Hypertrophic cardiomyopathy (HCM) is the most common heart disease in cats, thickening the heart's main pumping chamber and quietly raising the risk of heart failure, sudden blood clots, and death — often before any owner notices a thing. Roughly 15 percent of apparently healthy cats screened by echocardiography have HCM (Payne et al., 2015, JVIM). Early HCM usually causes no symptoms at all, which is why a new heart murmur, a gallop sound, or fast resting breathing matters so much. If your cat is breathing more than 30 to 35 breaths per minute while asleep, or has sudden hind-leg weakness, treat it as an emergency.
Last reviewed: June 2026
What Hypertrophic Cardiomyopathy Actually Is
HCM is a disease in which the muscular wall of the left ventricle — the heart's main pumping chamber — becomes abnormally thick. That thickening makes the chamber stiff, so it cannot relax and fill properly between beats. Blood backs up into the left atrium, which stretches and enlarges, and pressure rises into the lungs. The 2020 ACVIM consensus statement classifies cats by stage, from subclinical disease with no signs through to overt congestive heart failure or arterial thromboembolism (Luis Fuentes et al., 2020, JVIM (ACVIM Consensus)). The underlying cause is often genetic: a sarcomeric protein mutation has been identified in Maine Coon and Ragdoll cats, and a hereditary basis is suspected across many breeds and domestic shorthairs.
How Common It Is and Which Cats Are at Risk
HCM is far more common than most owners realize. A large screening study of apparently healthy cats found that about 15 percent had echocardiographic evidence of cardiomyopathy, most of it HCM (Payne et al., 2015, JVIM). Males are affected more often than females, and risk climbs with age. Predisposed breeds include the Maine Coon, Ragdoll, British Shorthair, Sphynx, and Persian, but the majority of patients are ordinary domestic shorthair and longhair cats. As described in Nelson & Couto's Small Animal Internal Medicine, HCM must always be distinguished from heart muscle thickening secondary to hyperthyroidism, high blood pressure, or acromegaly, because those mimics are treatable and the management differs.
The Early Signs Owners Can Actually Catch
The hardest truth about HCM is that early disease is usually silent — many cats look completely normal right up until a crisis. There is no reliable cough the way there is in dogs with heart disease. What you can watch for is the resting respiratory rate: a healthy sleeping cat takes fewer than 30 breaths per minute, and a sustained rate above 30 to 35 is one of the earliest external clues that fluid is building in or around the lungs. Other signs include reduced activity or hiding, reluctance to play, a drop in appetite, and faster fatigue. A heart murmur, a gallop rhythm, or an irregular beat detected at a routine exam is frequently the first hint, which is why the AAFP-AAHA feline life stage guidelines recommend careful cardiac auscultation at every wellness visit (AAFP-AAHA Feline Life Stage Guidelines, 2021).
The Two Emergencies: Heart Failure and Saddle Thrombus
Two complications turn HCM from a silent finding into a life-threatening event. The first is congestive heart failure, where fluid floods the lungs or the chest cavity and the cat struggles to breathe — you may see rapid, labored, or open-mouth breathing, an extended head and neck, and blue-tinged gums. The second is arterial thromboembolism, often called a saddle thrombus, where a clot forms in the enlarged left atrium and lodges where the aorta splits to the back legs. The result is sudden, agonizing paralysis of one or both hind limbs, cold and pale foot pads, and loud crying. Both are true emergencies requiring immediate veterinary care; open-mouth breathing in a cat is never normal.
How Vets Diagnose HCM
Definitive diagnosis requires an echocardiogram — an ultrasound of the heart that directly measures the thickness of the ventricular wall and the size of the left atrium. Before that, your vet may run a blood NT-proBNP test, which rises when heart muscle is stressed and helps decide whether a murmur warrants cardiology referral, and chest X-rays to look for fluid. Blood pressure measurement and a thyroid panel are essential to rule out the treatable mimics. An ECG is added if the rhythm is irregular. The ACVIM consensus stresses that a murmur alone does not confirm HCM, because many cats with murmurs have structurally normal hearts and some cats with severe HCM have no murmur at all (Luis Fuentes et al., 2020, JVIM (ACVIM Consensus)).
Treatment and What the Future Looks Like
There is no cure for HCM and no drug proven to reverse the muscle thickening, so treatment is tailored to the stage. Subclinical cats with no signs are often simply monitored with periodic echocardiograms and home respiratory-rate tracking. Cats judged to be at high clot risk — those with a severely enlarged left atrium or sluggish blood flow on ultrasound — are usually started on the anti-clotting drug clopidogrel, which has been shown to reduce recurrent clot events. Cats in heart failure are treated with furosemide to clear lung fluid, sometimes with pimobendan or an ACE inhibitor, and need careful rechecks. Prognosis varies enormously: a cat with mild subclinical HCM may live years with a normal lifespan, while a cat presenting in heart failure or with a saddle thrombus has a guarded outlook measured in months. The single most useful thing an owner can do is track sleeping breathing rate at home and act fast when it climbs.
When to See a Vet
Call your vet today if:
- You hear that your cat has a newly detected heart murmur or irregular heartbeat
- Your cat's resting or sleeping breathing rate is consistently above 30 to 35 breaths per minute
- Your cat is suddenly less active, hiding, or eating noticeably less
- A predisposed breed (Maine Coon, Ragdoll, Sphynx) has never been screened for HCM
- Your cat tires quickly or seems short of breath after mild activity
Go to the ER immediately if:
- Your cat is breathing with its mouth open, panting, or gasping
- Breathing is rapid and labored with the head and neck extended
- One or both hind legs suddenly become paralyzed, cold, or painful
- Your cat is crying out, collapsed, or has blue or grey gums
- You see sudden, severe distress with rapid breathing and weakness together
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Frequently Asked Questions
What are the first signs of HCM in cats?
Early HCM usually has no visible signs at all, which is what makes it dangerous. The most catchable early clue is a sleeping breathing rate above 30 to 35 breaths per minute. A heart murmur or gallop sound found at a checkup, reduced activity, hiding, or faster fatigue can also point to it. Many cats are diagnosed only after a crisis.
How do I count my cat's resting respiratory rate?
Wait until your cat is fully asleep or completely relaxed, then count each rise and fall of the chest as one breath for 30 seconds and double it. A normal sleeping rate is under 30 breaths per minute. A sustained rate above 30 to 35, especially if it is climbing day to day, is an early warning of fluid around the lungs and warrants prompt veterinary attention.
How much does it cost to diagnose and treat feline HCM?
Initial vet exam typically runs $50 to $150 in the US, and an NT-proBNP blood test adds about $100 to $200. A cardiology echocardiogram runs roughly $400 to $700, with chest x-rays adding $150 to $400. Ongoing heart medications cost around $30 to $80 per month. A saddle thrombus or heart-failure ER crisis can reach $1,500 to $5,000, so catching HCM early through screening is dramatically cheaper.
Is hypertrophic cardiomyopathy in cats hereditary?
In several breeds, yes. A specific genetic mutation causing HCM has been identified in Maine Coon and Ragdoll cats, and a hereditary basis is suspected in many others, including Sphynx, British Shorthair, and Persian cats. Genetic tests exist for the known Maine Coon and Ragdoll mutations. Breeding cats from predisposed lines should be screened by echocardiogram before being bred.
Can a cat with HCM live a normal life?
Many cats with mild, subclinical HCM live for years with a normal or near-normal lifespan and never progress to heart failure. The outlook depends heavily on the stage at diagnosis: cats with a severely enlarged left atrium, those who have thrown a clot, or those already in heart failure have a much more guarded prognosis. Regular monitoring helps catch progression early.
Why does my cat with HCM need clopidogrel?
Clopidogrel is an anti-platelet drug that makes blood clots less likely to form. Cats with an enlarged left atrium and sluggish blood flow are at real risk of a saddle thrombus, a sudden, painful clot that paralyzes the hind legs. Clopidogrel has been shown to reduce the chance of a repeat clot, so vets often prescribe it preventively in higher-risk HCM cats.
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