Understanding Heart Murmurs in Cats

Why it happens, what it means and what you should do

Published on: November 9, 2025

Reviewed on: November 9, 2025

Author: Dave Evans MA VetMB PgC(SADI) PgC(SAC) MRCVS

Has your vet heard a murmur in your cat’s heart? Perhaps your pet appears completely healthy and you had no prior indication that anything was wrong. Hearing the words “heart murmur” can be alarming and raise a lot of questions: Is it serious? Should you be worried?

With over 20 years of experience as a veterinary cardiologist, I’ve encountered this tricky dilemma many times. In this article I will explain why a murmur in a cat should always prompt further investigation and discuss how, in many cases, your cat may remain healthy despite the murmur. The key lies in diagnosing the underlying cause.

1. What is a Heart Murmur?

A heart murmur occurs when blood flow inside the heart becomes turbulent rather than smooth. Imagine a quiet, powerful river flowing steadily. Then picture that river rushing through rapids or under a narrow bridge. The sound changes dramatically. That change in sound is caused by turbulence.

In a cat’s heart, anything that disturbs smooth flow through the chambers, valves or great vessels can lead to turbulence and a murmur. Normally, a cat’s heart produces a steady “lub-dub, lub-dub” sound when auscultated with a stethoscope. With a murmur you might hear a whooshing or swishing noise.

Because cats have fast heart rates - especially at the vet - and the structures are small, accurately detecting murmurs takes skill and practice.

2. What Causes Heart Murmurs?

There are several reasons why a cat may have a heart murmur:

Innocent murmurs. Often seen in kittens, these murmurs are considered harmless and result from high flow speeds through a small heart. They often disappear as the kitten grows.
Flow murmurs. Caused by turbulent flow without underlying disease. These may appear in different parts of the heart, especially the outflow tracts.
Underlying heart disease. The most important consideration for cats over six months of age is heart muscle disease—particularly hypertrophic cardiomyopathy (HCM). Other causes include congenital heart defects and, less commonly in cats than dogs, leaky valves or infection-related valve changes.
Other conditions. Anemia or systemic infection can sometimes contribute to a murmur.

Your vet may also mention the grade of the murmur, usually on a scale of 1-6. While a louder murmur (higher grade) might seem alarming, grading has limited reliability in cats because murmurs can vary with heart rate and myocardial stress. Some cats with loud murmurs have little disease, while others with minimal murmurs may have serious heart muscle disease.

3. Hypertrophic Cardiomyopathy (HCM)

HCM is the most common cause of a murmur in cats. It involves thickening of the heart muscle due to genetic protein mutations at the cellular level. The thickened muscle becomes stiff and less able to fill properly during diastole.

Even though HCM is common, not all affected cats will have a murmur. That’s because whether a murmur is heard depends on where the thickening occurs. If it affects the left ventricular outflow tract, for instance, turbulent flow may result. But if the thickening is elsewhere - say, the apex of the heart - a murmur may be absent.

In short: some cats with HCM have no murmur; some cats with murmurs have minimal disease. It presents a diagnostic challenge.

4. Dynamic Murmurs

Many murmurs in cats are what we call “dynamic” murmurs. These tend to occur when an obstruction develops in the outflow tract, often linked to HCM, and the severity of the murmur changes with heart rate and systolic function.

The left ventricular outflow tract in a normal heart is a wide open superhighway of rapidly flowing blood. As a normal heart contracts and relaxes, the outflow tract remains unobstructed. In contrast, a heart with HCM may thicken in the outflow region and create narrowing. As blood is forced through that narrowing, flow becomes turbulent and a murmur results.

Because this narrowing can increase when the heart rate is faster, a murmur may come and go. Some cats may only show a murmur during excitement or stress.

5. Diagnosing Heart Murmurs

When your vet hears a murmur, further investigation is often warranted. Here’s how:

Auscultation (listening with a stethoscope) is step one. Sometimes murmurs are intermittent or hard to detect.
Gallop rhythm or arrhythmia may also be heard and are more likely to indicate disease than the murmur alone.
Echocardiogram (echo) is the gold standard. It lets us see heart chamber size, muscle thickness, valve leaks, and flow dynamics.
Pro-BNP blood test can help in areas without ready access to echo. A negative result suggests mild disease; a positive result should prompt an echo.
Chest x-rays can check heart size and lung health, but are less specific for HCM because thickening often occurs inwardly.
ECG may be used if rhythm disturbance or clot formation is suspected.

6. Symptoms

Many cats with heart murmurs show no obvious signs because they hide illness well. Subtle clues may include sudden stopping during activity or open-mouth breathing. When HCM progresses, more serious problems may arise, such as congestive heart failure (fluid in the lungs) or thromboembolism (blood clots blocking circulation to the legs). These conditions require urgent veterinary attention.

7. Treatment and Management

Management depends on the underlying cause. Some murmurs require no treatment, while others need specific medications.
Treatment options may include:

Diuretics to reduce lung fluid buildup when heart failure occurs.
Anti-clotting medications like clopidogrel, rivaroxaban or aspirin to reduce thromboembolism risk.
Beta-blockers in select cases of dynamic obstructive disease.
ACE inhibitors less frequently used now but may reduce remodelling in some cases.

While we currently have no proven preventive medication for HCM in cats, ongoing research into drugs like mTOR inhibitors and mavacamten offers hope for the future. Early investigation and diagnosis remain the best strategy.

8. What Can You Do as a Pet Owner?

If your cat has been told they have a murmur, don’t panic. Here are the steps you can follow:

Follow your vet’s advice on investigation and monitoring. Seek an echo by a suitably qualified cardiologist if possible.
Keep up with heart screening and regular check-ups.
Monitor your cat’s sleeping respiratory rate (a simple free method to watch for changes).
Note any changes in energy or behaviour, keep your cat at a healthy weight with appropriate diet.
Stay proactive and engage in follow-up investigations - usually repeat echoes may be beneficial over time.

With proper care, many cats can lead happy and healthy lives despite having a murmur.

Conclusion

A heart murmur is not a diagnosis; it’s a clue that calls for further investigation. Its loudness does not reliably reflect disease severity. Some cats with murmurs may have minimal underlying disease, while others without murmurs may have serious problems. The key is timely diagnosis - via echocardiogram or other appropriate testing. With this information, you can work proactively alongside your vet to support your cat’s heart health.

Check out my video on the same topic if you want a more in depth explanation:

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