Affiliate & editorial disclosure: PetInsuranceLab may earn a commission when you request a quote or buy a product through links on this page. This does not affect our editorial independence or the accuracy of the information below.

Dog Heart Murmur Symptoms in 2026

A heart murmur, on its own, doesn't look like much of anything. Most dogs with one eat, play, and sleep completely normally -- the only "symptom" is a sound your vet picks up during a routine exam, not something you'd ever notice at home. What actually matters is what happens after that sound is heard: how it's graded, whether it fits the innocent-puppy pattern or the adult-disease pattern, and why the volume knob on a stethoscope tells you far less than most owners assume.

How Murmurs Are Graded -- And What Grade Actually Means

Vets describe a murmur's intensity on a standard 1-to-6 scale. Grade 1 is so faint it's only audible in a dead-quiet room; Grade 2 is soft but consistently there; Grade 3 is moderate and easy to pick up; Grade 4 is loud with no palpable vibration; Grade 5 is loud enough to feel as a vibration (a "thrill") through the chest wall; and Grade 6 is loud enough to hear even with the stethoscope lifted slightly away from the chest. Innocent, physiologic murmurs in puppies are almost always Grade 1-2 and disappear entirely by 4-5 months of age as the heart finishes maturing -- a fast heart rate and a young animal's naturally hyperdynamic circulation are the usual explanation, not disease.

Grading scale per VCA Animal Hospitals and standard veterinary cardiology references.

Why Grade Isn't the Severity Score Most Owners Assume It Is

Here's the detail that trips people up: a Grade 5 murmur is not automatically worse disease than a Grade 2. VCA Animal Hospitals is explicit that "the loudness of a heart murmur does not always indicate how severe the underlying disease is" -- the sound depends as much on how fast blood is moving through the leaky or narrowed valve as on how damaged the valve actually is. Veterinary cardiology literature backs this up further: auscultation reliably separates a mild leak from a non-issue, but it's considered unreliable for telling a moderate valve leak apart from a severe one just by ear. A quiet murmur can sit on top of significant heart enlargement; a dramatically loud one can, at least for a while, mean less than it sounds.

That's why mitral valve disease (MMVD) -- VCA's "most common cause of an acquired heart murmur in dogs," especially in smaller breeds -- is staged with an echocardiogram and the ACVIM's B1/B2/C/D system, not by ear. Stage B1 means a murmur with no detectable heart enlargement; B2 means the same murmur but with measurable enlargement on imaging, which is the point current cardiology guidance recommends starting pimobendan, well before any outward sign of illness. Stage C is active congestive heart failure, and D is advanced heart failure. A dog can sit in B1 or B2 for years on an unchanged Grade 3 murmur -- the number on the stethoscope doesn't move the same way the disease does underneath it.

The Breeds Where a Murmur Is the Rule, Not the Exception

For most dogs a murmur is incidental. For a Cavalier King Charles Spaniel, it's closer to an expectation. A long-running 1993 clinical study found roughly 56% of Cavaliers under age 5 already had an MMVD murmur, and reported 100% of Cavaliers age 10 or older had one -- figures still cited by breed-health researchers today, with more recent surveys generally converging on "over half by 5, well over 90% by 10." Other small and toy breeds carry elevated risk too (Dachshunds, Poodles, Chihuahuas, and Shih Tzus among them), but no breed's numbers approach the Cavalier's near-universal rate. That's also why insurers and breeders increasingly screen young Cavaliers specifically for a murmur before breeding -- a clean heart at 5 years old in this breed is the exception being documented, not the default being assumed.

If your dog's murmur has been confirmed as MMVD rather than innocent, the one number veterinary cardiologists actually ask owners to track at home isn't the murmur -- it's sleeping respiratory rate. A prospective study of 51 dogs with stable, medically controlled left-sided congestive heart failure found their sleeping and resting rates stayed under 30 breaths per minute; a rate that climbs and stays above that while the dog is genuinely asleep is one of the earliest signs fluid is starting to build up in the lungs, often before any cough or labored breathing is visible. Counting a full minute of chest rises with a phone timer costs nothing and catches the shift long before a vet visit would.

A pet vital-signs monitoring smart collar can track resting respiratory rate automatically overnight instead of relying on a manual count, which is especially useful for breeds like Cavaliers where MMVD is close to guaranteed with age.

Does Pet Insurance Cover a Dog's Heart Murmur?

Only if it's noted for the first time after your policy's waiting period has already passed. Most insurers define a pre-existing condition broadly -- any sign or symptom a vet or owner noticed before coverage began, not only a confirmed diagnosis -- and a murmur picked up at a routine wellness check counts. Because MMVD and most other murmur-causing conditions are progressive rather than curable, there's typically no symptom-free window that resets the exclusion the way there can be for a one-off, fully resolved issue. See our full pet insurance for heart murmur guide for exactly how that pre-existing-condition timing plays out provider by provider, including which companies handle a congenital murmur differently from an acquired one.

Frequently Asked Questions

Last updated: October 10, 2026