Quick answer: A heart murmur is an extra sound a veterinarian hears through a stethoscope when blood moves turbulently through the heart. It is a finding, not a diagnosis. Some murmurs are harmless, others signal a structural change, so the sound needs context: your pet’s age, species, physical exam, and often chest imaging or an echocardiogram.
What a murmur actually is
A healthy heart is a quiet pump. Blood moves through four chambers and past four valves in an orderly way, and a stethoscope picks that up as two clean sounds per beat, the familiar lub and dub. A murmur is the sound of that orderliness breaking down somewhere. When blood becomes turbulent instead of smooth, the turbulence itself is audible, usually as a soft whoosh layered over or between the normal beats.
That distinction matters more than almost anything else in this article. The stethoscope is not detecting damage. It is detecting flow. Turbulent flow can come from a valve that no longer closes cleanly, a valve that has narrowed, an opening between chambers that should have sealed shut early in life, a thickened heart wall crowding the outflow path, or simply blood moving faster or thinner than usual for reasons that have nothing to do with the heart’s structure at all.
So when a veterinarian says they hear a murmur, the honest translation is: something is making flow turbulent, and the next job is to work out what. That is a starting point for questions, not an answer.
Why some murmurs are innocent and some are not
Plenty of murmurs turn out to be clinically unimportant. Puppies and kittens quite often have a soft murmur that fades on its own as they grow, and a young animal’s murmur that disappears by the next visit is a common and reassuring story. Anemia thins the blood and makes it move more turbulently. Fever, pain, excitement and a racing heart rate can all do the same thing temporarily. A very lean, deep-chested athletic dog can produce a soft flow murmur simply because there is less tissue between the heart and the stethoscope. A cat who arrives furious in a carrier may murmur one week and be silent the next, because the murmur was tied to the heart rate, not to the heart.
Other murmurs point at real structural change. In older small-breed dogs, the most common reason a new murmur appears is gradual degeneration of a heart valve, which starts leaking a little and then rather more over a period of years. In young animals, a murmur that persists past the first few months can reflect a congenital defect in how the heart formed. In cats, a murmur can accompany thickening of the heart muscle.
What separates these groups is never the sound alone. It is the sound plus the age, the species, the breed, whether the murmur is new or has been noted for years, whether it has grown louder, and what the rest of the physical exam shows about rhythm, pulse quality and breathing.
What the grade does and does not tell you
Murmurs are usually graded for loudness, most often on a scale written as one through six, where one is barely audible in a quiet room and six can be felt as a vibration through the chest wall. Owners understandably read that number as a severity score. It is not one.
The grade describes volume, and volume describes turbulence, not consequence. A genuinely loud murmur can come from a defect that never troubles the animal. A quiet murmur, particularly in a cat, can sit alongside meaningful structural change. Please do not let anyone, including the internet, convert a grade into a prognosis for your pet. The number is one line in a much longer assessment.
Why many cats with heart disease have no murmur at all
This is the part cat owners are most often not told, and it is the reason this article exists.
In cats, the link between murmurs and heart disease is loose in both directions. A large share of cats with an audible murmur turn out to have nothing structurally wrong. More importantly, a cat can have significant change in the heart muscle and still have a completely quiet chest on examination. A normal listen in a cat is genuinely reassuring, but it is not proof of a healthy heart.
Cats compound this by being excellent at hiding effort. A dog with a struggling heart usually keeps trying to do the things it has always done, so the struggle becomes visible. A cat simply does less. It stops making the jump to the counter, it picks a lower place to sleep, it plays in shorter bursts, and every one of those changes reads as a cat getting older rather than a cat getting sick. Cats also rarely cough with heart disease the way dogs do, so the single sign owners are watching for is often the one sign that never comes.
The practical consequence is that feline heart disease can look like nothing at all and then declare itself abruptly, as sudden difficulty breathing, or as sudden pain and weakness in the back legs. Both are emergencies. Neither is a failure of attention on the owner’s part. It is simply how the disease behaves, and it is why trends noticed at home and a yearly examination both matter more in cats, not less.
What you can notice at home
Counting a resting respiratory rate
If you take one practical thing from this article, take this one. A resting respiratory rate is the single most useful number an owner can collect, it costs nothing, and it carries no risk.
Watch your pet while they are fully asleep or lying deeply relaxed. Count one breath each time the chest rises and falls together. Count for a full minute, or count for thirty seconds and double it. Do it a handful of times over a couple of weeks while your pet is well, and write the numbers down in a note on your phone. What you are building is your pet’s own baseline.
Two rules make the number meaningful. First, it only counts at true rest. A pet who has just played, who is warm, excited, frightened or panting will breathe faster for completely ordinary reasons, which is exactly why our post on keeping pets calm during fireworks matters here: a stressed animal’s breathing and heart rate climb on their own, and a count taken during that state tells you nothing about the heart. Second, this is a number to report, not a number to interpret. If your pet’s resting rate drifts upward from its usual figure and stays there over several days, that is worth a phone call and worth bringing the log to the appointment. It is not something to diagnose at the kitchen table.
Exercise tolerance and coughing in dogs
Dogs tend to show change through stamina. The signs are quiet ones: wanting to turn back early on a walk they have done for years, sitting down partway up an incline, lagging on the return leg, or taking noticeably longer to settle once they are home. Daly City helps a little here, because the weather stays cool and foggy for much of the year, so heat is less often available as the easy explanation when a dog suddenly wants to cut a walk short. On a route with any real climb in it, a change in stamina shows up early.
A soft, persistent cough in a dog is worth mentioning too, especially one that shows up at night, shortly after lying down, or on first waking. Coughs have many causes, plenty of them nothing to do with the heart, so a cough is a reason to be examined rather than a conclusion. The same goes for fainting or wobbling during excitement, a belly that seems to be filling out while the rest of the body is not, and appetite or weight that drifts down over months.
Write down what you see and when it started. A clear owner history changes an appointment more than most people expect.
What a workup can involve
Once a murmur is heard, the questions are where the turbulence is coming from, whether the heart’s structure has changed, and whether that change is currently affecting the animal. A cardiology workup is built from several pieces rather than a single test.
It starts with a thorough physical: listening over both sides of the chest, timing the murmur within the beat, assessing rhythm and pulse quality, and listening to the lungs for anything suggesting fluid. Blood pressure may be measured where it is relevant. General bloodwork and urinalysis look at the whole animal, since anemia and other systemic conditions can produce a murmur on their own.
From there, pet diagnostics can add imaging. Chest X-rays show heart size and silhouette and reveal how the lungs look. An ECG records the electrical rhythm when the beat sounds irregular. An echocardiogram, an ultrasound scan of the heart, is the study that actually shows chamber sizes, valve motion and the direction and speed of blood flow, which is how a murmur is finally explained rather than just described. In some cases a veterinarian will recommend referral to a veterinary cardiologist.
What follows depends entirely on what those tests show. For a number of pets the answer is a recheck interval and a resting respiratory rate log, and nothing more. Your veterinary team will talk you through the findings and the options for your individual animal.
Why the annual exam is where most murmurs turn up
Almost nobody arrives at a veterinary clinic because they suspect a murmur. Murmurs are found incidentally, by a stethoscope during a visit booked for something completely different: a vaccine appointment, a limp, an itchy ear, a dental check, or the examination before a planned procedure under anesthesia. Nothing you could have observed at home would have revealed it, because a murmur produces no symptom by itself. It is a sound, and only a stethoscope hears it.
That is the honest argument for the yearly physical examination even when a pet seems perfectly well, and for twice-yearly visits as pets age, which is how a wellness program is normally structured. It is also a useful contrast with the problems that do announce themselves. Our post on environmental allergies and itchy skin in Daly City dogs describes a condition your dog will make impossible to ignore. A murmur is the opposite kind of problem, and the exam is the only place it surfaces.
There is a second reason it is worth catching early. A murmur noticed before a planned anesthetic procedure can change how that procedure is approached, which is one of the quieter reasons the pre-anesthetic examination is not a formality.
When breathing changes cannot wait
Some signs are not a matter for the next available appointment. Labored breathing, breathing that visibly uses the abdomen, a cat breathing with its mouth open, gums that look pale, gray or blue, collapse or fainting, and in cats a sudden inability to use the back legs accompanied by obvious distress all warrant immediate veterinary attention. Call ahead if you can, so the team can prepare before you arrive.
Daly City Pet Hospital sees pet emergency and urgent care cases every day from 8 AM to 7 PM. For anything that happens outside those posted hours, a 24-hour veterinary facility is the right destination, and it is worth deciding now which one you would drive to, rather than working it out at eleven at night with a struggling pet in the car.
Frequently Asked Questions
Does a heart murmur mean my dog or cat has heart disease?
No. A murmur means blood is moving turbulently somewhere in the heart, which has many possible causes. Some are structural, some are temporary, and some are entirely harmless. The murmur tells your veterinarian to look further; it does not, on its own, establish that your pet has heart disease.
Can a heart murmur disappear?
Yes, some do. Soft murmurs in puppies and kittens frequently fade as the animal grows. Murmurs caused by fever, anemia, pain or a high heart rate at the clinic can resolve once the underlying reason resolves. Murmurs arising from a structural change in the heart generally persist and are monitored over time.
My cat’s heart sounded normal. Does that rule out heart disease?
Not entirely. Cats can have meaningful heart muscle change with no audible murmur at all, which is why a quiet chest in a cat is reassuring rather than conclusive. It is also why changes in a cat’s activity, sleeping position, play tolerance and resting breathing rate are worth reporting even when the examination sounded fine.
Does a louder murmur mean a worse outcome?
No. The grade describes how loud the murmur is, not how sick the animal is. Loud murmurs can come from defects that never trouble a pet, and quieter murmurs can accompany significant change, particularly in cats. Loudness is one observation among many and should never be read as a prognosis.
How do I count my pet’s resting respiratory rate at home?
Wait until your pet is asleep or lying completely relaxed, never straight after activity, excitement or stress. Count one breath for each full rise and fall of the chest, either across a full minute or across thirty seconds doubled. Record it a few times while your pet is well, then report any sustained rise to your veterinary team.
Should a murmur change plans for a dental procedure or surgery?
It can. A murmur found during a pre-anesthetic examination is a reason for your veterinarian to look more closely before proceeding, and the findings may influence how the procedure and the monitoring are planned. That conversation belongs with the veterinary team who has examined your pet and reviewed the results.