What Causes Heart Murmurs in Dogs

Hearing the words “heart murmur” at the vet can feel scary, especially if your dog is acting totally normal. I get it. As a veterinary assistant, I have seen how fast a routine exam can turn into a moment of worry.

The encouraging news is that a heart murmur is not a diagnosis by itself. It is a sound your veterinarian hears when blood flow through the heart is more turbulent than usual. Some murmurs are harmless, some need monitoring, and some point to underlying heart disease or another medical issue that needs treatment.

Quick note: This article is for education and cannot replace an exam with your veterinarian, especially if your dog has symptoms.

What a heart murmur is

A normal heart sound is often described as “lub-dub.” A murmur is an extra “whooshing” sound heard between those beats. It happens when blood flow becomes less smooth, for reasons that range from completely benign to urgent.

Your veterinarian typically describes a murmur using:

• Grade (1 to 6): How loud it is. Grade 1 is very soft; Grade 6 is very loud and may be felt as a vibration on the chest wall.
• Timing: Systolic (most common), diastolic (less common), or continuous.
• Location: Where it is loudest on the chest, which can hint at which valve or area is involved.
• Quality: Soft, harsh, musical, etc.

One important nuance: a louder murmur does not always mean more severe disease, and a soft murmur can still be significant. Body shape, chest thickness, and the direction of blood flow can all affect what we hear through a stethoscope.

Main causes in dogs

1) Innocent or flow murmurs

Some dogs have murmurs that are not linked to structural heart disease. These are often called innocent, physiologic, or flow murmurs. They can occur when blood is moving faster than usual, even if the heart is normal.

Common situations include:

• Puppy murmurs: Mild murmurs can be heard in young puppies and may disappear as they grow.
• Stress or excitement: A fast heart rate during a vet visit can accentuate flow sounds.
• Pregnancy (in intact females): Increased blood volume can contribute to flow murmurs.
• Fever: Increased metabolic demand can change blood flow dynamics.

2) Valve disease

The most common cause of a newly detected murmur in a small-to-medium adult or senior dog is often degenerative mitral valve disease (also called myxomatous mitral valve disease). In simple terms, a valve starts to get “leaky,” so some blood moves backward when it should not, creating turbulence you can hear as a murmur.

Key points:

• Often affects small breeds and mixes, especially as they age.
• May be present years before symptoms appear.
• Can progress to congestive heart failure in some dogs, which is treatable but needs close veterinary guidance.

3) Congenital defects

Some murmurs are caused by structural problems a dog is born with. These are often detected in puppies or young dogs, though mild cases can be found later.

Examples include:

• Patent ductus arteriosus (PDA): A vessel that should close after birth stays open, causing abnormal blood flow.
• Pulmonic stenosis: Narrowing near the pulmonary valve, forcing the heart to pump harder.
• Aortic stenosis: Narrowing near the aortic valve.
• Ventricular septal defect (VSD): A hole between the ventricles.

Some congenital issues can be corrected or improved with specialized procedures. Early detection matters.

4) Dilated cardiomyopathy (DCM)

Dilated cardiomyopathy (DCM) is a disease where the heart muscle becomes weaker and the chambers enlarge. Murmurs may occur, but some dogs with DCM may have little to no murmur early on and instead show rhythm issues.

DCM is more common in large and giant breeds, though it can occur in any dog. Your veterinarian may recommend chest X-rays, echocardiography, and sometimes rhythm monitoring if DCM is suspected.

5) Heartworm disease

Heartworm disease can damage the heart and pulmonary vessels. It is not one of the most classic “murmur causes,” but in some cases it can contribute to abnormal heart sounds or murmurs, especially when pulmonary hypertension or right-sided heart changes develop. This is one reason year-round prevention and regular testing are so important, even for indoor dogs.

6) Anemia

Anemia can cause a functional murmur because thinner blood and compensatory faster flow create turbulence. The heart itself may be structurally normal, but the underlying cause of the anemia needs attention.

Potential causes include parasites, bleeding, immune-mediated disease, chronic illness, and more. A simple blood test can help identify anemia.

7) Infection or inflammation

Less commonly, infections can affect heart valves, leading to turbulence and murmurs. One example is infective endocarditis (infection of the heart’s inner lining or valves). This is uncommon but serious and may be associated with fever, lethargy, and shifting lameness.

8) Masses and fluid around the heart

In older dogs, your vet may consider masses in or around the heart as part of the overall picture. More commonly, fluid around the heart (pericardial effusion) causes muffled heart sounds rather than a classic murmur. That said, some masses can affect blood flow or heart function in ways that lead to abnormal sounds, and imaging is how we sort it out.

Causes by age

Puppies

• Innocent puppy murmur that resolves over time
• Congenital defects like PDA, pulmonic stenosis, VSD
• Anemia or parasites (especially if not on a deworming plan)

If your puppy has a murmur, ask your vet whether it sounds innocent and when to recheck. Many vets recommend a recheck in a few weeks, or earlier referral if the murmur is loud, continuous, or accompanied by symptoms.

Adult dogs

• Early valve changes (especially in small breeds)
• Heartworm disease (depending on geography and prevention history)
• DCM (especially in larger dogs)
• Anemia due to illness, parasites, or bleeding

Senior dogs

• Degenerative valve disease is extremely common
• DCM in predisposed breeds
• Secondary murmurs from chronic disease, anemia, or high blood pressure
• Masses affecting the heart or surrounding structures

When a murmur is a problem

Many dogs with murmurs feel fine, especially early on. Still, it is important to watch for signs that could suggest heart disease is progressing or the body is struggling to compensate.

• Coughing, especially at night or after rest
• Faster breathing at rest
• Exercise intolerance or tiring more quickly
• Weakness or collapsing episodes
• Decreased appetite
• Bloated belly (fluid accumulation)
• Blue or pale gums
• Weight loss or muscle loss

One more helpful reminder: not all coughing is heart-related. Small breed dogs commonly cough from tracheal collapse, chronic bronchitis, or other airway issues. This is exactly why diagnostics matter, since treatment depends on the true cause.

If you see labored breathing, repeated collapse, or your dog cannot get comfortable, treat it as urgent and seek veterinary care right away.

How vets find the cause

Because a murmur is a sound and not a final diagnosis, the next step is figuring out why it is there and whether it matters medically for your specific dog.

Common tests

• Physical exam: Murmur grade, rhythm, pulses, gum color, and lung sounds.
• Chest X-rays: Looks at heart size and shape and checks lungs for fluid.
• Echocardiogram (cardiac ultrasound): The best test to define valve function, chamber size, and blood flow patterns, and to identify many congenital problems.
• Electrocardiogram (ECG): Evaluates electrical rhythm and detects arrhythmias.
• Bloodwork: Checks for anemia, infection, kidney and liver health, thyroid disease, and more.
• Heartworm test: Important in most dogs, even those on prevention, based on your vet’s recommendations.
• Blood pressure: Hypertension can worsen cardiac workload.
• Cardiac biomarkers (in some cases): Tests such as NT-proBNP may help assess cardiac stress, especially when symptoms overlap with respiratory disease.

Depending on your dog’s situation, your veterinarian may start with X-rays and bloodwork and then recommend an echocardiogram, or they may recommend an echo sooner. Not every clinic has ultrasound available right away, and that is okay. A step-by-step plan is still a good plan.

What to do at home

You do not have to wait helplessly between vet visits. Here are practical, evidence-based steps that truly help.

Track resting respiratory rate

When your dog is asleep or deeply relaxed, count breaths for 30 seconds and multiply by two. Many healthy dogs are often under 30 breaths per minute at rest, but it can vary with size, temperature, sleep stage, and stress.

• Pick a quiet time daily for a week and learn your dog’s normal baseline.
• Trends matter more than a single reading. If your dog’s resting rate is consistently rising, or if you repeatedly see numbers above about 30 to 35 breaths per minute, contact your veterinarian for guidance.

Keep a simple symptom journal

• Cough frequency and timing
• Energy level on walks
• Appetite changes
• Any fainting or wobbliness

Support health with the basics

• Maintain a healthy weight: Extra pounds increase cardiac workload.
• Appropriate exercise: Gentle, consistent activity is often better than weekend overexertion. Ask your vet what is safe.
• Dental care: Good dental care supports overall health. Severe periodontal disease can contribute to systemic inflammation, and veterinarians tend to be extra cautious about oral infection in dogs with known heart disease. Brushing and professional cleanings matter.

Be cautious with supplements and diet changes

It is tempting to add a supplement the moment you hear “murmur,” but heart conditions differ, and some supplements can interact with medications or be inappropriate for certain diseases. If you want to change diet or add omega-3s, taurine, or other supplements, do it with your veterinarian’s guidance, especially if your dog is on heart meds.

What happens next

After a murmur is found, the next steps usually fall into one of these paths:

• Monitor and recheck: Common for soft murmurs in puppies, suspected flow murmurs, or very early valve disease without heart enlargement.
• Baseline testing: Many dogs benefit from baseline chest X-rays, bloodwork, blood pressure, and a heartworm test, then repeat checks over time.
• Cardiology referral: Often recommended for loud murmurs, continuous murmurs, suspected congenital disease, concerning arrhythmias, or when an echocardiogram is needed to guide decisions.
• Medication if needed: If a dog develops congestive heart failure or certain high-risk changes, vets may prescribe medications that reduce fluid buildup, support heart function, or control blood pressure or rhythm. The exact plan depends on the diagnosis.

Your veterinarian will also tell you how often to recheck. Some dogs are monitored every 6 to 12 months, while others need much closer follow-up.

Common questions

Can a dog live a normal life with a heart murmur?

Yes, many can. Some murmurs never progress. Others need monitoring and medication, and dogs can still enjoy great quality of life for years with the right plan.

Does a louder murmur always mean worse disease?

Not always. Grade is helpful, but it is not a perfect measure of severity. That is why imaging, especially echocardiography, plus X-rays in many cases, can be so valuable.

Will my dog need medication?

It depends on the cause and whether there is heart enlargement, congestion, high blood pressure, rhythm abnormalities, or symptoms. Many dogs with early valve disease do not need meds right away, but they do benefit from monitoring.

Should I limit exercise?

Some dogs can keep normal activity. Others need moderated exercise. The safest answer is: follow your veterinarian’s guidance based on diagnostics.

When it is urgent

Please contact a veterinarian immediately if your dog has:

• Difficulty breathing, open-mouth breathing, or breathing that seems “effortful”
• Collapse or fainting
• Blue-tinged gums or tongue
• A sudden, persistent cough with weakness
• A dramatically swollen abdomen or severe restlessness

These can be signs of heart failure, dangerous arrhythmias, or other serious conditions that should not wait.

Takeaway

A heart murmur is a clue, not a verdict. The cause might be as simple as a temporary flow murmur, or it might signal valve disease, heartworm-related changes, DCM, anemia, or a congenital issue. The best next step is usually a thoughtful diagnostic plan, paired with home monitoring that helps you catch changes early.

If your dog was just diagnosed with a murmur, ask your veterinarian two questions at your next visit: What grade is it? and What is the recommended next test to find the cause? Those answers will guide everything else in a calm, step-by-step way.