Heart Murmurs In Cats: Grades, Causes & What They Mean

If your vet mentions during a routine exam that they've heard a heart murmur, it's natural to feel a jolt of worry. Here's the reassuring nuance most owners aren't told upfront: a heart murmur is a sound, not a diagnosis. It tells your vet that something is causing turbulent blood flow in the heart — but the underlying cause can range from completely harmless to something that genuinely needs treatment, and figuring out which one it is requires a bit more investigation.

This article is for general information only and isn't a substitute for veterinary care. A heart murmur diagnosis should always be followed up with your vet's specific guidance for your cat.

What a Heart Murmur Actually Is

A heart murmur is an unusual "whooshing" sound heard alongside the heart's normal "lub-dub" rhythm, caused by turbulent blood flow within the heart or the vessels leading out of it. Your vet detects it with a stethoscope during a routine physical exam — it's not something you'd typically notice at home.

The Grading System: 1 Through 6

Vets grade murmurs on a scale of loudness, from Grade I (softest, sometimes only detectable intermittently or in one specific spot) to Grade VI (loudest, audible even without a stethoscope pressed to the chest, sometimes felt as a vibration through the chest wall).

Here's the part that surprises a lot of owners: the grade doesn't reliably predict severity. A soft Grade I murmur can occasionally indicate a serious underlying condition, while a loud Grade V or VI murmur can sometimes come from a relatively minor issue. The grade tells your vet how easily the murmur can be heard — not automatically how serious the cause is. That's why further investigation, not just the grade itself, is what determines the real picture.

What Causes a Heart Murmur in Cats

"Innocent" or physiologic murmurs — Sometimes heard in young kittens as an incidental finding, these often resolve on their own as the kitten matures and typically don't indicate underlying heart disease.

Hypertrophic cardiomyopathy (HCM) — One of the most common causes of heart murmurs in adult cats, this is a thickening of the heart muscle. Certain breeds — Maine Coons and Ragdolls in particular — are more prone to moderate-to-severe HCM, while Persians, American Shorthairs, and British Shorthairs see it somewhat less severely but still more often than the general population.

Other cardiomyopathies and congenital heart defects — Including conditions like patent ductus arteriosus (a vessel that fails to close properly after birth) or septal defects. Siamese cats have a higher rate of certain congenital heart issues.

Anemia — A drop in red blood cells can cause the blood to flow differently, sometimes producing a murmur even when the heart's structure itself is normal.

Hyperthyroidism — An overactive thyroid, common in older cats, can increase heart workload and contribute to a murmur.

High blood pressure (hypertension) — Can create turbulent flow patterns detectable as a murmur.

Heartworm disease and endocarditis (infection of a heart valve) — Less common but recognized causes.

Stress — Some cats develop a temporary, stress-related murmur during a vet visit that isn't present when they're calm at home, which is one more reason your vet may want to reassess at a follow-up visit if a murmur is borderline.

Signs That Suggest a Murmur Is Clinically Significant

Many cats with a murmur show no outward symptoms at all, especially in earlier stages. When symptoms are present, they commonly include:

Poor appetite
Weight loss (or stunted growth in a kitten)
Breathing problems or increased respiratory effort
Pale gums
Lethargy or weakness

If your cat shows any of these alongside a known or newly discovered murmur, that's a reason to follow up with your vet promptly rather than waiting for a routine recheck.

How Vets Investigate Further

Since the grade alone doesn't confirm the cause, your vet may recommend:

Bloodwork to check for anemia, hyperthyroidism, or other systemic issues
Chest X-rays to assess heart size and lung condition
Echocardiogram (cardiac ultrasound) — the most definitive way to see the heart's actual structure and function, and typically the test that provides a clear answer about what's causing the murmur
Blood pressure measurement

A referral to a veterinary cardiologist is common for a more detailed workup, particularly for murmurs that are new, changing, or paired with any of the symptoms above.

Does a Heart Murmur Mean My Cat Can't Have Surgery or Anesthesia?

Not automatically. A murmur alone doesn't rule out anesthesia — what matters is understanding the underlying cause first, since that informs how your vet manages anesthesia safely. This is a conversation worth having directly with your vet before any planned procedure.

What About Life Expectancy?

This depends entirely on the underlying cause, not the murmur itself. An innocent or stress-related murmur with no structural heart disease generally has no impact on lifespan. A murmur linked to significant cardiomyopathy or another serious condition carries a more variable outlook depending on severity and how well it responds to treatment — which is exactly why proper diagnostic workup matters so much before drawing any conclusions.

Frequently Asked Questions

My cat's murmur was described as "innocent" — does that mean I don't need to follow up at all? Even an innocent murmur is often worth a recheck at a future visit, particularly in kittens, to confirm it resolves as expected and doesn't represent something that needs closer monitoring.

Can stress alone really cause a murmur, or is that just an excuse? It's a genuinely recognized phenomenon — some cats develop a temporary murmur from the stress of a vet visit that isn't present when calm. This is one reason your vet might suggest a follow-up exam rather than drawing conclusions from a single visit.

Is a heart murmur more common in older cats? Yes, murmurs tend to be more prevalent in middle-aged and senior cats, often linked to acquired conditions like hypertrophic cardiomyopathy or hyperthyroidism, whereas murmurs in younger cats are more often congenital or innocent in nature.