Heart Murmurs in Dogs and Cats: What Owners Should Know

There are few sentences that stop an owner's own heart quite as effectively as a veterinarian lowering the stethoscope and saying, "I can hear a murmur." The word carries a weight that often outstrips what has actually been found, and many owners leave the consulting room imagining the worst before the conversation has finished.

In reality, a heart murmur is a finding, not a diagnosis. It describes a sound — nothing more. That sound can mean anything from a completely harmless phenomenon in a growing puppy to an early warning of a condition that will need lifelong monitoring. The murmur itself tells your veterinarian that something is worth investigating; it does not, on its own, tell them what.

This guide explains what a murmur physically is, how veterinarians grade what they hear, the common causes in dogs and in cats and how they differ, what diagnostic testing usually involves, the signs of heart disease that owners can watch for at home, and the practical realities of living with a pet who has a murmur.

Important: This article shares general educational information for pet owners and is not a substitute for an in-person veterinary examination, diagnosis, or treatment. For any medical concern or emergency, contact your veterinarian promptly.

What a Heart Murmur Actually Is

A healthy heartbeat produces two crisp sounds through a stethoscope, usually described as "lub-dub," created by heart valves closing in sequence. Blood moving through the heart in an orderly way is essentially silent between those two sounds.

A murmur is an additional sound — typically a whoosh, swish, or hum — caused by turbulent blood flow somewhere in or near the heart. Just as water running smoothly through a pipe is quiet while water forced through a narrowing or leaking past a faulty seal becomes audible, blood that is not moving smoothly makes noise.

Turbulence can arise for several reasons: a valve that no longer seals properly and allows some backflow, a narrowed opening blood must squeeze through, an abnormal connection between chambers or vessels, or simply blood that is thinner than normal and therefore flows less smoothly. Each of these has very different implications, which is exactly why a murmur prompts further questions rather than answering them.

How Veterinarians Grade Murmurs

Veterinarians describe murmur intensity on a scale from one to six. The grade refers only to how loud the sound is and where it can be heard, and it is recorded so that changes over time can be tracked.

• Grade I. Very soft, difficult to hear, and often only detectable in a completely quiet room after careful listening.
• Grade II. Soft but heard readily once the stethoscope is in the right place.
• Grade III. Moderate and easily heard, typically at a similar volume to the normal heart sounds.
• Grade IV. Loud, and heard over a wide area of the chest.
• Grade V. Very loud, and accompanied by a vibration — a "thrill" — that can be felt with a hand on the chest wall.
• Grade VI. Loud enough to be heard with the stethoscope lifted slightly off the chest, with a palpable thrill.

A crucial caveat: loudness does not reliably indicate severity. Some quiet murmurs accompany significant disease, and some loud murmurs come from conditions that are comparatively manageable. This is particularly true in cats, where serious heart disease can exist with only a soft murmur or none at all. The grade is one data point among several, not a verdict.

Innocent and Physiologic Murmurs

Not all murmurs indicate structural heart disease. Young puppies and kittens sometimes have what are called innocent or physiologic murmurs — soft, low-grade sounds that reflect a rapidly growing cardiovascular system rather than any defect. Many of these fade and disappear on their own as the animal matures.

Murmurs can also appear temporarily in situations that change the properties or speed of blood flow. Significant anaemia, fever, an overactive thyroid, pregnancy, or simple excitement and a racing heart rate during a stressful vet visit can all produce audible turbulence in a structurally normal heart.

This is one reason your veterinarian may recommend rechecking a young animal's murmur rather than pursuing extensive testing immediately. A soft murmur in a thriving, energetic puppy is approached quite differently from a new murmur in a middle-aged dog, and time itself is a useful diagnostic tool.

Common Causes in Dogs

In adult and senior dogs, the most frequently encountered cause of a new murmur is degenerative change in the mitral valve — the valve between the left atrium and left ventricle. Over time the valve tissue thickens and no longer closes completely, allowing a portion of blood to leak backwards with each beat. This produces a characteristic murmur on the left side of the chest.

This condition is especially common in small and toy breeds and tends to be slowly progressive, which is why many dogs live comfortably with it for years under monitoring. Some dogs never progress to clinical signs at all; others eventually develop changes that need active management.

• Degenerative valve disease. The leading cause in older, often smaller dogs; typically gradual in its progression.
• Dilated cardiomyopathy. A disease of the heart muscle itself, seen more often in certain larger breeds.
• Congenital defects. Structural abnormalities present from birth, usually detected in puppies at early vaccination visits.
• Anaemia or systemic illness. Non-cardiac causes of turbulence that resolve when the underlying issue is treated.
• Heartworm disease. In regions where it occurs, this parasite can affect the heart and pulmonary vessels; prevention is discussed with your veterinarian.

Why Cats Are Different

Cats deserve a section of their own, because feline heart disease behaves in ways that regularly surprise owners. The most significant point is that the relationship between murmurs and disease is much less reliable in cats than in dogs.

Some cats have loud murmurs and structurally normal hearts. Others have serious underlying disease and no audible murmur at all. Murmurs in cats can also come and go depending on stress level and heart rate during the examination, meaning a murmur heard one day may be absent the next.

The most common form of feline heart disease involves thickening of the heart muscle, which reduces the heart's ability to fill properly. It can develop quietly, and the first sign an owner notices is sometimes a sudden and serious episode of laboured breathing or hind limb weakness. Because of this, veterinarians often recommend imaging rather than relying on listening alone when there is any concern in a cat, and any cat with sudden breathing difficulty should be treated as an emergency.

What Diagnostic Testing Involves

When a murmur warrants investigation, your veterinarian will select from several tools. The aim is to determine whether the heart is structurally normal, and if not, what the abnormality is and how much it is affecting function.

• Physical examination. Listening carefully to timing, location, and character, and checking pulses, gum colour, and lung sounds.
• Echocardiogram. An ultrasound of the heart and the single most informative test, showing chambers, valves, and blood flow in real time.
• Chest radiographs. X-rays that show heart size and shape and, importantly, whether there is fluid in or around the lungs.
• Electrocardiogram. A recording of the heart's electrical activity, used when rhythm abnormalities are suspected.
• Blood pressure and blood tests. To identify contributing factors such as high blood pressure, thyroid disease, kidney disease, or anaemia.
• Cardiac biomarkers. Blood tests that can help indicate heart muscle stress, sometimes used as a screening step, particularly in cats.

Not every pet needs every test. Your veterinarian may also refer you to a veterinary cardiologist for advanced imaging and interpretation, which is a normal and constructive step rather than a sign that things are dire.

Signs of Heart Disease to Watch for at Home

A murmur by itself causes no symptoms. What matters clinically is whether the heart is still meeting the body's needs. These are the changes worth noticing, and any of them should prompt a call to your veterinarian.

• Coughing. Particularly a persistent cough in dogs, or one that is worse at night or after lying down.
• Increased breathing effort. Faster breathing at rest, visible abdominal effort, or open-mouth breathing in a cat, which is always urgent.
• Reduced exercise tolerance. Tiring sooner on walks, stopping to rest, or a general reluctance to do things that were previously easy.
• Fainting or collapse. Sudden weakness or a brief loss of consciousness, especially around exertion or excitement, requires prompt assessment.
• Restlessness at night. Difficulty settling, repeated repositioning, or an inability to lie comfortably on the chest.
• Appetite and weight changes. Gradual loss of condition or a swollen, distended abdomen.
• Sudden hind limb weakness in cats. A painful, sudden loss of use of the back legs is an emergency and needs immediate veterinary care.

Counting Resting Respiratory Rate at Home

One of the most genuinely useful things an owner of a pet with heart disease can do is learn to count the sleeping or resting respiratory rate. It is simple, free, and often detects a developing problem earlier than anything the pet visibly shows.

Wait until your pet is fully asleep or resting calmly, not panting and not recently active. Watch the chest rise and fall — one rise plus one fall counts as a single breath. Count for thirty seconds and double the number, or count a full minute for accuracy. Record it, and build up a picture of your individual pet's normal baseline over a week or two.

Ask your veterinarian what number should prompt a call for your specific pet, because thresholds are individual. A consistent, meaningful rise above your pet's own established baseline is a signal to contact the clinic rather than wait, and having weeks of home data makes that conversation far more useful.

Living With a Pet Who Has a Murmur

For a great many pets, a murmur means regular monitoring and very little else. Your veterinarian will set a recheck schedule, and the most valuable thing you can do is keep to it, because the trend over time is what matters most.

Never start, stop, or adjust any cardiac medication on your own, and never give human heart or blood pressure medicines to a pet. If treatment is prescribed, consistency in timing and dose is important, and any side effects should be reported rather than managed by skipping doses.

• Keep weight healthy. Excess weight makes the heart work harder; your veterinarian can advise on a suitable target.
• Discuss diet. Some pets with heart disease benefit from specific dietary adjustments, but these should be guided by your veterinarian rather than chosen independently.
• Adjust exercise sensibly. Many pets continue gentle, regular activity; let your pet set the pace and avoid overheating.
• Keep other care current. Dental health, parasite prevention, and routine wellness care all remain part of supporting an animal with heart disease.
• Reduce stress where you can. Calm handling and predictable routines help, particularly with cats.

Any supplement should be discussed with your veterinarian before use. Supplements are complements to, never substitutes for, proper veterinary diagnosis, prescribed treatment, and monitoring — and no supplement should be expected to treat or manage heart disease.

Myths and Facts

Myth: A loud murmur always means severe disease.

Fact: Grade reflects loudness, not severity. Imaging, not volume, determines how serious the underlying situation is.

Myth: A murmur means my pet has a short time left.

Fact: Many pets live for years with a murmur, and some murmurs are entirely innocent. Prognosis depends on the cause, the stage, and the response to management.

Myth: If my pet seems fine, the murmur can be ignored.

Fact: Heart disease often progresses silently before signs appear. Scheduled rechecks exist precisely to catch changes during that quiet phase.

Myth: Cats with normal breathing cannot have heart disease.

Fact: Feline heart disease can be present with very subtle or no outward signs, which is why veterinarians rely on examination and imaging rather than appearance alone.

Frequently Asked Questions

Can a heart murmur go away?

Innocent murmurs in young animals frequently disappear as they grow. Murmurs caused by anaemia, fever, or thyroid disease can resolve when the underlying condition is addressed. Murmurs from structural heart disease generally do not resolve, though they can remain stable for long periods.

Is a murmur painful for my pet?

The murmur itself is a sound and causes no pain. Some consequences of advanced heart disease, such as difficulty breathing, are distressing, which is why monitoring and timely treatment matter.

My dog needs a dental procedure — is anaesthesia safe with a murmur?

Many pets with murmurs undergo anaesthesia safely with appropriate assessment beforehand. Your veterinarian will decide what pre-anaesthetic testing is needed and tailor the protocol accordingly. Do not skip necessary dental care without discussing it first.

Should I limit my dog's exercise?

This depends entirely on the diagnosis and stage. Many pets continue enjoying regular gentle activity. Ask your veterinarian for guidance specific to your pet rather than restricting activity unnecessarily.

How often should a murmur be rechecked?

That varies with the cause, the grade, the species, and whether there are any signs. Your veterinarian will set an interval and adjust it as the picture becomes clearer.

Hearing that your pet has a murmur is unsettling, but it is best understood as information rather than a sentence. Follow the recommended testing, learn to count resting breaths at home, keep the recheck appointments, and contact your veterinarian promptly if breathing changes. Most owners find that the uncertainty is far harder than the day-to-day reality.