Heart Murmur in Cats: What It Means and What to Do Next

Short answer: a heart murmur in a cat is a sound, not a diagnosis. Some murmurs are innocent — they come and go with stress or heart rate and mean nothing. Others are the first audible sign of heart disease, most often hypertrophic cardiomyopathy (HCM). No one can tell which kind your cat has by listening alone: that takes an echocardiogram, sometimes preceded by an NT-proBNP blood test. And the uncomfortable converse is also true — roughly a third of cats with HCM have no murmur at all.

What a murmur actually is

A murmur is turbulence: blood moving noisily where it normally moves silently. Anything that changes flow can cause one — a thickened heart wall, a leaky valve, anemia, an overactive thyroid, or simply a fast, stressed heart in an exam room.

Veterinarians grade murmurs from I (barely audible) to VI (audible with the stethoscope barely touching the chest). The grade describes loudness, not severity of disease. A soft murmur can accompany serious disease; a loud one can be innocent. This is why the grade alone should never end the conversation.

Innocent or significant?

Some murmurs in cats are physiologic — they appear under stress or a high heart rate and vanish when the cat calms down. They are common in young cats and in perfectly healthy adults.

The murmurs that matter are the ones that reflect structural disease. In cats, the most common structural cause is HCM, in which the muscle of the left ventricle thickens and stiffens. Other causes include other cardiomyopathies, congenital defects in kittens, and systemic conditions — hyperthyroidism, hypertension, anemia — that change how blood flows.

Age changes the odds. A new murmur in a cat over seven, or in a breed with known cardiac risk — Maine Coon, Ragdoll, British Shorthair, Sphynx, Persian — deserves a workup rather than a shrug.

The tests that actually answer the question

NT-proBNP is a blood test that rises when heart muscle is stretched or stressed. It is a screening tool: a normal result makes significant disease less likely; a raised one says "image this heart". It does not diagnose anything by itself.

Echocardiography is the answer. An ultrasound of the heart measures the walls directly, shows how the chambers fill, and finds the thickening of HCM — or rules it out. It is the only way to know whether a murmur matters, and in HCM it is also how progress is tracked over time.

Radiographs and ECGs have supporting roles, but neither can see wall thickness. If the budget allows one test after a suspicious murmur, it should be the echo.

If the echo finds HCM

First: a diagnosis of subclinical HCM — disease present, no symptoms — is not the catastrophe it once was. Many cats live for years at that stage. It is, however, the stage where knowing matters most, because monitoring and management decisions belong there, and because it is the stage where the first evidence-backed intervention now exists: a delayed-release form of rapamycin (sirolimus) was associated with slower progression of wall thickening in a placebo-controlled feline trial, and holds the first conditional FDA approval for managing the condition. We laid out that evidence honestly — including what it does not show — in our guide to rapamycin and HCM in cats.

Treatment decisions in cardiac cases belong with a veterinary cardiologist. Nothing about a murmur changes that.

What to do this week

If your cat has a newly discovered murmur: don't panic, and don't file it away either. Ask your veterinarian two questions — "could this be structural?" and "would NT-proBNP or an echo settle it?" — and book accordingly. A murmur that is checked and innocent buys you peace of mind. A murmur that is checked and real buys you time, which in feline heart disease is the thing worth the most.