Hypertrophic cardiomyopathy (HCM) is the most common heart disease in cats, affecting roughly 15 percent of the general cat population on screening echocardiogram (Payne et al., 2015, JVIM). The American College of Veterinary Internal Medicine stages HCM A through D based on whether the heart looks structurally normal, has thickened walls without signs, or has already caused congestive failure or a saddle clot. Most cats live for years in stages A and B1 with no medication; cats in stages B2, C, and D need active management because the risk of clots and heart failure climbs sharply.
Last reviewed: June 2026
What HCM Actually Is
HCM is a primary disease of the heart muscle in which the left ventricle wall thickens for no obvious reason, leaving less room for blood to fill the chamber. Over time the left atrium stretches, slow blood flow in the atrium predisposes to clots, and high filling pressures back up into the lungs as congestive heart failure. A 2015 screening study of apparently healthy cats found echo-confirmed HCM in 14.7 percent — meaning roughly 1 in 7 cats has at least mild disease, often silent for years (Payne et al., 2015, JVIM). Maine Coon and Ragdoll lines carry known sarcomeric gene mutations, but most affected cats are domestic shorthairs without a clear genetic test.
The ACVIM A-B-C-D Staging Framework
The 2020 ACVIM consensus on classification of cardiomyopathies in cats divides disease into four stages that mirror the canine MMVD framework (Luis Fuentes et al., 2020, JVIM). Stage A is a cat at risk by breed or family history but with a normal echocardiogram. Stage B1 is a cat with mild ventricular thickening (wall thickness 6 millimeters or slightly above) but a normal-sized left atrium and no signs. Stage B2 is a cat with moderate to severe thickening AND an enlarged left atrium — these cats have a real annual risk of clots and heart failure. Stage C is a cat that has had at least one episode of congestive heart failure or arterial thromboembolism. Stage D is refractory heart failure that no longer responds to standard medications.
Stage B1: Watching, Not Treating
B1 cats have measurable thickening but a normal left atrium and no clinical signs. Most cardiologists do not start medication at this stage because no study has shown a benefit. The 2018 REVEAL study followed over 1,000 cats with subclinical HCM and reported that progression to congestive heart failure or clots was uncommon in B1 cats, around 7 percent over 5 years, but rose to roughly 30 to 40 percent in B2 cats over the same period (Fox et al., 2018, JVIM). The takeaway: a B1 cat is a recheck cat. Most cardiologists repeat the echo every 6 to 12 months and look for left atrial enlargement, the single most important prognostic marker.
Stage B2: When the Left Atrium Gets Big
Once the left atrium enlarges past about 1.6 times the aortic root, the cat moves into B2 and the risk picture changes. The same REVEAL data set showed that B2 cats develop heart failure or clots in roughly 30 percent of cases over 5 years, and median time to a clinical event was about 3 to 5 years (Fox et al., 2018, JVIM). At this stage many cardiologists start clopidogrel for clot prevention because the FATCAT trial showed clopidogrel cut recurrent clot risk roughly in half compared to aspirin. ACE inhibitors and beta-blockers are sometimes added based on individual factors, but the evidence is mixed and decisions are made case by case.
Stage C: Heart Failure or a Saddle Clot
Stage C cats have crossed the line into clinical disease. The two ways this presents are dramatic and impossible to miss. Congestive heart failure shows up as labored open-mouth breathing, fast respiratory rate at rest (often over 40 breaths per minute), hiding, and refusal to eat. Arterial thromboembolism — the so-called saddle thrombus — shows up as sudden hindlimb paralysis with cold, painful, pulseless back legs and loud crying. Both are emergencies. In-hospital survival of the first heart failure episode is roughly 50 to 75 percent with appropriate care; for cats surviving a clot event, median survival is around 6 to 12 months on clopidogrel and heart failure medication, per the 2020 ACVIM consensus (Luis Fuentes et al., 2020, JVIM).
How HCM Is Diagnosed
Echocardiogram by a cardiologist is the only definitive test. A heart murmur or gallop sound at a routine exam raises suspicion, but roughly one-third of cats with HCM have no audible murmur. A chest x-ray cannot reliably diagnose early HCM but is used to look for fluid in the lungs when a sick cat presents. NT-proBNP is a blood test that helps screen cats: a low value makes significant HCM unlikely, but it does not stage disease. Once HCM is confirmed, the cardiologist measures left ventricular wall thickness, left atrial size, and looks for sluggish swirling blood (smoke) in the atrium that predicts clot formation.
Resting Respiratory Rate at Home
The single most useful home monitor is a sleeping or resting respiratory rate. A normal cat breathes 16 to 30 times per minute while sleeping. A consistent rate above 30, and certainly above 40, means fluid is backing up into the lungs and the cat needs same-day veterinary attention. Owners can count breaths over 30 seconds and double the number; many cardiologists ask owners to log this 2 to 3 times a week in B2 and C cats. The 2021 AAFP-AAHA Feline Life Stage Guidelines, 2021 endorse home respiratory rate monitoring as a core part of senior cat heart screening.
When to See a Vet
Call your vet today if:
- Sleeping respiratory rate above 30 breaths per minute on two separate counts
- Hiding, reduced appetite, or new lethargy in a cat with known HCM
- Newly detected heart murmur or gallop sound at any age
- Mild cough or throat-clearing in a cat with known HCM (uncommon but worth a recheck)
- Maine Coon, Ragdoll, or other at-risk breed kitten with a family history — schedule a baseline echo
Go to the ER immediately if:
- Open-mouth or labored breathing at rest
- Resting respiratory rate over 50 breaths per minute
- Sudden hindlimb paralysis, especially with cold paws or loud crying — this is a saddle clot
- Collapse, weakness, or pale or blue gums
- A previously stable HCM cat who suddenly stops eating, hides, and breathes hard
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Frequently Asked Questions
How is HCM in cats staged A through D?
Stage A is an at-risk breed cat with a normal heart. Stage B1 is mild ventricular thickening with a normal left atrium and no signs. Stage B2 is moderate to severe thickening with an enlarged left atrium and a meaningful clot and heart failure risk. Stage C is a cat that has already had heart failure or a clot, and stage D is refractory failure that no longer responds to standard medication.
How much does HCM workup and treatment cost?
Initial vet exam typically runs $50 to $150 in the US, and basic bloodwork plus NT-proBNP adds $150 to $350. A cardiologist echocardiogram is $400 to $800. Hospitalization for a first heart failure episode runs $1,500 to $4,000, and ER care for a saddle clot is often $2,500 to $6,000. Ongoing medications average $40 to $120 per month. Catching B2 cats early through screening is dramatically cheaper than ER treatment.
What is the life expectancy of a cat with HCM?
Stage B1 cats often live a normal lifespan. Stage B2 cats have median survival of roughly 3 to 5 years from the time of moderate left atrial enlargement, though many do well longer. Cats that survive the first heart failure episode have a median survival of 12 to 18 months; cats surviving a clot event live a median of 6 to 12 months, depending on response to clopidogrel and heart failure medication.
Should every cat get a screening echo?
No. The 2020 ACVIM consensus recommends echo screening for at-risk breeds (Maine Coon, Ragdoll, Sphynx, British Shorthair, Persian) and any cat with a heart murmur, gallop sound, or elevated NT-proBNP. Routine echo screening of asymptomatic domestic shorthairs without a murmur is not recommended because the yield is low.
Can stress trigger heart failure in an HCM cat?
Yes. Anesthesia, IV fluids given too fast, hospital stress, and heat stress can all push a B2 cat into heart failure. This is why pre-anesthetic NT-proBNP or screening echo is recommended in at-risk breeds before elective surgery, and why HCM cats need conservative IV fluid rates and quiet recovery.
What is a saddle thrombus and how do I prevent it?
A saddle thrombus is a blood clot that forms in the enlarged left atrium, breaks loose, and lodges where the aorta splits to feed the back legs. The cat suddenly cannot use the hind end, the paws are cold and pulseless, and the cat screams in pain. Prevention in B2 cats is clopidogrel, which roughly halves recurrent clot risk based on the FATCAT trial.
Does diet or supplement therapy help HCM?
No specific diet has been shown to slow HCM progression. Taurine deficiency causes a different disease (dilated cardiomyopathy) and is rare in cats on commercial diets. Omega-3 fatty acids may modestly help cats already in heart failure. Avoid high-sodium treats and table food in cats with heart failure to prevent fluid overload.
Is cat HCM hereditary?
Often yes, especially in Maine Coons and Ragdolls, where specific MYBPC3 gene mutations are well documented. Genetic testing is available for these breeds. Most domestic shorthairs do not have an identifiable mutation, but the disease still runs in families — siblings and parents of an affected cat should be screened.
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