Hypertrophic Cardiomyopathy in Cats: Signs, Stages, Treatment and Prognosis

Hypertrophic Cardiomyopathy in Cats: Signs, Stages, Treatment and Prognosis

By Dr Duncan Houston

A cat with hypertrophic cardiomyopathy can look completely healthy for years. Another may suddenly arrive at an emergency hospital struggling to breathe or unable to use their back legs.

That contrast is what makes hypertrophic cardiomyopathy, commonly shortened to HCM, so confusing and frightening for owners. A thickened heart wall is important, but it does not tell the whole story. The size and function of the left atrium, evidence of blood stagnation, abnormal heart rhythms and whether heart failure has developed are often more useful for understanding immediate risk.

Many cats with mild HCM continue to enjoy a good quality of life for years. The key is confirming the diagnosis properly, ruling out conditions that can imitate HCM and monitoring for the complications that change the urgency.

Quick Answer

Hypertrophic cardiomyopathy is a disease in which the muscular wall of the left ventricle becomes abnormally thick. This can make the heart stiff and less able to relax and fill normally, eventually increasing pressure within the left atrium and lungs.

Some cats remain symptom-free, while others develop congestive heart failure, arterial blood clots, abnormal heart rhythms or sudden death. Echocardiography is the main diagnostic test, and treatment depends on the cat’s stage rather than wall thickness alone.

Open-mouth breathing, marked breathing effort, collapse or sudden painful weakness or paralysis of the back legs is an emergency.

What Is Hypertrophic Cardiomyopathy?

Hypertrophic cardiomyopathy is the most commonly diagnosed feline cardiomyopathy phenotype. It is characterised by abnormal thickening of part or all of the left ventricular wall without the chamber being dilated.

The left ventricle is the main pumping chamber that sends oxygenated blood around the body. In HCM, the ventricle may still contract strongly, but the thickened muscle can become stiff and relax poorly between beats.

This impaired relaxation can lead to:

• Reduced filling of the left ventricle

• Increased pressure inside the heart

• Enlargement of the left atrium

• Blood stagnation within the left atrium

• Fluid accumulation in or around the lungs

• Abnormal heart rhythms

• Formation of blood clots

HCM should be diagnosed only after other causes of ventricular thickening, particularly systemic hypertension and hyperthyroidism, have been investigated. For this reason, cardiologists often describe an HCM phenotype until secondary causes have been excluded.

How Common Is HCM in Cats?

HCM is common enough that it can be found during screening of apparently healthy cats.

In one large prospective study of 780 apparently healthy cats, 14.7% were diagnosed with HCM on echocardiography. That is roughly one in seven cats in that particular population. Prevalence increased with age, although younger cats could also be affected. Male sex, increasing age, higher body condition score and the presence of a heart murmur were associated with a greater likelihood of HCM.

That figure should not be applied rigidly to every country, breed or household. The prevalence depends on the age, genetics and selection of the cats being studied.

HCM can occur in:

• Domestic shorthair and domestic longhair cats

• Maine Coons

• Ragdolls

• British Shorthairs

• Sphynx cats

• Bengals

• Persians

• Norwegian Forest Cats

• Other pedigree and mixed-breed cats

A pedigree may increase suspicion, but most cats diagnosed in general veterinary practice are not necessarily purebred.

Is a Thick Heart Wall Always HCM?

No.

A thickened left ventricular wall is an echocardiographic finding, not automatically a final diagnosis.

Conditions that can cause or imitate an HCM phenotype include:

• Systemic hypertension

• Hyperthyroidism

• Acromegaly

• Aortic stenosis or another cause of pressure overload

• Severe dehydration causing an apparently smaller ventricular chamber

• Myocardial inflammation or injury

• Infiltrative disease

• Transient myocardial thickening

• Measurement variation or an incorrectly timed measurement

Blood pressure and thyroid testing are particularly important in middle-aged and senior cats. Other investigations are selected according to the cat’s history, examination and echocardiographic appearance.

What is transient myocardial thickening?

Transient myocardial thickening can closely resemble HCM on an initial echocardiogram. It is thought to involve temporary myocardial oedema or injury rather than permanent hypertrophy.

It has most often been recognised in younger cats, sometimes following a major illness, anaesthesia, trauma, inflammation or another significant physiological stress. The heart-wall measurements can later return towards normal over several months.

A 2025 study found transient myocardial thickening in 16 of 53 cats with an HCM phenotype that underwent serial echocardiography. Their median maximum wall thickness decreased from approximately 8.3 mm to 5.4 mm over a median of 3.5 months. This does not mean every young cat with thickened walls has a temporary problem, but it shows why repeat imaging can be important when the clinical context is unusual.

What Happens to the Heart in HCM?

The consequences of HCM depend on more than how thick the ventricle becomes.

Impaired relaxation

The thickened muscle can become stiff. The ventricle therefore requires greater pressure to fill with blood between contractions.

Left atrial enlargement

The left atrium receives blood from the lungs and passes it into the left ventricle. If the ventricle becomes difficult to fill, pressure backs up into the left atrium.

Over time, the atrium may enlarge.

Left atrial enlargement is one of the most important findings in feline HCM because it is associated with a greater risk of:

• Congestive heart failure

• Blood stagnation

• Intracardiac thrombus formation

• Arterial thromboembolism

• Reduced long-term survival

The atrial measurement and its change over time often matter more clinically than a small difference in ventricular wall thickness.

Congestive heart failure

When pressure rises sufficiently, fluid can escape from the blood vessels.

Cats may develop:

• Pulmonary oedema, meaning fluid within the lungs

• Pleural effusion, meaning fluid in the space surrounding the lungs

• Both conditions at the same time

Either can prevent the lungs from expanding and exchanging oxygen normally.

Arterial thromboembolism

Slow or turbulent blood flow in an enlarged left atrium can promote clot formation. Part of a clot may detach, travel through the aorta and lodge where the aorta divides towards the hind legs.

This is commonly called feline arterial thromboembolism, aortic thromboembolism, FATE or a saddle thrombus.

It can cause:

• Sudden severe pain

• Crying or distress

• Weakness or paralysis of one or both hind legs

• Cold paws or limbs

• Pale, grey or blue paw pads

• Weak or absent femoral pulses

• Rapid breathing

• Collapse

This is an emergency and is often the first recognised sign of underlying heart disease. In the FAT CAT trial, only 15% of cats surviving an arterial thromboembolic event had a recorded history of heart disease before that event.

Abnormal heart rhythms

HCM may be associated with atrial or ventricular arrhythmias. Some cause no visible symptoms, while others may result in:

• Weakness

• Exercise intolerance

• Brief collapse

• Fainting

• Sudden death

An electrocardiogram or longer rhythm recording may be recommended when an arrhythmia is heard, a cat has collapsed or the echocardiogram suggests increased rhythm risk.

What Is Obstructive HCM?

Some cats with HCM develop dynamic obstruction to blood leaving the left ventricle.

This usually occurs when the mitral valve moves abnormally towards the outflow tract during contraction, a movement called systolic anterior motion. The narrowed outflow tract produces turbulence and may create a heart murmur.

This presentation is sometimes called:

• Hypertrophic obstructive cardiomyopathy

• HOCM

• HCM with dynamic left ventricular outflow tract obstruction

The degree of obstruction can change with heart rate, stress, dehydration and excitement. A very anxious cat may therefore have a louder murmur or greater obstruction during one examination than another.

Obstruction can influence medication choices, but it does not automatically mean a cat has a worse prognosis. In the large REVEAL study, cats with obstructive and non-obstructive preclinical HCM did not have significantly different rates of cardiovascular complications or death.

What Are the Signs of HCM in Cats?

Many cats with early HCM show no visible symptoms.

The disease may be discovered after:

• A heart murmur is heard

• A gallop sound is detected

• An irregular rhythm is found

• A screening echocardiogram is performed

• A preanaesthetic blood test shows an increased cardiac biomarker

• A related cat is diagnosed with HCM

• Breed screening is performed

When complications develop, the signs can appear suddenly.

Signs of congestive heart failure

Watch for:

• Faster breathing while asleep

• Increased abdominal movement with each breath

• Difficulty settling or lying down

• Sitting upright with the elbows held away from the chest

• Extending the head and neck to breathe

• Open-mouth breathing

• Reduced appetite

• Lethargy

• Hiding

• Weakness

• Collapse

Signs of arterial thromboembolism

Watch for:

• Sudden crying or severe distress

• Sudden inability to stand

• Dragging one or both hind legs

• One or both back legs becoming cold

• Pale or blue paw pads

• Rapid breathing

• Sudden weakness in one front leg, although this is less common

• Collapse

Signs of an arrhythmia or reduced circulation

These may include:

• Brief fainting episodes

• Sudden weakness

• Reduced activity

• Collapse

• Unexplained episodes of disorientation

• Sudden death

Cats generally do not show the same gradual exercise intolerance that owners may notice in dogs. Their daily activity can be difficult to assess because sleeping for most of the afternoon is not exactly unusual feline behaviour.

Does a Heart Murmur Mean a Cat Has HCM?

No.

A murmur is the sound of turbulent blood flow. It can occur because of HCM, an outflow obstruction, anaemia, hyperthyroidism, congenital disease or increased blood-flow velocity in a stressed but otherwise healthy cat.

In the CatScan study:

• 40.8% of apparently healthy cats had a murmur.

• Approximately 70% of those murmurs were considered functional.

• The positive predictive value of a murmur for HCM varied from 17.9% to 42.6%.

A murmur therefore increases suspicion but does not confirm HCM.

The opposite is also important: a cat can have HCM without a murmur.

An echocardiogram is warranted more strongly when a murmur is accompanied by:

• A gallop rhythm

• An arrhythmia

• Fainting

• Abnormal breathing

• A family history of HCM

• A known pathogenic genetic variant

• A louder or persistent murmur

• Planned breeding

• A procedure involving anaesthesia or substantial fluid therapy

How Worried Should You Be?

Risk level	What it may look like	What to do
Lower immediate risk	HCM found on screening, no symptoms, normal or minimally enlarged left atrium	Follow the cardiologist’s monitoring schedule. Treatment may not yet be required
Increased risk	Moderate or severe left atrial enlargement, reduced atrial function, spontaneous echo contrast or an intracardiac clot	Discuss antithrombotic treatment and closer monitoring with a cardiologist
Severe	Previous or current congestive heart failure, previous arterial thromboembolism, important arrhythmia or fainting	Ongoing medication and structured veterinary follow-up are required
Critical	Open-mouth breathing, marked breathing effort, sudden painful hind-leg paralysis, collapse or blue-grey gums	Go to an emergency veterinary hospital immediately

The ACVIM staging system separates symptom-free cats into lower-risk stage B1 and higher-risk stage B2 because two cats with the same ventricular wall measurement may have very different near-term risks.

When Is HCM an Emergency?

Go to an emergency veterinary hospital immediately if your cat develops:

• Open-mouth breathing

• Rapid breathing with obvious effort

• Strong abdominal movement during breathing

• An inability to lie down or settle

• Blue, grey or very pale gums

• Sudden collapse

• Sudden painful weakness or paralysis of one or more legs

• Cold hind paws

• Repeated fainting

• Severe weakness or rapid deterioration

Do not wait for the breathing rate to reach a particular number if the effort is abnormal.

What should you do during a breathing emergency?

• Keep your cat as calm and still as possible.

• Place them in a carrier with good ventilation.

• Avoid wrapping them tightly in blankets.

• Do not force food, water or oral medication.

• Do not repeatedly remove them from the carrier to count breaths.

• Call the emergency clinic while travelling so the team can prepare oxygen and minimal-stress assessment.

• Use an extra dose of a diuretic only when your veterinarian has already provided a specific written emergency plan.

Stress and restraint can worsen respiratory distress. In hospital, stabilisation may need to occur before a full examination, radiographs or echocardiogram.

What should you do if the back legs suddenly stop working?

Treat this as an emergency even if you are unsure whether the cause is a clot, spinal injury or trauma.

Keep the cat warm and confined, but do not massage the legs or apply direct heat. Feline arterial thromboembolism is intensely painful and requires immediate analgesia, cardiovascular assessment and supportive treatment.

How Is HCM Diagnosed?

The diagnosis should combine the echocardiogram with the cat’s blood pressure, age, medical history and relevant laboratory testing.

1. Veterinary Examination

Your veterinarian will assess:

• Heart rate and rhythm

• Presence and grade of a murmur

• Gallop sounds

• Pulse quality

• Breathing rate and effort

• Gum colour

• Body condition

• Thyroid enlargement

• Signs of systemic disease

• Limb temperature and pulses when thromboembolism is suspected

A normal examination does not completely rule out HCM.

2. Echocardiography

An echocardiogram is an ultrasound examination of the heart and is the principal test for diagnosing and staging HCM.

It allows assessment of:

• Left ventricular wall thickness

• Pattern and distribution of hypertrophy

• Left ventricular chamber size

• Left atrial size

• Left atrial function

• Mitral valve movement

• Left ventricular outflow obstruction

• Systolic and diastolic function

• Spontaneous echo contrast

• Intracardiac thrombus

• Other cardiomyopathy phenotypes

• Congenital or acquired heart disease

A maximum end-diastolic wall thickness of 6 mm or greater is commonly used as evidence of hypertrophy in an adult cat, but the measurement must be interpreted alongside body size, image quality, loading conditions and the rest of the examination. Borderline measurements should not be treated as a diagnosis in isolation.

Does the cat need a cardiologist?

A veterinarian experienced in echocardiography may be able to identify important disease. A veterinary cardiologist provides the most detailed assessment when:

• Measurements are borderline

• The cat is intended for breeding

• Significant outflow obstruction is present

• The left atrium is enlarged

• A clot or spontaneous echo contrast is suspected

• The cat has had heart failure, collapse or thromboembolism

• Treatment decisions are complex

• Another cardiomyopathy phenotype is possible

3. Blood Pressure Measurement

Systemic hypertension can cause left ventricular hypertrophy and may coexist with primary HCM.

Blood pressure should be measured with minimal stress and repeated when the result is unexpectedly high. Senior cats and cats with kidney disease, hyperthyroidism or endocrine disease require particular attention.

4. Thyroid Testing

Hyperthyroidism can cause:

• Rapid heart rate

• Heart murmur

• Ventricular thickening

• High blood pressure

• Increased cardiac workload

Total thyroxine testing is especially important in middle-aged and older cats. Treating hyperthyroidism may improve cardiac changes when the thickening is secondary rather than primary HCM.

5. Thoracic Radiographs

Chest X-rays can help identify:

• Pulmonary oedema

• Pleural effusion

• Changes in heart shape

• Other causes of respiratory signs

• Concurrent airway or lung disease

Radiographs do not reliably rule out early HCM. A cat can have substantial myocardial disease while the cardiac silhouette still appears normal.

A cat in severe respiratory distress may need oxygen, point-of-care ultrasound or thoracocentesis before conventional radiographs are attempted.

6. Electrocardiography

An ECG records the electrical activity of the heart.

It may be recommended when:

• An irregular rhythm is heard

• The heart rate is unusually fast or slow

• The cat has collapsed

• Atrial fibrillation is suspected

• Ventricular premature complexes are present

• Anti-arrhythmic treatment is being considered

A short ECG can miss intermittent rhythm abnormalities. Holter or patch monitoring may occasionally be needed.

7. NT-proBNP and Cardiac Troponin

NT-proBNP is released in response to myocardial stretch. It can help identify cats that require further cardiac assessment and may support differentiation between cardiac and non-cardiac causes of respiratory distress.

However, it is not a replacement for echocardiography.

Studies have produced differing results depending on the population and test used. NT-proBNP is generally better at detecting more severe disease than mild or borderline HCM, and normal results can miss some cats with early disease. Abnormal results should therefore prompt interpretation alongside examination and echocardiography.

Cardiac troponin I may increase with myocardial injury. It can provide useful supporting information but does not independently confirm HCM.

What Are the Stages of Feline HCM?

The ACVIM system stages feline cardiomyopathy according to risk and clinical complications.

Stage	What it means	Typical management focus
Stage A	Increased risk but no current echocardiographic evidence of cardiomyopathy	Screening and monitoring
Stage B1	Cardiomyopathy is present, but the cat has no symptoms and is at lower near-term risk	Monitoring, usually without routine cardiac medication
Stage B2	No symptoms, but findings indicate a higher risk of heart failure or thromboembolism	Clot-risk assessment, closer follow-up and selected preventive treatment
Stage C	Current or previous congestive heart failure or arterial thromboembolism	Active cardiac treatment and long-term monitoring
Stage D	Heart failure is refractory to conventional stage C treatment	Specialist-directed adjustment and advanced palliative management

Not every cat passes predictably through every stage. Some remain in stage B1 for life. Others first present with an arterial thromboembolism or acute heart failure without any previously recognised murmur.

What Makes a Cat Stage B2?

Moderate or severe left atrial enlargement is the main feature that moves a symptom-free cat into a higher-risk group.

Additional findings that may increase concern include:

• Reduced left atrial function

• Spontaneous echo contrast, sometimes described as smoke-like swirling blood

• A visible intracardiac thrombus

• Marked ventricular hypertrophy

• Left ventricular systolic dysfunction

• Important arrhythmias

• Regional wall-motion abnormalities

• A gallop rhythm

• Previous fainting

The decision should be made from the complete echocardiographic and clinical picture rather than one measurement.

How Is HCM Treated?

There is no universal medication plan for every cat with HCM.

Treatment is selected according to:

• Disease stage

• Left atrial size

• Evidence of blood stagnation

• Presence of outflow obstruction

• Heart rhythm

• Systolic function

• History of heart failure

• History of thromboembolism

• Blood pressure

• Kidney function

• The cat’s ability to take medication

Treatment for Stage A

Stage A cats do not have current echocardiographic evidence of cardiomyopathy.

Management may include:

• Periodic veterinary examinations

• Echocardiographic screening for at-risk breeds

• Blood pressure monitoring

• Thyroid testing as the cat ages

• Genetic counselling for breeding cats

• Maintaining a healthy body condition

Routine cardiac medication is not indicated solely because a cat belongs to a predisposed breed.

Treatment for Stage B1

Most stage B1 cats are monitored rather than automatically medicated.

The follow-up interval is individualised but may involve repeat echocardiography approximately every 6 to 12 months. Longer intervals may be reasonable for stable mild disease, while borderline or changing findings may justify earlier reassessment.

Routine use of beta blockers has not been shown to prolong survival in cats with preclinical HCM. In one five-year study, atenolol did not significantly improve all-cause or cardiac survival compared with no atenolol treatment. It may still be prescribed for selected cats with severe dynamic outflow obstruction or clinically important arrhythmias.

ACE inhibitors, diuretics and antithrombotic medication are not automatically required for every stage B1 cat.

Treatment for Stage B2

Stage B2 cats are symptom-free but have a greater risk of developing heart failure or thromboembolism.

Clopidogrel is commonly recommended when left atrial enlargement, poor atrial function, spontaneous echo contrast or another finding indicates meaningful thromboembolic risk.

Clopidogrel reduces platelet activity, but it does not guarantee that a clot will never form. Higher-risk cats may require a different or combined antithrombotic strategy under specialist direction.

The balance between treatment benefit, bleeding risk, medication stress and the cat’s specific echocardiographic findings should be discussed with the treating veterinarian or cardiologist.

Sirolimus for Subclinical HCM

In March 2025, the United States Food and Drug Administration conditionally approved delayed-release sirolimus tablets for the management of ventricular hypertrophy in cats with carefully defined subclinical HCM. As of 2026, the product remains listed by the FDA as a conditionally approved animal drug for this indication.

This is an important development, but it requires careful interpretation.

The conditional approval was based on a reasonable expectation of effectiveness, supported partly by a 43-cat pilot study. The effectiveness endpoint was selected after exploratory analysis rather than before the study began. At 180 days, the labelled-dose group had a small mean reduction in maximum wall thickness while the placebo group had an increase.

Sirolimus:

• Is not a cure for HCM

• Has not yet been proven to prevent congestive heart failure

• Has not yet been proven to prevent arterial thromboembolism

• Has not yet been proven to extend survival

• Is not intended for cats with current or previous heart failure or arterial thromboembolism

• Is not intended for cats with severe left ventricular outflow obstruction

• Requires screening for liver disease

• Should not be used in cats with pre-existing diabetes mellitus or liver disease

• Requires veterinary prescription and monitoring

Eligibility should be determined from a full cardiology assessment. It should not be purchased or used simply because an echocardiogram contains the word “hypertrophy”.

Treatment for Congestive Heart Failure

A cat with acute congestive heart failure may require:

• Minimal handling

• Oxygen support when tolerated

• Injectable diuretic treatment

• Thoracocentesis if pleural fluid is preventing lung expansion

• Temperature support

• Careful sedation to reduce panic

• Monitoring of blood pressure and oxygenation

• Assessment for arrhythmias

• Antithrombotic treatment where indicated

Furosemide is the principal diuretic used to remove excess fluid. Once the cat is stable, the lowest effective long-term dose is usually sought because excessive diuresis can contribute to dehydration, electrolyte changes and reduced kidney perfusion.

Follow-up commonly includes:

• Kidney values

• Electrolytes

• Body weight

• Sleeping respiratory rate

• Blood pressure

• Repeat chest imaging or echocardiography when indicated

• Review of medication tolerance

Are ACE inhibitors routinely used?

ACE inhibitors have been used in selected cats with heart failure, but they are not automatically required for every cat with HCM and have not been proven to prevent progression in uncomplicated preclinical disease.

Is pimobendan used in cats with HCM?

Pimobendan may be considered in selected cats, particularly when there is reduced systolic function or evidence of low cardiac output and no important dynamic outflow obstruction.

Evidence is mixed. A prospective randomised exploratory study of cats with HCM and recent congestive heart failure did not identify a significant improvement in the 180-day outcome. Medication choice should therefore be based on the individual haemodynamics rather than treating pimobendan as a standard drug for every HCM cat.

Treatment for Arterial Thromboembolism

Acute treatment may include:

• Strong opioid analgesia

• Antithrombotic medication

• Management of concurrent heart failure

• Oxygen support when needed

• Fluid and electrolyte management

• Monitoring for acute kidney injury

• Monitoring for reperfusion injury

• Physiotherapy after circulation begins to return

• Nursing support for urination, temperature and mobility

The prognosis varies according to:

• Number of limbs affected

• Whether motor function remains

• Body temperature

• Presence and severity of heart failure

• Kidney function

• Potassium changes

• Response during the first 24 to 72 hours

• The underlying cardiac disease

Historically high rates of euthanasia have complicated interpretation of survival data. Recent reviews report that approximately 30% to 40% of treated cats may survive to hospital discharge, although outcomes vary substantially between populations and hospitals. Cats that survive the initial event may regain meaningful limb function, but recurrence remains a concern.

Preventing another clot

Clopidogrel has stronger evidence than aspirin for secondary prevention.

In the randomised FAT CAT trial, the median time to recurrent arterial thromboembolism was 443 days in cats receiving clopidogrel compared with 192 days in cats receiving aspirin. Clopidogrel also prolonged the combined time to recurrence or cardiac death.

Other antithrombotic drugs may be selected for individual high-risk cats. No medication removes the risk completely.

How Do You Monitor a Cat’s Breathing at Home?

Sleeping respiratory rate is one of the most useful home measurements for cats with known cardiomyopathy or previous heart failure.

How to count it

1. Wait until your cat is deeply asleep and not purring.

2. Watch the chest or abdomen.

3. One rise and fall equals one breath.

4. Count for 30 seconds.

5. Multiply by two.

6. Record the result.

You can also count for a full minute when the movement is difficult to see.

Most cats with stable, medically controlled congestive heart failure have a mean sleeping respiratory rate below 30 breaths per minute. In one study, the median sleeping rate in stable cats was 20 breaths per minute.

What number is concerning?

Use the threshold supplied by your veterinarian because the cat’s personal baseline matters.

As a general guide:

• Consistently below 30 breaths per minute while deeply asleep is usually reassuring.

• A repeated increase above your cat’s normal baseline should prompt contact with your veterinary team.

• Repeated readings above 30 breaths per minute while genuinely asleep deserve veterinary advice, particularly in a cat with previous heart failure.

• A rate above 40 breaths per minute, obvious effort or open-mouth breathing should be treated as urgent.

Do not delay emergency care to obtain a perfect count when the cat is visibly struggling.

Common counting errors

Inaccurate results occur when the cat is:

• Awake

• Purring

• Dreaming or twitching

• Hot

• Stressed

• Recently active

• Sniffing

• Being handled

Record the number at roughly the same time and under similar conditions whenever possible.

What Else Should You Monitor at Home?

Keep track of:

• Sleeping respiratory rate

• Breathing effort

• Appetite

• Body weight

• Energy and social behaviour

• Fainting or weakness

• Hind-leg movement and temperature

• Medication doses

• Vomiting or diarrhoea

• Water intake and urination

• Difficulty administering medication

Contact your veterinary team if:

• The breathing rate is persistently rising

• Appetite falls

• Your cat vomits repeated medication doses

• Body weight changes quickly

• Weakness or collapse occurs

• Medication administration has become impossible

• Your cat appears dehydrated

• Urination decreases despite receiving a diuretic

Do not increase, decrease or stop cardiac medication without advice unless you have been given a written adjustment plan.

What Should You Do If a Murmur Is Found?

If your cat is breathing normally and otherwise well, an incidental murmur is not usually an emergency.

The sensible next steps are:

1. Ask whether a gallop rhythm or arrhythmia was also heard.

2. Review your cat’s age, breed and family history.

3. Measure blood pressure.

4. Test thyroid function where age or clinical signs justify it.

5. Arrange echocardiography, particularly when the murmur is persistent, louder, accompanied by another abnormal sound or relevant before anaesthesia.

6. Discuss NT-proBNP testing if an echocardiogram is not immediately available, while understanding that it cannot rule out every mild case.

A murmur should lead to investigation, not automatic medication.

What Should You Do After an HCM Diagnosis?

1. Ask which stage your cat is in

“Your cat has HCM” is only the beginning of the useful information.

Ask whether your cat is:

• Stage B1

• Stage B2

• Stage C

• Stage D

2. Ask about the left atrium

Important questions include:

• Is it normal, mildly enlarged or markedly enlarged?

• Is its function reduced?

• Is spontaneous echo contrast present?

• Is there a visible thrombus?

• Has its size changed since the previous scan?

3. Ask about outflow obstruction

Find out whether systolic anterior motion or dynamic left ventricular outflow obstruction is present because this may affect medication choices.

4. Establish a breathing baseline

Count the sleeping respiratory rate on several calm nights and record the results.

5. Confirm the recheck interval

A stage B1 cat may be rechecked in approximately 6 to 12 months. A stage B2 cat or one with changing measurements may need closer monitoring.

6. Tell other veterinarians about the diagnosis

Mention the heart disease before:

• Anaesthesia

• Sedation

• Intravenous fluid therapy

• Blood transfusion

• Major surgery

• Corticosteroid treatment

• Hospitalisation for another illness

The anaesthetic and fluid plan may need adjustment.

7. Ask for an emergency plan

Owners of cats with previous heart failure should know:

• Which signs require immediate transport

• Whether an extra diuretic dose should ever be given

• Which emergency clinic to use

• Which medications must not be missed

• How to transport the cat with minimal stress

Common Mistakes Owners Make

Assuming every murmur means HCM

Many feline murmurs are functional. Echocardiography is needed to determine what is actually happening.

Assuming no murmur means no heart disease

HCM can be silent. Some cats first present with heart failure or thromboembolism despite having no previously detected murmur.

Focusing only on wall thickness

The left atrium, blood-flow pattern, rhythm, systolic function and disease stage often provide more useful risk information than a single millimetre measurement.

Giving every HCM cat the same medications

Atenolol, clopidogrel, furosemide and pimobendan have different purposes. A medication that is useful in one stage may be unnecessary or harmful in another.

Waiting for coughing

Owners familiar with canine heart disease may wait for a cough. In cats, increased sleeping respiratory rate and breathing effort are more useful warning signs of congestive heart failure.

Counting breaths while the cat is awake or purring

This creates misleadingly high numbers and unnecessary panic.

Giving extra diuretic without a plan

Too little diuretic may fail to control fluid. Too much can worsen dehydration, kidney perfusion and electrolyte abnormalities. Dose adjustments should follow a veterinary plan.

Assuming a negative genetic test rules out HCM

Current tests detect specific breed-associated variants. They do not test for every possible genetic or non-genetic cause.

Assuming one echocardiogram predicts the entire future

HCM can progress, remain stable or, in selected cats with transient myocardial thickening, partially or completely regress. Serial assessment provides information that one scan cannot.

Genetic Testing for HCM

Two MYBPC3 variants currently have the strongest evidence and are recommended for routine breed-specific testing:

• A31P in Maine Coon cats

• R818W, historically also called R820W, in Ragdoll cats

A 2024 evidence review classified these two variants as pathogenic and advised routine genetic testing specifically for the associated Maine Coon and Ragdoll populations. Several other commercially offered variants were classified as likely pathogenic or of uncertain significance and require more cautious interpretation.

What does a positive test mean?

A positive result means the cat carries a specific variant associated with increased HCM risk.

It does not prove that the cat currently has echocardiographic HCM. Penetrance is incomplete, which means not every carrier develops the same degree of disease.

Cats carrying two copies generally have a greater risk of severe or earlier disease than cats carrying one copy.

What does a negative test mean?

A negative result means the cat does not carry the specific variant tested.

It does not mean:

• The cat cannot develop HCM

• Another unidentified variant is absent

• Echocardiographic screening is unnecessary

• The cat is automatically suitable for breeding

Most feline HCM cannot currently be explained by one commercially testable mutation.

Should Cats With HCM Be Bred?

A cat with echocardiographically confirmed HCM should not generally be used for breeding.

Cats carrying two copies of a recognised pathogenic variant should also be removed from breeding programmes.

Management of a heterozygous carrier within a small pedigree population is more complicated. Removing every carrier immediately may reduce genetic diversity and unintentionally increase other inherited problems. Decisions should therefore be made through a structured breed-health programme involving:

• Echocardiographic screening

• DNA testing

• Pedigree analysis

• Selection of a negative breeding partner where breeding is retained

• Avoidance of producing homozygous kittens

• Removal of clinically affected cats

• Ongoing screening of offspring

A veterinary cardiologist and geneticist familiar with the breed should guide these decisions. A DNA result is useful information, not a complete breeding plan.

Can HCM Be Prevented?

Most primary HCM cannot currently be prevented in an individual cat.

There is no proven supplement, special diet or exercise routine that prevents inherited myocardial hypertrophy.

Practical risk management includes:

• Maintaining a healthy body condition

• Screening predisposed breeds

• Avoiding breeding affected cats

• Using recognised breed-specific DNA tests appropriately

• Measuring blood pressure in adult and senior cats

• Testing for hyperthyroidism when appropriate

• Treating hypertension and endocrine disease

• Investigating persistent murmurs, gallops and arrhythmias

• Monitoring cats with known HCM

• Informing veterinarians before anaesthesia or fluid therapy

Do not start severe salt restriction without veterinary advice. Cats with heart disease still require a complete, palatable diet, and maintaining food intake is often more important than pursuing extreme dietary changes.

What Is the Prognosis?

There is no single life expectancy for every cat with HCM.

Some cats remain symptom-free and live a normal or near-normal lifespan. Others progress to congestive heart failure, arterial thromboembolism or sudden death.

The REVEAL study followed 1,008 cats with preclinical HCM or obstructive HCM. During the study period:

• 30.5% developed congestive heart failure, arterial thromboembolism or both.

• 27.9% experienced cardiovascular death.

• Estimated risk of heart failure was 7% at one year, 19.9% at five years and 23.9% at ten years.

• Estimated risk of arterial thromboembolism was 3.5% at one year, 9.7% at five years and 11.3% at ten years.

• A minority remained remarkably long-lived, with 10% reaching nine to fifteen years after study entry.

These figures describe a large mixed population, not the exact future of one individual cat.

Findings associated with a more guarded prognosis

Greater concern is associated with:

• Moderate or severe left atrial enlargement

• Progressive atrial enlargement

• Reduced left atrial function

• Spontaneous echo contrast

• An intracardiac thrombus

• Previous arterial thromboembolism

• Congestive heart failure

• Important arrhythmias

• Fainting

• Systolic dysfunction

• Poor response to treatment

• Concurrent kidney, thyroid or systemic disease

Left atrial size repeatedly appears as one of the most useful prognostic findings.

A median survival is not a deadline

Survival figures from referral studies are influenced by:

• Disease severity at presentation

• Whether the cat already had heart failure

• Whether thromboembolism occurred

• Treatment availability

• Owner decisions

• Concurrent disease

• Euthanasia decisions

• Study design

A cat should therefore not be assigned a countdown based on one published median.

The more useful questions are:

• Is the cat still subclinical?

• Is the left atrium enlarging?

• Has fluid accumulation occurred?

• Is clot risk increasing?

• Is the breathing rate stable?

• Is the cat eating, moving and interacting normally?

• Is treatment controlling the disease without causing unacceptable adverse effects?

Will My Cat Be Okay?

Many cats with HCM remain comfortable for long periods.

A stage B1 diagnosis does not mean that heart failure is imminent. It means the cat has a condition that deserves monitoring.

Stage B2 carries greater risk, but identifying that stage provides an opportunity to discuss clot prevention, closer surveillance and newer treatment options.

Cats that develop heart failure or thromboembolism have a more guarded outlook, but some respond well enough to regain a meaningful quality of life. The treatment decision should consider the cat’s breathing, mobility, appetite, stress, concurrent disease and response during the first days and weeks.

The aim is not simply to make an echocardiographic measurement smaller. It is to keep the cat breathing comfortably, moving normally and enjoying life.

Frequently Asked Questions

Can a cat with HCM live a normal lifespan?

Yes. Some cats with mild, stable HCM remain symptom-free and live a normal or near-normal lifespan. Risk rises when the left atrium enlarges or when heart failure, thromboembolism, fainting or important arrhythmias develop.

Can HCM in cats be cured?

Primary HCM currently has no cure. Treatment focuses on monitoring disease progression, reducing clot risk, controlling heart failure and treating clinically important obstruction or arrhythmias. Transient or secondary myocardial thickening may improve when the underlying cause resolves, which is why diagnosis matters.

Does a heart murmur mean my cat has HCM?

No. Murmurs can be functional or caused by several other conditions, and some cats with HCM have no murmur. Echocardiography is required to confirm the diagnosis.

What sleeping breathing rate is too high?

Most stable cats have a sleeping respiratory rate below 30 breaths per minute. A repeated increase above your cat’s baseline or repeated readings above 30 should prompt veterinary advice. Any obvious effort, open-mouth breathing or rate above approximately 40 while resting should be treated as urgent.

Does a gene-negative Maine Coon or Ragdoll still need heart screening?

Yes. A negative result rules out only the specific variant tested. It does not exclude other genetic causes or future HCM. Breeding cats should continue to receive appropriate echocardiographic screening.

Final Thoughts

Hypertrophic cardiomyopathy is common, but it does not behave the same way in every cat.

The most important question is not simply how thick the ventricular wall is. What matters most is whether the left atrium is enlarging, whether blood flow is becoming stagnant, whether the cat has developed heart failure and whether there is evidence of clot or rhythm risk.

An incidental murmur in a comfortable cat is not usually an emergency, but it deserves investigation. Open-mouth breathing, obvious respiratory effort, collapse or sudden painful hind-leg paralysis requires immediate care.

Once HCM has been diagnosed, ask for the stage, the left atrial measurements, the clot-risk assessment and a clear recheck plan. Establish a sleeping respiratory-rate baseline and learn the signs that should trigger faster action.

Many cats remain stable for years. The safest approach is neither panic nor complacency. It is accurate diagnosis, stage-based treatment and careful monitoring of the signs that genuinely change the risk.

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If you are unsure whether your cat’s breathing, heart murmur or echocardiogram findings are urgent, ASK A VET™ can help you understand the warning signs and prepare the right questions for your veterinary or cardiology team.