Stage B2 HCM in Cats
By Dr. Kevin Toman, DVM · Medically reviewed November 1, 2026 · 40+ years in clinical practice · 300+ cats with HCM treated
Short answer: Stage B2 HCM means your cat's heart wall is too thick and the heart shows higher-risk changes, most often an enlarged left atrium. Your cat has no symptoms yet. B2 cats have a higher chance of heart failure or a blood clot than B1 cats. It is not a death sentence, but it is the stage where a clear plan matters most.
Why B2 is the most important stage to act on
Think of B2 as a yellow light. Your cat looks fine. But the heart is showing strain.
This is the stage where good care can make the biggest difference. Your cat hasn't had a crisis yet, and the goal is to keep it that way.
B1 vs. B2: what's the difference?
| Stage B1 | Stage B2 | |
|---|---|---|
| Heart wall | Thicker than normal | Thicker than normal |
| Symptoms | None | None |
| Left atrium | Normal or only slightly larger | Enlarged, or other risk signs |
| Risk of heart failure or clot | Lower | Higher |
| Usual recheck | About every 12 months | About every 6 months, or as your vet advises |
| Blood-thinning medicine | Usually not needed | Often recommended |
Why an enlarged left atrium matters
The left atrium is the small chamber that collects blood from the lungs. It passes that blood into the main pumping chamber.
When the main chamber gets thick and stiff, blood backs up. The left atrium stretches to hold it. That causes two problems:
- Higher pressure can push fluid into the lungs. That's heart failure.
- Blood moves slowly in a big atrium. Slow blood can form a clot. A clot can travel and block blood flow, often to the back legs. Vets call this a saddle thrombus, or ATE.
How to read your cat's echo report
Ask your vet for a copy of the report. Look for these terms:
| On the report | What it means | Why it matters |
|---|---|---|
| LVWd or MaxLVWT | How thick the heart wall is | Shows how much hypertrophy (thickening) there is |
| LA:Ao ratio | Left atrium size compared to the aorta | A higher number means a bigger left atrium |
| LA diameter (mm) | Left atrium width | Another way to measure left atrial size |
| LA FS% or LA function | How well the left atrium squeezes | Weak squeeze raises clot risk |
| SEC or "smoke" | Swirling, slow blood seen on ultrasound | A warning sign for clots |
| SAM or LVOTO | A blockage where blood leaves the heart | May change which drugs are used |
From Dr. Kevin's 400+ cats with HCM:
The first sign your cat is headed from stage B1 to B2 HCM is usually left atrial size, measured either by LA diameter or by comparing it to the aorta.
Why is this important? Because B2 kitties have a much higher incidence of throwing clots, so we need to start them ASAP on clopidogrel or riveroxaban to minimize their risk.
They should also be continued on Felycin-CA1. The stakes are now higher.
Does a B2 cat have symptoms?
Usually not. Most B2 cats:
- Eat normally
- Play normally
- Sleep normally
- Breathe normally at rest
That's what makes B2 tricky. No symptoms does not mean mild disease. Only an echocardiogram, or cardiac ultrasound. can see what's going on inside.
How B2 is diagnosed
- Echocardiogram (heart ultrasound). The main test. It measures the wall and the left atrium.
- Blood pressure. High blood pressure can thicken the heart wall.
- Thyroid test. An overactive thyroid can do the same.
- Won't work: an ECG (heart rhythm test), chest X-rays or an NT-proBNP blood test.
Treatment options for Stage B2
Every B2 plan should be built for that cat. Here are the main tools:
| Treatment | What it does | Evidence |
|---|---|---|
| Clopidogrel/Riveroxaban | Makes blood less likely to clot | To lower repeat clot risk in cats that already had a clot. Widely recommended for B2 cats with a large left atrium. |
| Felycin-CA1 (sirolimus) | Targets the wall thickening itself | FDA conditionally approved for cats with HCM and no symptoms, which includes B2. |
| Check kidneys and BP | Common complications | High BP is a common cause of heart disease. Both high BP and HCM can impact kidney function. |
| Repeat echocardiograms | Track change over time | Standard of care |
Why B2 cats are often good Felycin candidates: Felycin is FDA conditionally approved for cats with subclinical HCM, meaning a thick heart wall with no symptoms. B2 cats fit that description. Learn more here.
Felycin does not replace clopidogrel or other cardiac drugs. They do different jobs, and many B2 cats may need both. Ask your vet.
Outside the US?
We ship Felycin routinely to the US, UK, and Canada. You start by sending your vet's prescription, or by enrolling your cat in our Cat Longevity Protocol. You can purchase Felycin from us and get guaranteed delivery, although it sometimes takes a few days.
If you live in the EU, Taiwan, Hong Kong, or China you can purchase Felycin from us. You start by enrolling your cat in our Cat Longevity Protocol. You provide the shipping, and we cannot guarantee delivery but we can absolutely send it your way.
What to watch at home
Keep a simple log of:
- Sleeping breath rate
- Appetite
- Energy and play
- Weight
- Medicines given
Go to an emergency vet now for open-mouth breathing, hard breathing, sudden back-leg weakness, cold back legs, or crying out in pain.
Frequently asked questions
Is Stage B2 HCM serious?
Yes. It's higher-risk heart disease without symptoms. It needs a careful plan, but many cats stay stable for a long time.
Does B2 mean my cat has heart failure?
No. B2 cats have never had heart failure or a clot. Once that happens, a cat moves to Stage C.
Can a B2 cat go back to B1?
Sometimes the left atrium shrinks on a later scan. That's good news, but the disease is still there. Keep up with rechecks.
Should my B2 cat take clopidogrel?
It's often recommended when the left atrium is enlarged. Your vet will decide based on the echo results.
Can my B2 cat take Felycin?
B2 cats have no symptoms, so they fall within Felycin's US label. Your vet must first rule out liver disease and diabetes.
Your next step
Have a B2 echo report? Send it to Dr. Kevin for a review → He'll explain what the numbers mean and which options fit your cat.
Free guide: HCM University
References
- Luis Fuentes V, et al. ACVIM consensus statement on cardiomyopathies in cats. J Vet Intern Med. 2020;34(3):1062–1077.
- Hogan DF, et al. FAT CAT study: clopidogrel vs. aspirin after ATE. J Vet Cardiol. 2015;17 Suppl 1:S306–S317.
- US FDA. Conditional approval of Felycin-CA1. March 2025.
- Kaplan JL, et al. RAPACAT trial. J Am Vet Med Assoc. 2023;261(11):1628–1637.
This article is for education only. It does not replace an exam, diagnosis or treatment plan from your own veterinarian. Prices and regulations change; confirm locally.