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Diagnostics2026-06-09 · 10 min read

Cat Heart Murmur: When It Means HCM, When It Doesn't, and What to Do Next

What a heart murmur in a cat actually means — HCM vs physiologic murmurs, NT-proBNP screening, echocardiogram timing, thyroid and blood pressure rule-outs, staging, and what to ask your veterinarian.

Ran Chen
Ran Chen
Founder, VetMedGuide. Life-sciences operator and 10× global market-access lead.
Published

Your veterinarian heard a heart murmur during your cat's annual exam. The immediate questions are: what does this mean, how urgent is it, and what should happen next?

A heart murmur is an abnormal sound — a whooshing or swishing — heard through a stethoscope alongside the normal heartbeat. It means blood is flowing turbulently through the heart. But in cats, a murmur is not always a sign of heart disease. Many healthy cats have innocent (physiologic) murmurs that never cause problems. At the same time, many cats with serious heart disease — including hypertrophic cardiomyopathy (HCM), the most common feline heart condition — have no murmur at all.

This article explains what a cat heart murmur means, what tests help sort benign from serious, what conditions cause murmurs in cats, and what the pathway from murmur to diagnosis to management looks like.

How common are heart murmurs in cats — and how many mean disease?

Heart murmurs are relatively common in adult cats. Studies have found murmurs in roughly 16–44% of cats presented to veterinary teaching hospitals. But the majority of those murmurs are physiologic — meaning they come from normal blood flow patterns, not from structural heart disease.

In one study of 100 cats presented to a teaching hospital, heart murmurs were detected in 44 cats, but only 10 were diagnosed with actual heart disease. In another study of 103 cats, murmurs were detected in 16 cats, but only 5 had confirmed cardiac disease.

At the same time, HCM affects up to 15% of all cats (roughly 1 in 7) — a rate significantly higher than in humans (1 in 300–500). Many cats with HCM have no audible murmur, particularly in early stages.

This creates a diagnostic challenge: murmurs are common but mostly benign, while HCM is common but often silent on auscultation. The key question after a murmur is detected is whether it represents structural disease or is simply a benign flow sound.

What causes a heart murmur in cats

Physiologic (innocent) murmurs

Many healthy cats have murmurs caused by normal blood flow dynamics — no structural heart disease. These physiologic murmurs are often dynamic: they may get louder when the cat is stressed or excited (because catecholamine release increases contractility) and softer when the cat is calm. They typically do not progress and do not require treatment.

Dynamic right ventricular outflow tract obstruction (DRVO) is a common cause of benign murmurs in cats. It does not indicate heart disease.

Hypertrophic cardiomyopathy (HCM)

HCM is the most frequently diagnosed cardiac condition in cats. It causes abnormal thickening (hypertrophy) of the left ventricle — the heart's main pumping chamber — which impairs the heart's ability to relax and fill with blood. Over time, this can lead to atrial enlargement, congestive heart failure, blood clots, and sudden death.

HCM is diagnosed by echocardiographic identification of left ventricular wall thickness exceeding 6 mm during diastole (excluding papillary muscles). For cats under 3 kg, a 5-mm threshold may be more appropriate.

Obstructive HCM (oHCM) is a subtype involving left ventricular outflow tract obstruction, often from systolic anterior motion (SAM) of the mitral valve. Cats with oHCM often have murmurs and may be detected earlier.

Other causes of heart murmurs in cats

  • Hyperthyroidism: Overactive thyroid hormone causes a hyperdynamic circulatory state that can produce tachycardia, murmurs, and left ventricular thickening that mimics HCM. Treating the thyroid often reverses the cardiac changes.
  • Systemic hypertension (high blood pressure): Can cause left ventricular thickening that looks like HCM on echocardiography. Blood pressure measurement is essential before diagnosing primary HCM.
  • Anemia: Reduced blood viscosity from anemia can produce a flow murmur.
  • Congenital heart disease: Ventricular septal defects (VSD), patent ductus arteriosus (PDA), and other structural abnormalities can cause murmurs, usually detected in kittens or young cats.
  • Other cardiomyopathies: Dilated cardiomyopathy (DCM), restrictive cardiomyopathy (RCM), and arrhythmogenic right ventricular cardiomyopathy (ARVC) are less common than HCM but can also produce murmurs.

Because hyperthyroidism and hypertension can both mimic HCM, both must be ruled out before a primary HCM diagnosis is made.

Breed risk for HCM

While HCM occurs in all cat breeds — including domestic shorthairs — certain breeds carry a higher genetic risk:

  • Maine Coon: MYBPC3 mutation (A31P). Genetic testing is available and recommended for breeding cats.
  • Ragdoll: MYBPC3 mutation (R820W). Genetic testing is available.
  • Sphynx, Bengal, British Shorthair, Persian: Increased prevalence reported, though specific genetic mutations are less well characterized.

Positive genetic test results indicate increased risk but do not guarantee the cat will develop clinical HCM. Annual screening with NT-proBNP or echocardiography is recommended for genetically at-risk cats starting at 2–3 years of age.

The diagnostic pathway: what happens after a murmur is found

Step 1: Physical exam and history

Your veterinarian will assess the murmur's grade (I–VI), location, timing, and whether it changes with heart rate. They will also evaluate respiratory rate and effort, body condition, thyroid gland size, and overall health status.

Signs that increase concern for significant heart disease:

  • Gallop rhythm (an extra heart sound)
  • Arrhythmia (irregular heartbeat)
  • Rapid or labored breathing
  • History of fainting, sudden weakness, or hind-limb paralysis (possible blood clot)

Step 2: Blood pressure and thyroid testing

Before pursuing cardiac imaging, your veterinarian should check:

  • Blood pressure: Systemic hypertension can cause left ventricular thickening that mimics HCM. If blood pressure is elevated, treating the hypertension may resolve the cardiac changes.
  • Total T4 (thyroxine): Hyperthyroidism is extremely common in older cats and can cause cardiac changes including murmurs, tachycardia, and left ventricular hypertrophy. If T4 is elevated, treating the thyroid condition is the priority.

These are baseline tests that should be done in any cat with a murmur, especially cats over 8 years old.

Step 3: NT-proBNP biomarker

NT-proBNP (N-terminal pro–B-type natriuretic peptide) is a blood biomarker that reflects cardiac stretch and stress. It is not a definitive diagnostic test, but it helps triage which murmurs warrant echocardiography.

How it helps:

  • Normal NT-proBNP (< 100 pmol/L): Suggests that significant heart enlargement is unlikely. A physiologic murmur is more probable. Echocardiography may not be urgent.
  • Elevated NT-proBNP (> 100 pmol/L): Suggests that structural heart disease may be present. Echocardiography is recommended.
  • Markedly elevated NT-proBNP (> 270 pmol/L): In a cat with respiratory signs, this strongly suggests congestive heart failure as the cause.

Important caveat: NT-proBNP is eliminated by the kidneys. In cats with azotemia (elevated kidney values), NT-proBNP may be elevated even without heart disease, which reduces its diagnostic accuracy.

A patient-side SNAP Feline proBNP test provides rapid qualitative results in the exam room, while quantitative NT-proBNP requires submission to a reference laboratory.

Step 4: Echocardiography (echo)

Echocardiography — ultrasound of the heart — is the gold standard for diagnosing HCM and other structural heart diseases. It allows the veterinarian or veterinary cardiologist to:

  • Measure left ventricular wall thickness
  • Assess left atrial size (an important prognostic indicator)
  • Detect systolic anterior motion of the mitral valve (SAM)
  • Evaluate diastolic function
  • Identify blood clot risk

Echo is ideally performed by a board-certified veterinary cardiologist, though some general practitioners with advanced training can perform screening studies. If echo is not available, NT-proBNP and thoracic radiographs provide alternative — though less precise — information.

Step 5: Additional tests if needed

  • Thoracic radiographs (chest X-rays): Assess heart size (vertebral heart score), lung fields for fluid (CHF), and pulmonary patterns. Helpful when echo is not available or to evaluate for congestive heart failure.
  • Electrocardiogram (ECG): Identifies arrhythmias that may accompany HCM.
  • Genetic testing: For Maine Coon and Ragdoll cats, MYBPC3 mutation testing helps assess risk.

HCM staging

When HCM is diagnosed, it is staged to guide monitoring and treatment decisions. The staging system parallels the system used for myxomatous mitral valve disease in dogs:

Stage Definition Typical management
A At-risk (e.g., breed predisposition or genetic mutation) but no evidence of disease Annual screening
B1 Structural HCM present (LV thickening) but normal left atrial size, no clinical signs Annual echo recheck
B2 LV thickening with moderate left atrial enlargement or other risk factors, but no clinical signs Consider medication; recheck every 3–6 months
C Current or past congestive heart failure, or arterial thromboembolism Active medical management; close monitoring
D Refractory CHF despite standard therapy Specialist-level management

Most cats diagnosed with HCM are in Stage B1 at the time of detection. Not all B1 cats progress. Regular monitoring is essential to catch progression early.

Treatment overview

Treatment depends on stage:

Stages A and B1: Generally no medication is recommended for non-obstructive HCM. Annual monitoring with echo or NT-proBNP. Atenolol may be considered for obstructive HCM with significant SAM.

Stage B2: Medication may be started, particularly if left atrial enlargement is present. Clopidogrel (18.75 mg once daily) is often prescribed for thromboembolism prevention. Felycin-CA1 (sirolimus delayed-release tablets) received conditional FDA approval for management of LV hypertrophy in cats with subclinical HCM — its label contraindications include diabetes mellitus, liver disease, aortic thromboembolism, and congestive heart failure.

Stages C and D: Active treatment of congestive heart failure typically includes furosemide (diuretic), an ACE inhibitor or similar afterload reducer, and clopidogrel for clot prevention. Management requires close veterinary supervision and frequent rechecks.

What a cat heart murmur might mean for pet insurance

If your cat has a heart murmur diagnosed before you purchase pet insurance, the murmur — and any condition related to it — will likely be classified as pre-existing. Some policies distinguish between curable and incurable pre-existing conditions; heart disease generally falls in the incurable category.

If you are considering pet insurance, enrolling your cat before a murmur is detected is the only reliable way to ensure cardiac conditions are covered. Once a murmur is documented in the medical record, related claims may be denied.

What to ask your veterinarian

  • "Does the murmur change when my cat calms down? Is it dynamic?"
  • "Should we check blood pressure and thyroid levels before pursuing further cardiac testing?"
  • "Would an NT-proBNP test help us decide whether my cat needs an echocardiogram?"
  • "If my cat needs an echo, can it be done here, or should we see a veterinary cardiologist?"
  • "Based on what you hear today, is this likely a physiologic murmur, or should we investigate further?"
  • "If HCM is found, what stage is it, and how often should we recheck?"

Sources

Topics
CardiologyDiagnostic Imaging
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