Educational content for pet owners and veterinary teams — not a substitute for an in-person veterinary exam.
Your dog went in for a booster. The vet listened to his chest a little longer than usual, moved the stethoscope forward, listened again, and said: "He has a heart murmur."
Most owners hear that as a diagnosis. It isn't. A murmur is a finding — a sound telling your vet that blood is moving turbulently somewhere in or near the heart. It doesn't say what's causing the turbulence or how serious it is. Those answers come from the workup that follows.
A heart murmur in dogs is the sound of turbulent blood flow heard through a stethoscope, graded from I to VI based on how loud it is. It is a clinical sign, not a diagnosis. Puppies can have soft "innocent" murmurs that fade as they grow. In adult dogs, a newly detected murmur most often points to a structural problem — degenerative mitral valve disease in small breeds, dilated cardiomyopathy in large breeds, or a congenital defect. A murmur is not heart failure, though some causes progress toward it. Diagnosis requires an echocardiogram; grade alone does not predict outlook.
Dog heart murmur grades: what I through VI actually mean
Vets grade murmurs on a six-point scale (the Levine scale) describing loudness and how the sound behaves — nothing else. The grade is recorded so the next vet who listens can tell whether it changed.
| Grade | What the vet hears | Practical note |
|---|---|---|
| Grade I | Very faint; needs a quiet room and focused listening to find | Easily missed on a busy day; often logged as "possible murmur" |
| Grade II | Soft, but heard immediately over its loudest point | Still quieter than the normal heart sounds |
| Grade III | Moderate; roughly as loud as the normal heart sounds | Where owners usually start hearing concern in the vet's voice |
| Grade IV | Louder than the heart sounds, but no vibration felt through the chest wall | Audible over a wide area |
| Grade V | Loud, plus a palpable thrill — a buzz you can feel with a hand on the chest | The thrill is the dividing line between IV and V |
| Grade VI | Palpable thrill, and audible with the stethoscope lifted slightly off the chest wall | Uncommon; almost always significant structural disease |
Two things owners get wrong here. First, loudness is not severity. A dog with a small congenital defect can have a dramatic grade V murmur and live an ordinary life, while a dog in advanced dilated cardiomyopathy may have a soft grade II — or none at all — because a weak heart pushes blood with less force. Second, the grade is partly the listener: if two vets grade the same dog II and III, that's normal variation, not a decline.
What does carry information: a murmur that gets louder over months in a dog with known valve disease, and where on the chest it's loudest. Left apex points toward the mitral valve. Left base suggests aortic or pulmonic outflow. Right side raises tricuspid disease or a septal defect.
What causes murmurs — and why heart murmur in puppies is a different conversation
The likely cause tracks age, size, and breed more than anything else.
Puppies: innocent murmurs. Growing puppies commonly have soft, low-grade murmurs with no structural disease behind them — the textbook explanation is a small, fast heart moving blood briskly through a relatively narrow outflow tract. These are usually grade I to II at the left base, and textbook references expect them to fade as the puppy grows, commonly by around four to six months. A murmur still there after that window, or one that's loud, harsh, or continuous, gets an echocardiogram — not more waiting. The dangerous version of "innocent murmur" is using the phrase to skip the workup on a congenital defect. Anemia and fever also produce murmurs that vanish once the cause resolves; if the gums look wrong too, see our guide to pale gums in dogs.
Small older dogs: myxomatous mitral valve disease (MMVD). The one most owners eventually meet. The mitral valve leaflets thicken and stop sealing, so blood leaks backward into the left atrium with each beat — that leak is the murmur, loudest at the left apex. It's the most common acquired heart disease in dogs, heavily weighted toward small breeds: Cavalier King Charles Spaniels famously, plus Dachshunds, Chihuahuas, Miniature Poodles, and terriers. It progresses slowly, often over years.
Large dogs: dilated cardiomyopathy (DCM). The muscle weakens and the chambers stretch. Dobermans, Great Danes, Boxers, Irish Wolfhounds, and Cocker Spaniels are classically over-represented. DCM is sneakier — the murmur may be soft or absent, and the first sign can be an arrhythmia, a collapse, or sudden death in a dog who "seemed fine." A big dog with a new murmur or an irregular rhythm gets imaged and rhythm-checked rather than watched.
Congenital defects. Patent ductus arteriosus (a continuous, machinery-like murmur), subaortic stenosis, pulmonic stenosis, and ventricular septal defects show up in young dogs. Some are correctable, and outcomes often hinge on catching them before the heart remodels around the problem.
Murmur vs heart failure: not the same thing
A murmur is a sound. Heart failure is a state — the heart can no longer move blood well enough to keep fluid from backing up, so the lungs get wet and the dog struggles to breathe. Many dogs live with a murmur for years and never reach that point. The relationship is sequential, not equivalent.
The MMVD path runs roughly: valve leaks (murmur appears) → leak worsens → the left atrium and ventricle enlarge to accommodate it → enlargement crosses a threshold where medication changes the timeline → in some dogs, fluid backs up and clinical heart failure begins. Cardiologists formalize this with ACVIM staging — Stage A (at-risk breed, no murmur), B1 (murmur, no significant enlargement), B2 (murmur with enlargement), C (current or past heart failure), D (failure refractory to standard therapy).
Staging matters more than the grade because the B1-to-B2 line is where treatment decisions change — and you can't hear that line through a stethoscope. It's measured on imaging. A dog can sit at B1 for years. Rechecks exist to catch the crossing, not to re-listen to a noise. If your dog is already staged at C, our guide to heart failure in dogs covers day-to-day management.
What your vet will do after finding a murmur
A reasonable workup is stepwise, each step answering a different question.
Auscultation, properly. Grade it, locate the point of maximum intensity, note timing (systolic, diastolic, continuous), and check rhythm and pulse quality. Blood pressure gets checked here too, since systemic hypertension can produce or amplify a murmur and is treatable on its own — see normal blood pressure ranges in dogs.
Chest radiographs. These show heart silhouette size and whether there's fluid in the lungs. They answer "is this dog in failure right now," a different question from "why does this dog have a murmur."
ECG. Indicated when the rhythm is irregular, when the dog has fainted or collapsed, or in breeds where arrhythmia often precedes the murmur — a Doberman being screened for DCM, for example. A three-channel veterinary ECG machine in the exam room lets the team characterize an irregular beat during the same visit rather than sending the dog home with a shrug.
Echocardiogram. The test that makes the diagnosis. It shows which valve is leaking, how badly, whether the chambers have enlarged, and how well the muscle contracts — the measurements that set the ACVIM stage. Cardiac imaging needs a phased-array probe and color Doppler; a general-purpose linear probe won't do this job. Practices building that capability start with a veterinary ultrasound scanner supporting cardiac presets and doppler, then refer complicated cases onward.
Then a plan. Many B1 dogs get nothing but a recheck interval. B2 dogs are where medication meaningfully changes the timeline. If your vet can't yet say which one you're dealing with, that's what the echo is for.
What owners should watch for at home
One home measurement outperforms everything else: sleeping respiratory rate. Count your dog's breaths for a full minute while he's asleep and settled — not panting, not dreaming. A healthy resting dog sits comfortably under about 30 breaths per minute. Do it twice a week and write it down. A consistent upward drift, especially into the high 30s and 40s, is the earliest reliable signal that fluid is accumulating, and it often shows up before your dog looks unwell. Cardiologists ask for it because it works.
Also worth noting between visits: stopping on a walk he used to finish, a soft cough at night or after lying down, restlessness overnight, fainting or a wobbly collapse, and a belly filling out while the dog loses muscle over the spine. Fainting and labored breathing are same-day calls.
Frequently asked questions
Is a heart murmur in dogs serious?
Sometimes. The murmur is a sound; the seriousness lives in the cause. A puppy's innocent grade II and an eight-year-old Cavalier's grade IV mitral murmur are different situations described with similar words. Before an echocardiogram the honest answer is "we don't know yet" — which is why the workup is recommended rather than optional.
Can a dog live a normal life with a heart murmur?
Many do. Plenty of dogs with low-grade murmurs and no chamber enlargement go their whole lives without a cardiac symptom and die of something unrelated. Others, whose disease progresses to enlargement, need medication and monitoring but still have good years. Outlook is set by cause and stage, not grade — which is why nobody can responsibly hand you a dog murmur life expectancy number from the sound alone.
Do puppy heart murmurs go away?
Innocent ones typically do, and textbook references put resolution around four to six months of age. A murmur still present past that point, or one that's loud, harsh, or continuous rather than soft, should be imaged. "Wait and see" is reasonable for a soft murmur in a twelve-week-old puppy and unreasonable for a grade IV murmur in a six-month-old.
How often should a dog with a murmur be rechecked?
Your vet sets the interval by stage. Broadly: stable low-grade murmurs without enlargement get looked at every six to twelve months; dogs with known enlargement or on medication get seen more often; any dog whose home respiratory rate is trending up gets seen now, not at the scheduled recheck. The value of the recheck isn't the stethoscope — it's the imaging and the numbers.
For veterinary teams: murmur workups are one of the clearest arguments for keeping rhythm assessment in-house, since the murmur and the arrhythmia frequently arrive in the same patient and the fainting Doberman doesn't wait for a referral slot. Our veterinary ECG machine buying guide covers channel count, lead handling, and what matters when you're chasing a dropped beat on a wiggling patient.