"The Vet Heard a Murmur": What It Means and What It Doesn't

It is one of those sentences that can hijack a whole evening. Your pet went in for a routine vaccination or a check-up, the vet listened with the stethoscope a beat longer than usual, and out came the words: "There's a heart murmur." Suddenly the worry sets in, and the drive home is spent imagining the worst.

So let us steady the ship straight away. A heart murmur is a sound, not a diagnosis. It is a piece of information, a prompt to look a little closer, and on its own it tells you nothing certain about how long or how well your pet will live. Plenty of pets with a murmur have completely normal lives and never need a single tablet. Some murmurs do point to a problem worth managing. The whole job of the next few weeks is simply to work out which kind yours is. This article walks you through what the sound actually is, what the grade does and does not mean, why some murmurs are nothing to fear, and what should sensibly happen next.

What a murmur actually is

Inside a healthy heart, blood flows smoothly and quietly through the chambers and valves. When your vet listens, they hear the familiar "lub-dub" of the valves closing in turn, and not much else. A murmur is an extra sound layered over those normal beats: a whoosh, a swish or a soft blowing noise that fills part of the cycle. It is caused by turbulence, blood becoming choppy rather than flowing in clean lines, in the same way a calm river starts to babble when it runs over rocks or through a narrow gap.

The key thing to hold on to is that a murmur is heard, not felt, and it is a sound the heart is making, not damage in its own right. Turbulence can arise for several quite different reasons: a valve that no longer seals properly and lets blood leak backwards, a narrowed opening blood has to squeeze through, an abnormal connection between chambers, or simply blood moving faster or thinner than usual for reasons that have nothing to do with heart disease at all. That is precisely why the sound alone cannot tell you the cause. It tells your vet that flow is turbulent somewhere, and that it is worth finding out why.

What the grade means, and why louder is not always worse

If your vet mentions a number, they are grading how loud the murmur is, on a scale from 1 to 6. It is a long-standing convention, and it describes volume and the presence of a palpable vibration, not severity of disease. Roughly:

• Grade 1: very soft, often only audible after listening carefully in a quiet room.
• Grade 2: soft but heard fairly readily.
• Grade 3: moderate, easy to hear straight away.
• Grade 4: loud, with no vibration you can feel through the chest wall.
• Grade 5: loud, and now with a palpable "thrill", a buzz you can feel with a hand on the chest.
• Grade 6: very loud, with a thrill, audible even with the stethoscope lifted slightly off the chest.

Here is the part that surprises most owners, and it is genuinely reassuring: the grade is not a reliable measure of how serious the underlying problem is. A loud murmur is not automatically a worse murmur. A small, busy leak through a valve can be noisy out of all proportion, while a heart in real trouble can produce only a quiet sound, or sometimes barely any murmur at all. The relationship between loudness and disease severity is weak, which is exactly why your vet does not stage heart disease by the murmur grade alone. That said, the grade is not useless. In dogs in particular, a louder murmur does make significant underlying heart disease more likely, and softer murmurs are more often innocent (Côté et al., 2015; Ljungvall et al., 2014). It nudges the suspicion up or down, but it never decides the matter. The size of the heart, measured on a scan or an x-ray, tells you far more than the volume of the noise.

There is one more reason the grade can shift, and it catches people out. The same heart can sound a little different from one visit to the next, because a frightened, panting pet with a racing heart pushes blood through faster and turns up the volume. A murmur that was a grade 3 in a stressed cat at the clinic might sound softer once they have calmed down. So if the number changes slightly between appointments, do not read too much into it on its own.

When a murmur is nothing to worry about

A good number of murmurs are entirely harmless, and these go by various names: innocent, physiological or flow murmurs. They happen when blood moves faster or is thinner than usual, creating a little turbulence without any structural fault in the heart. They tend to be soft, usually grade 1 to 2, and they often come and go with the heart rate.

The classic example is the puppy or kitten. Young animals frequently have a soft innocent murmur that means nothing at all and fades away by around four to six months of age as they grow. It is so common that it is part of the normal soundscape of a wriggly, fast-hearted youngster. The catch is that a young animal can also have a murmur from a congenital heart defect, something they were born with, and those two possibilities can sound similar to begin with. So a murmur in a puppy or kitten is usually checked rather than dismissed, particularly if it is loud, if it persists past a few months, or if the youngster is not thriving. The reassuring headline is that most fade and turn out to be nothing; the sensible caveat is that a loud or lasting one in a young animal earns a proper look.

Murmurs can also appear when something elsewhere in the body is stirring up the blood. Anaemia, a low red blood cell count, makes blood thinner and noisier and can produce a flow murmur that resolves entirely once the anaemia is treated. A high temperature, an overactive thyroid (very common in older cats), pregnancy or simply a very athletic, slim-chested dog can all do something similar. In these cases the heart itself is structurally fine; it is just the conditions the blood is flowing under that have changed. Sort out the underlying cause and the murmur often quietens or disappears.

When a murmur does matter

The flip side is that a murmur can be the first audible clue to genuine heart disease, and this is the possibility your vet is sensibly ruling in or out. The picture differs between the species, and it is worth knowing which way the odds run.

In dogs, by far the most common cause of an acquired murmur, especially in older small-breed dogs like Cavalier King Charles Spaniels, dachshunds and many terriers and crosses, is degenerative mitral valve disease, where a heart valve thickens and starts to leak. It typically produces a murmur on the left side of the chest, and many dogs live happily for years in the early stages before it ever needs treatment. Larger breeds can develop a weakened, enlarged heart (dilated cardiomyopathy) instead. The point is that in dogs, a murmur is usually a reasonable signpost: if there is significant heart disease, there is often a murmur to go with it.

In cats, the relationship is far more slippery, and this is one of the most important things for a cat owner to understand. A murmur in a cat is a poor predictor of heart disease in either direction. Many perfectly healthy cats have a soft, dynamic murmur, often louder when they are stressed at the clinic, that signifies nothing. At the same time, a significant proportion of cats with the main feline heart disease, hypertrophic cardiomyopathy (a thickening of the heart muscle), have no murmur at all. Studies of apparently healthy cats have found murmurs to be common yet only weakly linked to actual disease (Payne et al., 2015; Wagner et al., 2010). What carries more weight in a cat is a gallop sound (an extra third heart sound) or an irregular rhythm, either of which is a stronger reason to investigate than the murmur itself (Luis Fuentes et al., 2020). The practical upshot: in a cat, a murmur is neither a clean all-clear nor a sentence, and listening alone simply cannot settle it.

What should happen next: imaging versus watchful waiting

Because the stethoscope can only take you so far, the next step is usually to gather more information, and how urgently depends on the whole picture, not the murmur in isolation. Your vet is weighing things like the species, the age and breed, how loud the murmur is, whether there is a gallop or an irregular beat, and above all whether your pet has any symptoms: coughing, getting tired or breathless on walks, fainting, or faster breathing at rest.

A pet who feels completely well, with a soft murmur and no other warning signs, may reasonably be monitored. That is not neglect; it is a measured response when the odds of a significant problem are low, and it usually means rechecking at the next routine visit and keeping an eye out for any change at home. By contrast, a louder murmur, an older patient, a high-risk breed, any symptoms, or the slipperiness of a feline murmur all tip the balance towards looking properly and sooner.

When your vet does decide to look further, the single most useful test is an echocardiogram, an ultrasound scan of the beating heart, because it shows the actual structure: which valve is leaking, whether the heart muscle is thickened or stretched, and whether the chambers have enlarged. A chest x-ray, an ECG for the rhythm, a blood pressure check (especially in cats) and a blood test called NT-proBNP can all add pieces too, and there are now blood tests that help decide whether a scan is worthwhile in the first place. We deliberately will not duplicate that here, because each of those tests, what it shows and what to expect on the day, has its own home in the tests explained. For a murmur, the headline is simple: the scan answers the question the stethoscope only raised.

It is also worth saying plainly that a murmur is a prompt to investigate, not an emergency in itself. Finding a murmur on a routine visit does not mean your pet is in danger that day. The thing that does warrant an urgent call is not the sound, it is the breathing.

Keeping watch at home while you find out

Whether your vet recommends a scan now or a watchful eye for now, there is one genuinely powerful thing you can do at home in the meantime, and it costs nothing. Counting your pet's resting breathing rate while they are asleep is the most sensitive early-warning sign we have that a heart is starting to struggle, often picking up trouble before any cough or tiredness appears. We will not run through the method here, because it deserves doing properly: the resting respiratory rate guide explains exactly how to count it and what is normal, and the breathing-rate tracker turns it into a quiet nightly habit you can show your vet. If you ever notice fast, laboured or open-mouthed breathing at rest, treat that as an emergency and ring your vet straight away, regardless of what the murmur grade was.

The questions to ask your vet today

You do not need to become a cardiologist before you leave the consulting room. A few plain questions will give you everything you need to know where you stand and what comes next:

• What grade is the murmur, and on this exam, does anything else stand out, a gallop, or an irregular rhythm?
• Given my pet's age, breed and species, how likely is this to mean real heart disease versus an innocent murmur?
• Do you recommend a scan or other tests now, or watching and rechecking? And what would make you change that plan?
• Are there any symptoms I should watch for at home, and what counts as an emergency?

Write the answers down before you go, because the worry has a way of erasing them on the way to the car. And then, gently, try to let the evening go. A murmur is the start of a question, not the answer to one, and far more often than owners fear, the answer turns out to be a reassuring one.

If you have just been told a murmur points to a leaky valve in your dog, the mitral valve disease explainer picks up that thread; if it is a cat and a thickened heart has been raised, feline HCM explained is the place to go next.