Cat Heart Murmur: Why HCM Needs an Echo, Not a Stethoscope

A heart murmur in a cat is a screening finding, not a diagnosis, and a quiet chest does not rule out heart disease. Between 30 and 50 percent of cats with hypertrophic cardiomyopathy (HCM) have no murmur at all on the day they are examined. The only test that confirms or excludes HCM is an echocardiogram, and the timing of that test matters more than the murmur grade: before anesthesia, before breeding, and the moment a sleeping cat's breathing rate starts to drift upward.

What a murmur is, and what grade 1 to 6 really means

A murmur is the sound of turbulent blood flow. In cats the usual causes are dynamic obstruction as blood accelerates past a thickened septum, systolic anterior motion of the mitral valve, or simply high flow in a thin, febrile, anemic or hyperthyroid cat. The 1 to 6 scale describes loudness: grade 1 is barely audible in a silent room, grade 3 is as loud as the normal heart sounds, grade 6 is audible with the stethoscope lifted off the chest wall.

Loudness correlates poorly with severity in this species. A grade 2 murmur in a 9-year-old Maine Coon can sit on 7 mm of ventricular wall, while a grade 4 murmur in a dehydrated kitten with a fast heart rate is often innocent flow noise. A murmur that appears only when the cat is stressed, or that comes and goes between visits, is a classic HCM pattern rather than a reason to relax.

Why so many affected cats sound normal

HCM thickens the ventricular wall concentrically, and obstruction is dynamic, meaning it worsens as heart rate rises. A cat resting at 180 beats per minute on an exam table may produce nothing audible. Others have diastolic dysfunction with no obstruction and no turbulence to hear. Add a thick chest wall in a long-haired cat and a 30-second auscultation in a noisy room, and silence becomes the expected result rather than evidence of health.

The reverse error is just as common: a murmur found in an older cat gets filed as "old cat heart" and never imaged, when the same murmur in a 7-year-old cat with a gallop sound or an irregular rhythm should trigger an echo within weeks.

The echo thresholds that decide the diagnosis

Echocardiography measures end-diastolic left ventricular wall thickness, usually reported as LVWd. Under 5 mm is normal for most cats. Between 5.0 and 5.9 mm is equivocal, and 6.0 mm or more in a cat of a predisposed breed is diagnostic of HCM. A single measurement on a single day is not enough: cats with equivocal walls are re-imaged in 6 to 12 months, because the disease is progressive and a cat at 5.5 mm today may be at 6.5 mm next year.

Two other numbers matter as much as wall thickness. A left atrium-to-aorta ratio above 1.5 signals atrial enlargement, which is the strongest routine predictor of congestive heart failure and of arterial thromboembolism. A Doppler gradient across the outflow tract above 30 mmHg confirms dynamic obstruction and changes both prognosis and drug choice.

Breeds, and what the MyBPC3 test can and cannot tell you

HCM is inherited in Maine Coons and Ragdolls through identified MyBPC3 mutations, and it is over-represented in Persians, Burmese, British Shorthairs and Sphynxes; the breed library lists the predisposed lines. A DNA test is a risk marker, not a diagnosis. A positive cat should be screened by echo starting at 1 year of age and every 12 months after that, and two positives should never be bred together. A negative cat can still develop HCM, because other mutations exist and are not on the panel. For that reason a clear swab never replaces imaging in a breed where the disease is common.

Before anesthesia, the echo is not optional

Any cat scheduled for a dental cleaning, mass removal or neuter with a grade 2 or louder murmur, an arrhythmia or a gallop should have an echo first. HCM cats are preload-dependent and handle tachycardia badly, so the anesthetic risk is not the murmur itself but unmasking heart failure with aggressive fluid rates. Knowing the wall thickness and atrial size beforehand lets the anesthetist choose a lower fluid rate, avoid drugs that spike heart rate, and monitor for the first sign of pulmonary edema.

How often to recheck

• Normal echo, no murmur, predisposed breed: every 2 to 3 years until age 6, then annually.
• Equivocal wall at 5.0 to 5.9 mm: repeat in 6 to 12 months.
• Confirmed HCM, normal atrium: every 6 to 12 months, plus a blood pressure check.
• Enlarged atrium or previous heart failure: every 3 to 6 months, with chest imaging as needed.

The one number an owner can measure at home

Sleeping respiratory rate is the cheapest monitoring tool in feline cardiology. Count chest movements for 30 seconds while the cat is asleep and flat, then double it. Do this at the same time each day for a week to learn the baseline.

Breaths per minute at rest	What it means	Action
Under 20	Comfortable	Continue routine checks
20 to 30	Normal range for most cats	Keep logging
30 to 40	Possible early congestion	Call the clinic within 24 hours
Over 40, or open-mouth breathing	Likely heart failure	Emergency visit now

Signs that mean the emergency room, not the next appointment

Open-mouth breathing, a resting rate above 40, gums that look gray or blue, collapse, or a hind leg that is cold, painful and dragging are all emergencies. The last one is arterial thromboembolism, and roughly half of affected cats die or are euthanized at the first episode; the survivors need pain control within hours, not days. Any cat that has been stable for a year and suddenly stops eating and hides for 24 hours deserves the same urgency, because appetite loss is often the first sign of decompensation in this species.

Five steps that change the outcome

1. Have every murmur grade 2 or louder imaged, and do not accept "watch it" without a wall measurement.
2. Book the echo before any anesthesia, not after a reaction.
3. Record sleeping respiratory rate weekly and bring the log to every visit.
4. Keep the cat lean; obesity raises the work of breathing and confuses the murmur exam.
5. Ask for a blood pressure reading at each recheck, since hypertension mimics and worsens the picture.

Breed screening, dental work and annual exams are the moments when HCM is found early, and early is the only stage where monitoring alone is still an option. Owners who track one number at home catch decompensation days before it becomes a crisis.

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