Your Dog Has a Heart Murmur? Here's What You Need to Know

During a routine annual wellness examination, your veterinarian places a stethoscope against your dog's chest, listens quietly for a few moments, and then turns to you with a solemn expression: "I'm hearing a heart murmur today."

For most pet parents, those words send an immediate wave of anxiety through the chest. We instantly picture heart failure, shortened lifespans, and restricted, quiet lives for our energetic companions.

First, take a deep breath.

While a heart murmur diagnosis requires attention and follow-up, it is not an immediate death sentence. In fact, thousands of dogs live long, vibrant, happy lives after receiving a heart murmur diagnosis. Understanding what a murmur actually is—and what it isn’t—is the most powerful tool you have to protect your dog’s health.

This comprehensive guide will demystify canine heart murmurs, walking you through how vets grade them, what causes them, the symptoms you must monitor at home, and how to help your dog thrive for years to come.

1. What Exactly Is a Dog Heart Murmur?

To understand a heart murmur, it helps to understand how a normal canine heart operates.

A healthy dog heart beats with a clean, rhythmic two-part sound: "lub-dub, lub-dub." The "lub" is the sound of the mitral and tricuspid valves closing as blood pumps out of the heart's upper chambers. The "dub" is the sound of the aortic and pulmonic valves closing as the heart fills with blood again.

A heart murmur is an extra, abnormal sound heard between or during these regular heartbeats. Instead of a clear "lub-dub," the veterinarian hears a swishing, whooshing, or rasping sound.

This swishing sound is caused by turbulent blood flow. Instead of blood moving smoothly in one direction through the heart's chambers and valves, the blood is swirling, flowing backward through a leaky valve, or forced through a narrowed blood vessel.

Structural vs. Benign Murmurs

• Structural Murmurs: Caused by actual physical defects or physiological changes in the heart itself, such as a damaged valve, a hole in the heart wall, or thickened muscle walls.
• Innocent (Benign) Murmurs: Common in young puppies (under 4 to 6 months old) whose cardiovascular systems are still growing. These murmurs often disappear on their own as the pup matures and carry no long-term health risks.
• Physiological (Non-Cardiac) Murmurs: Caused by conditions outside the heart that alter blood viscosity or flow speed, such as severe anemia, high fever, or severe dehydration.

2. Decoding Heart Murmur Grades: From I to VI

Veterinarians classify heart murmurs on a standardized scale from Grade I to Grade VI based on how loud the murmur sounds through a stethoscope.

It is important to note that loudness does not always equal severity. A very loud murmur can sometimes be caused by a minor hole that creates high-pressure blood turbulence, while a faint murmur might hide a serious, chronic valve issue.

• Grade I: Extremely faint and difficult to hear. It can usually only be detected in a completely quiet room by an experienced clinician listening to a specific area of the chest.
• Grade II: Soft, but easily heard with a stethoscope placed over the specific valve area involved.
• Grade III: Moderate intensity. This is the most common grade initially detected during routine checkups.
• Grade IV: Loud murmur that radiates broadly across both sides of the chest.
• Grade V: Very loud murmur that can be felt through the dog’s chest wall with your bare hand (a sensation known as a "palpable thrill").
• Grade VI: Extremely loud murmur accompanied by a prominent chest thrill. The murmur is so intense that it can be heard even when the stethoscope is lifted slightly off the dog's chest skin.

3. Common Causes of Heart Murmurs in Dogs

Heart murmurs can affect any dog at any stage of life, but the underlying causes often depend on the dog's age and breed size.

1. Myxomatous Mitral Valve Disease (MMVD)

MMVD accounts for roughly 75% of all canine heart disease cases. It primarily affects small to medium-sized older dogs, such as Cavalier King Charles Spaniels, Poodles, Chihuahuas, and Shih Tzus. Over time, the mitral valve thickens and weakens, preventing it from closing tightly. Blood leaks backward into the left atrium during each beat, creating a murmur.

2. Dilated Cardiomyopathy (DCM)

DCM is more common in large and giant breeds, including Doberman Pinschers, Great Danes, Boxer Dogs, and Irish Wolfhounds. In DCM, the heart muscle walls become thin, weak, and stretched out, making it difficult for the heart to pump blood efficiently throughout the body.

3. Congenital Heart Defects

Present at birth, these defects are often caught during a puppy's first vet exams:

• Patent Ductus Arteriosus (PDA): A fetal blood vessel fails to close after birth, forcing oxygenated blood back into the lungs.
• Subaortic Stenosis (SAS): A narrowing of the tissue just below the aortic valve, forcing the heart to work harder to push blood out to the body.

4. Key Symptoms to Monitor at Home

In the early stages of a heart murmur, most dogs show zero outward symptoms. They eat, play, run, and sleep completely normally. However, as heart conditions progress toward congestive heart failure (CHF), noticeable signs begin to emerge.

Keep a daily log if you observe any of the following symptoms:

• Persistent Coughing: Especially a dry, hacking cough that worsens at night or right after waking up.
• Elevated Resting Respiratory Rate: A healthy resting dog should take fewer than 30 breaths per minute while sleeping soundly.
• Exercise Intolerance: Your dog slows down prematurely on walks, stops to rest frequently, or refuses physical play.
• Lethargy and Weakness: Loss of vitality or sudden collapse during exertion.
• Pale or Blue-Tinged Gums: Indicating poor circulation and inadequate blood oxygenation.
• Abdominal Swelling: Fluid buildup in the abdomen (ascites) caused by poor circulation.

5. Next Steps: Diagnostics and Veterinary Treatment

A physical exam with a stethoscope can detect a murmur, but it cannot tell your vet why the murmur is happening. To build an accurate treatment plan, further diagnostics are required.

Diagnostic Testing

1. Echocardiogram (Cardiac Ultrasound): The gold standard in veterinary cardiology. An echocardiogram allows the vet to view the heart chambers in real-time, measure wall thickness, and assess valve movement.
2. Chest X-rays: Used to evaluate the overall size and shape of the heart and check for fluid accumulation in the lungs.
3. Electrocardiogram (ECG): Measures the electrical activity of the heart to check for irregular heart rhythms (arrhythmias).
4. Blood Tests (NT-proBNP): Biomarker tests that measure cardiac wall stretch and strain.

Medical Management

If the murmur is associated with progressive heart disease, your veterinarian or a board-certified veterinary cardiologist may prescribe medications such as:

• Pimobendan: Improves heart muscle contraction strength and dilates blood vessels to ease workload.
• ACE Inhibitors (e.g., Enalapril, Benazepril): Help relax blood vessels and lower systemic blood pressure.
• Diuretics (e.g., Furosemide): Help clear excess fluid buildup from the lungs and chest cavity.

6. Exercise Management: Balancing Health and Fun

One of the biggest questions pet parents ask after a heart murmur diagnosis is: "Can my dog still play?"

The answer depends on the underlying cause and stage of the heart condition. Complete bed rest is rarely recommended unless a dog is in active heart failure. Gentle, regular physical exercise keeps muscles strong, manages body weight, and reduces mental anxiety. However, intense, unmonitored exhaustion must be avoided.

Instead of high-impact, exhausting activities like long runs or strenuous fetch sessions in heat, pivot toward low-impact, mentally stimulating play in temperature-controlled environments.

For dogs with mild-to-moderate, managed conditions who still need an outlet for their energy without over-exerting themselves, controlled yard games are ideal.

Using a specialized outdoor training tool like the Warmmore Herding Ball for Australian Shepherds & Collies offers a great way to let working dogs move at their own pace. Designed as an 18-inch ball that dogs push gently across the grass using their chest and nose, it encourages self-directed play. Owners can manage session lengths easily—limiting play to short 5- to 10-minute bursts in cool weather—allowing dogs to satisfy their natural instincts while keeping their exertion levels safely monitored.

7. Summary: Living a Full Life with a Heart Murmur

Discovering that your dog has a heart murmur can feel overwhelming, but knowledge and proactive care make all the difference. By partnering closely with your veterinarian, getting appropriate diagnostic imaging, watching for subtle changes in resting breathing rates, and managing exercise, you can give your dog the best possible quality of life.

Remember that many dogs with heart murmurs live active, comfortable, and happy lives for many years. Focus on enjoying your daily moments together, supporting their needs with care, and keeping their environment joyful and stress-free.