Heart Murmur in Dogs: Causes, Grades and What Happens Next
Hearing your vet say they detected a murmur is worrying, but it is the start of finding answers.
What a murmur is
A murmur is a swishing sound heard with a stethoscope between normal heart sounds. It reflects turbulent blood flow, which may come from a leaky valve, a narrowed vessel, a hole in the heart wall or simply fast flow in a healthy heart.
Grades
Vets grade murmurs from I to VI based on loudness:
- I-II: soft
- III-IV: moderate to loud
- V-VI: very loud, sometimes with a palpable vibration
Loudness does not always match severity, so tests are needed.
Common causes
- Innocent or physiologic murmurs in some puppies, often disappearing as they grow
- Mitral valve disease, common in older small breeds such as Cavalier King Charles spaniels
- Congenital defects, such as patent ductus arteriosus and valve stenosis
- Dilated cardiomyopathy. See dilated cardiomyopathy in dogs
- Anemia and fever, which increase blood flow. See anemia in dogs
- Heartworm disease. See heartworm disease in dogs
- Endocarditis and other infections
Signs to watch for
Many dogs with murmurs show no signs at first. Over time you may notice:
- Coughing, especially at night
- Fast breathing at rest
- Reduced stamina
- Fainting
- Swollen belly
- Poor appetite or weight loss
See congestive heart failure in dogs.
Diagnostic tests
Your vet may recommend chest X-rays, an electrocardiogram, blood pressure measurement, bloodwork and an echocardiogram, which is often the most informative test. A veterinary cardiologist can interpret complex cases.
Management
Treatment depends on cause and stage. Options may include:
- No treatment with monitoring for early or innocent murmurs
- Heart medications such as pimobendan, ACE inhibitors or diuretics at appropriate stages
- Treating underlying causes, such as anemia or heartworm
- Surgery or catheter procedures for some congenital defects
- Diet and exercise adjustments
Home monitoring
Learn your dog's resting breathing rate while sleeping: sustained rates above about 30 breaths per minute are worth reporting. Log it a few times a week. See dog normal vital signs.
Exercise and lifestyle
Most dogs with early disease can exercise normally, but your vet may adjust activity. Keep your dog lean and avoid heat stress. See dog obesity risks.
Common mistakes
- Ignoring a murmur
- Skipping follow-up scans
- Missing a cough as a sign
- Giving medication without a diagnosis
Troubleshooting
- My puppy has a murmur: recheck at later vaccinations and ask about echocardiography.
- My dog coughs at night: report it promptly.
- My dog tires quickly: book a recheck.
A simple plan
- Get the murmur evaluated.
- Follow testing recommendations.
- Monitor breathing at home.
- Recheck on schedule.
Real-life examples
A puppy's soft murmur disappears at five months. A Cavalier with a murmur starts medication when scans show enlargement, and stays comfortable for years. A terrier with anemia has a murmur that resolves after treatment. Context matters.
Staying informed
Ask your vet for a written plan with recheck dates and warning signs.
Preparing for a cardiology visit
Bring a list of medications, a log of resting breathing rates, a video of any coughing or fainting and your dog's records. Ask about the cause, the stage, what tests are needed, how often to recheck and which signs mean you should call immediately. Written notes help you recall advice later.
Breeds and screening
Some breeds are prone to specific heart problems, such as mitral valve disease in Cavalier King Charles spaniels and dilated cardiomyopathy in dobermans. Breeders may screen parents, and owners can ask about regular heart checks as dogs age. Early detection allows treatment to begin at the best time.
The takeaway
A murmur is a clue. Follow up with testing, monitor at home and work with your vet.
Frequently asked questions
Is a murmur always serious?
No, but it needs evaluation.
What grades do murmurs have?
Vets grade from I (faint) to VI (very loud with a thrill).
Can puppies outgrow murmurs?
Some innocent murmurs go away by about four to six months.
Which tests help?
X-rays, ECG, blood pressure and echocardiography.
This article is general educational information, not veterinary advice. Every dog is different. If your dog is unwell, or you are unsure about a symptom, diet change or medication, contact your veterinarian.