HCM in Cats: Signs, Diagnosis & Home Monitoring

Hypertrophic cardiomyopathy is the most common heart disease in cats, and one of the most quietly dangerous. Most cats with HCM look completely healthy. Many never develop symptoms in their lifetime. But a smaller group will go on to congestive heart failure, a sudden, painful clot in the back legs, or sudden cardiac death. The frustrating part for owners is that you usually can’t tell which group your cat is in without a heart ultrasound, because the disease hides from a regular stethoscope check.

What HCM Is

In hypertrophic cardiomyopathy, the muscular wall of the left ventricle (the chamber that pumps blood to the body) becomes abnormally thick. The thickened muscle is stiffer and doesn’t relax properly between beats, so the heart can’t fill normally. Over time, the left atrium (the chamber feeding the ventricle) stretches as it works harder against the stiff ventricle, and an enlarged left atrium is the structural problem behind most of the bad outcomes in HCM: fluid backing up into the lungs (congestive heart failure), and blood pooling and clotting inside the atrium (which can shoot a clot down the aorta and lodge at the back legs).

Importantly, the thickening in HCM is not caused by another disease. It’s a primary problem with the heart muscle itself, often genetic. If thickening is caused by something else, like high blood pressure or hyperthyroidism, that’s classified separately, and treating the underlying cause can sometimes reverse it.

How Common Is It?

HCM is estimated to affect roughly 15% of the general cat population, with prevalence rising to about 29% in older cats (Luis Fuentes et al., 2020). The 15% figure traces back to a study of apparently healthy cats screened with echocardiography (Paige et al., 2009), so it includes many cats whose owners had no idea anything was wrong.

Most cats with HCM stay subclinical, meaning they have the disease on imaging but never develop symptoms. The 5-year cumulative cardiac mortality across all HCM cats in the consensus statement is about 23%. That number is for the whole HCM population, not a prediction for any individual cat. Some cats stay stable for many years; others progress.

Predisposed Breeds

Several breeds carry an above-average risk. The 2020 ACVIM consensus statement names Maine Coon, Ragdoll, British Shorthair, Persian, Bengal, Sphynx, Norwegian Forest, and Birman as predisposed. Mixed-breed cats also develop HCM; predisposition just means the odds are higher in those breeds.

The Genetics

Two specific mutations in the MYBPC3 gene have been validated and are recommended for breeding screens: A31P in Maine Coons (Meurs et al., 2005) and R820W in Ragdolls (Meurs et al., 2007). For these two breeds, a commercial DNA test exists and is endorsed by the consensus statement for breeding decisions. For non-Maine Coon, non-Ragdoll cats, no genetic test is currently recommended as a clinical tool. A Sphynx-specific variant (in the ALMS1 gene) has been reported and is offered commercially, but it post-dates the consensus and isn’t yet established as a screening test.

A few important caveats about the validated tests:

• Penetrance is incomplete. A cat carrying one copy of the mutation may never develop overt HCM. Cats carrying two copies (homozygotes) are at substantially higher risk and tend to develop disease earlier.
• A negative test does not rule out HCM. Other unidentified mutations and non-genetic causes exist. Maine Coons and Ragdolls without the validated mutation can still develop the disease.
• The test is most useful for breeders, who can avoid pairing two carriers. For pet owners, an echocardiogram tells you what’s happening today; a DNA test only tells you about risk.

Signs to Watch For

Most cats show no signs at all until something serious happens. There’s no early-warning cough, and you can’t really notice exercise intolerance in a cat that already sleeps 16 hours a day. When signs do appear, they’re usually signs of a complication, not of HCM itself.

Increased breathing rate or effort. This is the most useful early warning of congestive heart failure. A cat who used to breathe quietly while sleeping and is now visibly working at it (chest and belly heaving, breathing through an open mouth, hiding to breathe alone) needs emergency care. Open-mouth breathing in a cat at rest is always abnormal and almost always serious.

Sudden hind-leg paralysis with severe pain. This is aortic thromboembolism (ATE), sometimes called a saddle thrombus. A clot that formed in the enlarged left atrium has dislodged and lodged at the point where the aorta splits to the back legs. The legs go cold, the foot pads turn pale or blue, and the cat is in obvious, vocal pain. This is a medical emergency. It can happen in a cat who showed no prior signs of heart disease.

Fainting or collapse. Episodes of brief collapse, especially during stress or activity, can indicate a dangerous arrhythmia.

Gradual loss of appetite, weight loss, lethargy. Less specific but worth a vet visit, particularly in older cats or predisposed breeds.

If your cat has been diagnosed with HCM and is being managed by a vet, learning to count the resting respiratory rate is one of the highest-value monitoring habits you can build (more on that below).

Why a Stethoscope Isn’t Enough

Many owners assume that a routine vet visit, with a stethoscope on the chest, would catch a heart problem. For HCM, that assumption is a real risk. A heart murmur is heard in roughly 30-45% of healthy cats with no heart disease, and is heard in only some cats with HCM. In one screening study, the sensitivity of a murmur for detecting cardiomyopathy was 31% (Paige et al., 2009). That means almost 7 out of 10 affected cats had no detectable murmur on exam.

A gallop sound or an abnormal heart rhythm is more concerning when found, but they’re not sensitive screens either. A clean stethoscope exam is reassuring, but it can’t rule out HCM. Echocardiography is the only test that can.

How HCM Is Diagnosed

Echocardiogram (cardiac ultrasound) is the gold standard. A trained operator measures the thickness of the left ventricular wall, looks at how the chamber moves, and assesses the size of the left atrium. The size of the left atrium is the single most useful piece of information for risk stratification, because an enlarged left atrium is the engine of both heart failure and clot formation.

NT-proBNP is a blood test that becomes elevated when the heart muscle is stretched. Its main proven role is helping a vet decide whether a sick, breathing-heavily cat is in heart failure or has a primary lung problem; in that setting, it has good accuracy. As a screening test for healthy cats with mild-to-moderate HCM, the consensus statement explicitly does not recommend the quantitative assay. A cage-side SNAP test can be considered when a quick decision is needed and ultrasound isn’t available.

ECG (electrocardiogram) is used when an arrhythmia is suspected, not as a screen for HCM itself.

Chest X-rays show whether fluid has built up in or around the lungs, which is how heart failure is confirmed once symptoms appear.

Blood pressure and thyroid testing are standard parts of a cardiac workup, since hypertension and hyperthyroidism can cause secondary thickening that mimics HCM, and treating the cause can sometimes reverse it.

ACVIM Stages

The 2020 consensus statement classifies cats into five stages based on what’s actually present, not just on a diagnosis:

The stage matters because treatment depends almost entirely on it. A B1 cat usually just needs monitoring. A B2 cat may benefit from a clot-prevention drug. A C cat is on diuretics and active management.

Treatment by Stage

There’s no cure for HCM. The goal of treatment is to delay or reduce the consequences (heart failure, clot formation), not to reverse the muscle thickening.

Stage A and B1. No medication is currently shown to slow progression in cats with low-risk subclinical disease. The recommendation is monitoring: a follow-up echocardiogram on a schedule set by the cardiologist, and home tracking of resting respiratory rate.

Stage B2. When the left atrium is moderately to severely enlarged, the risk of clot formation is meaningfully higher. Clopidogrel is commonly used to reduce that risk. Discussion with a cardiologist about timing and dose is appropriate.

Stage C (after CHF or ATE). A diuretic, usually furosemide or torsemide, is the foundation for managing fluid in the lungs. Whether to add pimobendan in HCM has shifted in the last few years. It used to be considered off-limits for cats with dynamic outflow obstruction. The 2020 consensus suggests it can be considered when obstruction is absent, and is recommended when systolic function is reduced. The first prospective placebo-controlled trial of pimobendan in HCM cats with recent CHF (Schober et al., 2021) was negative on its primary endpoint, with hints of possible benefit in non-obstructive cases and possible harm in obstructive cases. The honest summary in 2026: pimobendan use in HCM is individualized by phenotype, and it’s a decision for the cardiologist managing your cat, not a default.

Stage C, after ATE. Acute care focuses on pain control, supportive care, and reassessment within hours, since outcomes vary widely. For preventing a second event, the FAT CAT trial showed clopidogrel was superior to aspirin: the median time to a recurrent thromboembolic event was 443 days on clopidogrel versus 192 days on aspirin (Hogan et al., 2015).

What Aortic Thromboembolism Looks Like

ATE deserves its own paragraph, because it’s the complication owners most need to recognize on sight. A clot that has been quietly forming in an enlarged left atrium suddenly breaks free, travels down the aorta, and lodges at the point where the aorta divides to feed the back legs. Within minutes the cat is unable to walk, the back legs are cold and stiff, the foot pads are pale or blue, and the cat is in severe pain (vocalizing, panting, distressed). It can happen with no prior warning, in a cat that seemed healthy.

Outcomes vary. In a UK general-practice series of 250 cats with ATE, 61% were euthanized at presentation, often because of the severity and the prognosis (Borgeat et al., 2014). Of cats given a chance, about 70% survived the first 24 hours. Among those who made it past the first week, the median survival was around 94 days, with some cats living a year or longer. The numbers are sobering, but they include cats whose owners chose euthanasia at presentation. With aggressive supportive care and good clot-prevention afterwards, some cats do come back, sometimes substantially. Recognising the signs and getting to an emergency vet quickly is the part you can actually influence.

For more on recognizing pain in cats, our guide to signs of pain in cats covers the subtler day-to-day cues; ATE is the opposite of subtle and needs an emergency vet, not a wait-and-watch.

Resting Respiratory Rate at Home

If your cat has been diagnosed with HCM (or any heart disease) and is being managed by a vet, the single most useful thing you can do at home is track the resting respiratory rate. In a healthy cat, the sleeping respiratory rate is consistently below 30 breaths per minute (Ljungvall et al., 2014). A trend toward higher rates, or a sustained reading above 30, is one of the earliest signs of fluid accumulating in the lungs, often hours to days before a cat looks visibly distressed (Porciello et al., 2016). Catching that early can be the difference between a planned diuretic adjustment and an emergency visit.

How to count: wait until your cat is asleep or fully relaxed (not purring; purring inflates the count). Watch the chest or belly rise and fall. Each rise-and-fall is one breath. Count for 30 seconds and double it, or for a full minute. Do it on the same general schedule each day. Track the numbers. Share the trend with your vet at follow-ups.

The validation of this monitoring approach is for cats with known heart disease, not as a screen for HCM in healthy cats. A single high reading in a healthy cat after a stressful afternoon doesn’t mean anything; what matters is the pattern in a cat who already carries a diagnosis.

If you’re already tracking other vitals like weight in older cats, see our senior cat care guide for how home monitoring fits into a wider screening routine.

Frequently Asked Questions

How long can a cat live with HCM? It depends entirely on the stage and the individual cat. Many cats with subclinical HCM live a normal lifespan and never develop symptoms. The 5-year cumulative cardiac mortality across all HCM cats is around 23%, but some cats remain stable for many years on monitoring alone. Cats who have had an episode of congestive heart failure or aortic thromboembolism have a more guarded prognosis, and the goal of treatment becomes maintaining quality of life as long as possible.

Can HCM be cured? No. Hypertrophic cardiomyopathy is a primary disease of the heart muscle, often genetic, and there’s no treatment that reverses the thickening. Treatment focuses on delaying or managing the complications (heart failure, clot formation). If thickening is caused by another condition like high blood pressure or hyperthyroidism, that’s classified separately, and treating the underlying cause can sometimes reverse it.

Should I get my cat genetically tested? Genetic testing is recommended for Maine Coons and Ragdolls used for breeding, where the goal is to avoid pairing two carriers. For pet owners of these two breeds, the test tells you about relative risk but doesn’t replace an echocardiogram. For other breeds, no genetic test is currently recommended as a clinical tool. A negative test does not rule out HCM.

Why didn’t my vet hear anything wrong at the annual exam? Heart murmurs are not a reliable screen for HCM. About 30-45% of healthy cats have a murmur, and many cats with HCM don’t have one. A clean stethoscope exam is reassurance, but it can’t rule out HCM. If your cat is a predisposed breed, has a heart murmur, gallop sound, or arrhythmia, or has had any episode that worried you, an echocardiogram is the only test that can confirm or rule out the disease.

HCM is hard because the same disease that does nothing in one cat can be life-changing in another, and you usually can’t tell which is which from the outside. If your cat is a predisposed breed or has any worrying finding, ask your vet about a baseline echocardiogram. Learn the emergency signs. And if your cat is already diagnosed, getting into the habit of counting the resting respiratory rate at home is one of the most useful things you can do.