Heart Murmurs in Dogs and Cats: What They Mean and When to Worry

Hearing that your dog or cat has a heart murmur can be alarming. The word 'murmur' sounds ominous, and it is natural to fear the worst. However, a heart murmur is not a disease in itself — it is a clinical finding, an abnormal sound detected by your veterinarian when listening to your pet's heart with a stethoscope. Understanding what a murmur actually is can help replace anxiety with informed action.

"I tell every worried pet owner the same thing when I hear a murmur: a murmur is a signpost, not a sentence. It tells us that blood is flowing turbulently through the heart — but the reason for that turbulence ranges from completely harmless to genuinely serious. Our job is to find out which it is, and that means listening carefully, grading accurately, and investigating when the murmur warrants it." — Dr. Sarah Chen, DVM

In a healthy heart, blood flows smoothly and silently through the chambers and valves. A heart murmur occurs when blood flow becomes turbulent — disrupted from its normal, smooth pattern — creating vibrations that produce an audible whooshing or swishing sound. This turbulence can be caused by a wide variety of conditions, some entirely benign and others indicating significant cardiac disease.

Heart murmurs are extremely common in veterinary practice. Studies suggest that up to 30–50% of apparently healthy cats and a significant proportion of dogs will have a detectable murmur at some point during their lives. In puppies, innocent (physiological) murmurs are particularly common and typically resolve by four to five months of age. In older dogs, degenerative valve disease becomes increasingly prevalent — roughly 75% of small-breed dogs over the age of twelve have some degree of mitral valve degeneration, often producing an audible murmur during routine veterinary check-ups.

The key question is not whether a murmur is present, but what is causing it and whether it requires treatment. This guide walks you through the grading system, common causes, and when a murmur warrants further investigation.

Veterinarians classify heart murmurs using a standardised grading scale from I to VI (1 to 6), based on the loudness and characteristics of the sound. While grade provides some indication of severity, it does not always correlate directly with the seriousness of the underlying condition — a grade II murmur can sometimes indicate more significant disease than a grade III, depending on the cause.

Grade I

A grade I murmur is the softest detectable murmur. It can only be heard in a very quiet room, often requiring the pet to hold their breath, and is localised to a small area on the chest wall. Grade I murmurs are easily missed and may not be detected at every examination. Many grade I murmurs are innocent — particularly in young, healthy animals — and may never require investigation unless other concerning signs are present.

Grade II

A grade II murmur is soft but readily heard when the stethoscope is placed over the heart in a quiet room. It remains localised and does not radiate widely across the chest. Like grade I murmurs, grade II murmurs can be innocent, though they warrant monitoring and may justify screening echocardiography in breeds predisposed to heart disease.

Grade III

A grade III murmur is moderately loud and easily detected. It is considered an intermediate-grade murmur and is typically the threshold at which most veterinarians recommend further cardiac investigation — particularly in middle-aged or older pets, or in breeds with known cardiac predisposition. A grade III murmur is more likely to indicate structural heart disease than grades I or II.

Grade IV

A grade IV murmur is loud and can be heard over a wide area of the chest. It is almost always associated with significant cardiac pathology and warrants thorough investigation including echocardiography. At this grade, the murmur is readily detectable even in a noisy consulting room.

Grade V

A grade V murmur is very loud and can be felt as a vibration (thrill) when the hand is placed on the chest wall over the heart. The presence of a palpable thrill is the distinguishing feature between grades IV and V. Grade V murmurs are always pathological and indicate significant turbulence within the heart.

Grade VI

A grade VI murmur is the loudest possible grade — it can be heard even with the stethoscope slightly lifted off the chest wall, and is accompanied by a strong palpable thrill. Grade VI murmurs indicate severe cardiac pathology and require urgent investigation and management.

In addition to grade, your vet will note the murmur's timing (systolic, diastolic, or continuous), location on the chest (left or right, and which intercostal space), and whether it radiates. These characteristics provide important clues about the underlying cause.

One of the most important distinctions in veterinary cardiology is between innocent (also called physiological or functional) murmurs and pathological murmurs caused by structural heart disease.

Innocent Murmurs in Puppies

Innocent puppy murmurs are remarkably common, occurring in up to 50–70% of puppies in some studies. These murmurs are caused by the normal turbulence of blood flowing through a rapidly growing cardiovascular system. They are typically soft (grade I–II, occasionally III), best heard over the left heart base, and are characteristically intermittent — they may be present at one visit and absent at the next, or more prominent when the puppy is excited or has a fever. Most innocent puppy murmurs resolve by four to five months of age as the heart and great vessels reach their adult proportions.

However, it is crucial not to assume that every puppy murmur is innocent. Congenital heart defects — such as subaortic stenosis (SAS), pulmonic stenosis, patent ductus arteriosus (PDA), and ventricular septal defect (VSD) — also produce murmurs in puppies. These congenital murmurs tend to be louder (grade III or higher), persistent, and may have specific characteristics. Any puppy with a murmur that persists beyond five months of age, is grade III or louder, or is accompanied by symptoms (exercise intolerance, stunted growth, breathing difficulty) should have an echocardiogram.

Innocent Murmurs in Cats

Functional murmurs are also very common in cats. Stress, anaemia, fever, hyperthyroidism, and even a high heart rate from the anxiety of a veterinary visit can produce a transient murmur in a structurally normal heart. One study found that over 40% of healthy cats had a murmur detected during a veterinary examination. This makes distinguishing innocent from pathological murmurs more challenging in cats than in dogs, and is one reason why echocardiography or cardiac biomarker testing is often recommended when a murmur is detected in a cat.

Pathological Murmurs

Pathological murmurs are caused by structural abnormalities of the heart valves, heart walls, or great vessels. In dogs, the most common cause is myxomatous mitral valve disease (MMVD) — degenerative thickening of the mitral valve that allows blood to leak backward during each heartbeat. MMVD predominantly affects small-breed dogs and becomes increasingly common with age. In cats, the most common pathological cause is hypertrophic cardiomyopathy, though the relationship between HCM and murmurs is inconsistent — many cats with HCM do not have audible murmurs, and many cats with murmurs do not have HCM.

The bottom line: a murmur detected by your vet is a starting point for investigation, not an automatic cause for alarm. The next step depends on your pet's age, breed, murmur characteristics, and overall health status.

The causes of heart murmurs differ significantly between dogs and cats, and understanding the most likely culprits helps guide appropriate investigation.

Dogs — Most Common Causes

• Myxomatous Mitral Valve Disease (MMVD) — the single most common cause of heart murmurs in adult dogs, particularly small breeds. The mitral valve leaflets progressively thicken and deform, allowing blood to regurgitate backward into the left atrium. Cavalier King Charles Spaniels are severely predisposed, with nearly all individuals affected by age ten. Dachshunds, Miniature Poodles, Chihuahuas, and Yorkshire Terriers are also commonly affected. MMVD produces a left-sided systolic murmur that typically increases in grade as the disease progresses
• Dilated Cardiomyopathy (DCM) — the most common acquired heart disease in large and giant breed dogs, DCM can produce a murmur due to mitral or tricuspid valve regurgitation as the enlarged heart stretches the valve structures
• Congenital defects — subaortic stenosis (common in Golden Retrievers, Rottweilers, and Boxers), pulmonic stenosis (Bulldogs, Terriers), patent ductus arteriosus (German Shepherds, Poodles), and ventricular septal defects each produce characteristic murmurs in puppies and young dogs
• Anaemia — reduced blood viscosity from any cause of anaemia increases turbulence and can produce a soft, systolic murmur. Treating the anaemia resolves the murmur

Cats — Most Common Causes

• Hypertrophic Cardiomyopathy (HCM) — the most common structural cause, though as noted, many cats with HCM lack audible murmurs. When present, HCM murmurs are typically caused by dynamic left ventricular outflow tract obstruction (systolic anterior motion of the mitral valve)
• Functional or stress-related murmurs — extremely common in cats during veterinary examinations. These are typically soft, systolic, and vary between examinations
• Hyperthyroidism — increased cardiac output and heart rate from thyroid excess frequently produces a murmur in older cats. The murmur may resolve once thyroid levels are normalised
• Anaemia — as in dogs, anaemia increases turbulence and can produce innocent murmurs in cats
• Congenital defects — ventricular septal defects and other congenital malformations are less common in cats than dogs but do occur

Given the wide range of possible causes, a thorough preventative care approach that includes regular cardiac assessment ensures that significant murmurs are identified and investigated appropriately.

Not every heart murmur requires immediate investigation. Your veterinarian will make recommendations based on the murmur grade, your pet's age and breed, and whether any symptoms are present.

When Investigation Is Recommended

Echocardiography (cardiac ultrasound) is the gold standard for evaluating heart murmurs and is recommended in the following situations:

• Any murmur grade III or higher in a dog or cat of any age
• Any murmur in a breed predisposed to heart disease (Cavalier King Charles Spaniels, Dobermans, Maine Coons, Ragdolls, and others)
• A murmur that increases in grade between examinations
• A puppy murmur that persists beyond five months of age
• Any murmur accompanied by symptoms — cough, exercise intolerance, breathing difficulty, collapse, or weight loss
• Before anaesthesia in any pet with a newly detected murmur, particularly cats (where cardiac biomarker screening may be an alternative first step)

What an Echocardiogram Reveals

An echocardiogram provides a comprehensive assessment of heart structure and function. It identifies which valves are affected, measures chamber sizes, assesses wall thickness and contractility, and quantifies the severity of any regurgitation. This information determines whether treatment is needed and, if so, which medications are most appropriate.

Treatment Depends on the Cause

An innocent murmur requires no treatment — only monitoring. If the murmur is caused by anaemia, hyperthyroidism, or another systemic condition, treating the underlying cause often resolves the murmur entirely.

For pathological murmurs caused by structural heart disease, treatment depends on the specific condition and its severity:

• MMVD in dogs (pre-clinical) — the landmark EPIC trial demonstrated that pimobendan significantly delays the onset of heart failure in dogs with MMVD and cardiac enlargement, extending the preclinical period by approximately fifteen months. Dogs meeting specific criteria (murmur plus echocardiographic evidence of enlargement) should start pimobendan
• MMVD in dogs (heart failure) — treatment with furosemide, pimobendan, and ACE inhibitors is well-established and can provide months to years of good quality life
• HCM in cats — beta-blockers or calcium channel blockers may be prescribed, with clopidogrel added for cats at risk of thromboembolism
• Congenital defects — some defects (PDA, pulmonic stenosis) can be corrected or improved with surgery or interventional catheterisation; others are managed medically

Whether your pet's murmur is innocent or pathological, ongoing monitoring ensures that any changes are detected promptly and managed appropriately.

Monitoring an Innocent Murmur

For puppies with suspected innocent murmurs, your vet will recheck at subsequent vaccination visits and at the six-month mark. If the murmur has resolved, no further cardiac monitoring is needed. If it persists, echocardiography is recommended to rule out a congenital defect. For cats with suspected functional murmurs, periodic auscultation during routine visits and, if indicated, cardiac biomarker testing or echocardiography will track any changes.

Monitoring a Pathological Murmur

Dogs and cats with structural heart disease require regular cardiac rechecks — the frequency depends on the severity. A dog with early MMVD and a grade II murmur may only need annual echocardiographic assessment, while a dog with advanced disease on multiple medications may be seen every two to four months. Your veterinary team will establish a monitoring schedule based on your pet's individual needs.

Home Monitoring Tips

As with other cardiac conditions, the sleeping respiratory rate is an invaluable home monitoring tool. Establish your pet's baseline by counting breaths over several nights when they are sleeping peacefully. Any consistent increase above baseline — particularly above 30 breaths per minute — warrants a call to your vet. Additional signs to watch for include coughing (especially at night or when lying down), reduced exercise tolerance, loss of appetite, and abdominal distension.

The Outlook

The prognosis for a pet with a heart murmur depends entirely on the underlying cause. Innocent murmurs carry an excellent prognosis — they are, by definition, benign. MMVD in small-breed dogs is a slowly progressive disease; many dogs live for years with a murmur before heart failure develops, and modern treatment can extend good quality life significantly once heart failure occurs. HCM in cats carries a more variable prognosis, as discussed in detail in our guide to heart disease in pets.

The most important takeaway is that a heart murmur is a reason for investigation, not panic. With proper diagnosis, monitoring, and treatment when needed, most pets with heart murmurs continue to enjoy active, comfortable lives. Your vigilance as an owner — combined with your veterinary team's expertise and financial planning for veterinary care — ensures the best possible outcome for your pet.