Your vet presses a stethoscope to your dog’s chest, pauses, and says the word no owner loves: murmur. Then, maybe, a number, “a grade 3.” Your stomach drops, and you start doing math on a scale you don’t understand, assuming a higher number means a sicker dog. Here’s the thing worth knowing before you spiral: that number is a measure of volume, not danger. A heart murmur is a clue that something may be worth a closer look. It is not, by itself, a diagnosis, and its grade tells you surprisingly little about how serious things are.
What a murmur actually is
A heart murmur is an extra whooshing sound layered over the normal heartbeat, usually caused by turbulent or unusually fast blood flow inside the heart. Vets sort murmurs into two broad camps. Physiologic (or “innocent”) murmurs are harmless, some soft ones are simply normal in puppies under about four to six months of age and disappear as the pup grows. Pathologic murmurs are the disease-related kind, and in dogs the common culprits are heart valves that have become leaky or narrowed, letting blood slosh where it shouldn’t.
The honest reality is that most murmurs do signal some underlying heart change. That might be a congenital abnormality a dog was born with, or an acquired heart disease that developed later in life, and those conditions run the full range from mild and lifelong-harmless to serious. Which is exactly why the murmur is a starting point, not an ending.
The grade measures loudness, not danger
Vets grade murmurs on a scale from 1 to 6 (written as I/VI through VI/VI). Roughly: grades I and II are soft, III is moderate, IV is loud, and V to VI are “thrilling,” loud enough that you can feel the vibration through the chest wall with a hand. It’s a useful shorthand for describing what a vet hears, and it helps track changes over time.
But here’s the trap owners fall into: grade does not reliably equal severity. A loud murmur can arise from a relatively benign cause, and a soft murmur can conceal genuinely serious disease. The volume reflects turbulence, not necessarily how much trouble the heart is in. There’s one important nuance to that rule, and I’ll come back to it, but the headline stands: the murmur, and its grade, are a red flag that possible heart disease is present. They can’t tell you how bad it is. Additional testing has to do that.
How the real answer gets found
If your vet identifies a murmur, the next step is figuring out what’s behind it, and the star test is an echocardiogram, an ultrasound of the heart. It shows the valves moving, the chambers filling and emptying, and the actual severity of any underlying problem in a way listening never can. Think of the murmur as a doorbell: it tells you someone’s at the door, but you have to open it to see who. Chest X-rays, blood pressure readings, and bloodwork often round out the picture.
What treatment looks like
The good news is that a murmur is frequently very manageable. Some types of heart disease stay mild for a dog’s entire life and never need treatment at all. Others produce symptoms that call for cardiac medications, lifestyle adjustments, or diet changes. Testing is what sorts your dog into the right bucket. Wherever a dog lands, if heart disease is confirmed, it means regular monitoring and periodic re-testing to guide care over time, because these conditions can evolve. Your vet will walk you through the prognosis and options based on what the tests show.
None of this is cause to panic. With the right treatment, consistent monitoring, and a good veterinary team, plenty of dogs live active, happy lives well after a murmur turns up. A murmur is information. Information is what lets you get ahead of a problem instead of being surprised by it.
What newer research adds
Remember that nuance I flagged? Here it is. While murmur grade is an unreliable guide to severity in general, research in small-breed dogs with degenerative (myxomatous) mitral valve disease, the most common acquired heart disease in dogs, has found that a louder murmur does tend to reflect more advanced disease in that specific population. So the “loudness doesn’t matter” rule bends for small dogs with leaky mitral valves, where a climbing grade is worth taking seriously.
That disease also shows how far cardiac care has come. Cavalier King Charles Spaniels are the poster breed: as many as 90 percent develop mitral valve disease by age 10. And thanks to a landmark trial known as the EPIC study, we now know that starting the medication pimobendan during a specific pre-symptomatic stage, an enlarged heart before congestive heart failure sets in, can meaningfully delay the onset of heart failure. That’s why catching disease early, via the echo your murmur prompts, genuinely changes outcomes. There’s even emerging work on machine-learning tools to help grade murmurs and flag which dogs most need that echocardiogram. The murmur is still just the doorbell. But we’ve gotten much better at answering the door in time.
“The grade tells me how loud, not how worried to be, and I wish that landed with more owners,” says Dr. Mara Chen. “A soft murmur can hide a real problem, and a loud one can be nothing. The murmur’s whole job is to earn your dog an echocardiogram. That’s the test that actually answers the question.”
References
- EPIC Study, Classification of canine heart disease
- Ljungvall I et al., “Murmur intensity in small-breed dogs with myxomatous mitral valve disease reflects disease severity”
- McDonald M et al., “A machine-learning algorithm to grade heart murmurs and stage preclinical myxomatous mitral valve disease in dogs” (JVIM, 2024)
