the report.
The line between B1 and B2 is the one that changes an owner's life. A B1 cat is usually monitored with repeat scans. A B2 cat is a candidate for clot prevention and closer surveillance, because the enlarged left atrium that defines the category is where clots form.
- **Stage A** — predisposed breed or family history, no disease detected.
- **Stage B1** — subclinical disease, low risk of near-term trouble.
- **Stage B2** — subclinical disease, but high risk of imminent heart failure or a clot.
- **Stage C** — heart failure or thromboembolism has already occurred.
- **Stage D** — signs refractory to standard treatment.
Is there finally a drug for subclinical HCM?
Yes, with a caveat in the approval itself. On 14 March 2025 the FDA conditionally approved Felycin-CA1, a delayed-release sirolimus tablet from TriviumVet given once weekly for management of ventricular hypertrophy in cats with subclinical HCM. It is the first drug ever approved for feline HCM. Conditional approval is not full approval.
It means the manufacturer has shown the drug is safe and has a reasonable expectation of effectiveness, and is still required to prove effectiveness while marketing it. Ask your vet to frame it that way rather than as a settled standard of care. The supporting evidence is the RAPACAT trial, published in *JAVMA* in November 2023. It randomised 43 client-owned cats with subclinical non-obstructive HCM to once-weekly delayed-release rapamycin or placebo for six months; the low-dose group had significantly lower left ventricular wall thickness than placebo, and the drug was well tolerated. Forty-three cats over six months is a small, short trial, and it measured wall thickness rather than survival.
After a clot, what actually prevents the next one?
Arterial thromboembolism — a clot thrown from the heart that lodges where the aorta divides, paralysing the hind legs — is the most feared complication of feline heart disease. The FAT CAT trial answered the prevention question directly. This double-blind study of 75 cats that had survived one thromboembolism compared clopidogrel at 18.75 mg per cat daily with aspirin at 81 mg per cat every 72 hours, reported by Hogan and colleagues in the *Journal of Veterinary Cardiology* in 2015.
Clopidogrel significantly reduced recurrent clots and improved survival, with a far longer median time to recurrence than aspirin. That result is why clopidogrel, not aspirin, is the default antiplatelet drug for a cat that has had a clot or sits in stage B2 with a dilated left atrium. Never dose aspirin at home on your own initiative. The feline dosing interval is 72 hours, not daily, and cats clear salicylate slowly enough that a human schedule is toxic.
The 30-breath rule, and how to use it
Cats with stable, medically controlled left-sided congestive heart failure keep their sleeping and resting respiratory rates below 30 breaths per minute, as Porciello and colleagues established in *The Veterinary Journal* in 2016. A rate that climbs above 30 while the cat is genuinely asleep suggests fluid is collecting in the lungs, often before any cough or open-mouth breathing appears.
The test costs nothing and takes 30 seconds. For a stage C cat on diuretics, it is the single most useful thing an owner does between appointments.
- Count only while the cat is asleep or fully at rest, never after play or a car ride.
- Watch the chest and count one rise-and-fall as one breath.
- Count for 30 seconds and double it, and record the number with the date.
- Take a baseline over several quiet days so you know your own cat's normal.
- Call your vet if the sleeping rate rises above 30, and go in the same day if it is climbing over consecutive counts.
Genetic tests, thyroid tests, and why one scan is not a diagnosis
The UC Davis Veterinary Genetics Laboratory tests Maine Coons for the MYBPC3 A31P variant and Ragdolls for MYBPC3 R820W. Cats with one copy are about 1.8 times more likely to develop HCM and those with two copies about 18 times more likely, but penetrance is incomplete. A negative genetic test does not rule out HCM, and echocardiography is still required — the DNA test informs breeding decisions more than it informs an individual cat's prognosis. For an older cat, thyroid disease belongs in the workup.
Foulex and colleagues reviewed 147 hyperthyroid cats in *JVIM* in 2026 and found 80% had a cardiomyopathy phenotype: 64% hypertrophic, 14% restrictive and 2% nonspecific. Thickening in a cat over six may be thyroid-driven rather than primary HCM, which is why a T4 test should accompany the echocardiogram. Some thickened feline hearts also recover. A *Veterinary Quarterly* study published in December 2025 followed 53 cats with HCM phenotypes using serial echocardiography and found 16 had transient myocardial thickening; maximum wall thickness fell from 8.3 ± 1.7 mm to 5.4 ± 0.9 mm after a median of 3.5 months, and all of those cats survived. If your cat was scanned during an illness, a stressful hospital stay or a dehydration episode, ask about a repeat scan in three to six months before accepting a lifelong diagnosis.
Frequently Asked Questions
My cat is a rescue of unknown breeding. Should I pay for a screening echocardiogram?
There is no formal recommendation to screen every cat, but the CatScan figure of 14.7% in ordinary non-purebred cats means mixed breeding is not protection. Screening is most defensible before an elective anaesthetic, in a male cat, and in an older cat, since prevalence rose with both age and male sex.
Can a cat with HCM be safely anaesthetised for a dental?
It depends on the stage. A B1 cat is generally a manageable anaesthetic risk; a B2 cat with a dilated left atrium is not a routine case, and the procedure should be planned with the cardiology findings in front of the anaesthetist rather than scheduled from a clean pre-op exam.
Does a cat with subclinical HCM need a special diet or a supplement?
Nothing in the current evidence supports a supplement for subclinical HCM. The only drug with an approval for this stage is the weekly sirolimus tablet, and its approval is conditional, so effectiveness is still being proven.
My cat's wall measurement changed between two vets. Which number is right?
Echocardiographic wall measurement carries real operator variation, and thickening can also genuinely come and go — 16 of 53 cats in the 2025 *Veterinary Quarterly* series had thickening that resolved. Ask for a repeat scan by the same cardiologist rather than averaging two numbers from different hands.