Heart murmurs in senior dogs and cats: grades, causes and when to worry

For most owners, the first time they hear the words heart murmur is in a quiet exam room at the end of an otherwise ordinary appointment. The veterinarian, stethoscope still pressed against the chest of your senior dog or cat, pauses a beat longer than usual. She listens with the small, focused stillness that always sets off a quiet alarm in your stomach. Then she says, gently, "I'm hearing a soft murmur today. It's probably nothing urgent, but I'd like to talk about what it means."

A heart murmur is not a diagnosis. It is a sound — an extra whoosh layered on top of the normal lub-dub of the heart — and like any symptom, it can mean almost nothing or it can mean a great deal. Some murmurs are harmless quirks of an aging body that will never affect your pet's life. Others are the very first whisper of a disease that, left unchecked, will define the years ahead. Sorting one from the other is the entire point of the conversation that follows that first stethoscope finding.

This guide explains what a heart murmur actually is, how veterinarians grade them from I to VI, the very different meaning of a murmur in a dog versus a cat, the workup that turns a stethoscope finding into a real diagnosis, the conditions most commonly hiding behind a murmur in senior pets, and the practical home monitoring that separates a murmur worth watching from one that needs urgent attention.

What a heart murmur actually is

A healthy heart sounds like a steady, rhythmic lub-dub. The first sound is the closing of the valves between the upper and lower chambers as the heart begins to squeeze; the second is the closing of the valves leading out of the heart as that squeeze ends. Between and around those two sounds, blood should flow silently and smoothly through chambers and vessels designed to move it without turbulence.

A murmur is the sound of turbulent blood flow. When blood does not move smoothly — because a valve is leaking, a chamber wall is too thick, a passage is narrowed, or the blood itself is unusually thin — it tumbles instead of streaming, and that tumbling produces an audible whoosh. The veterinarian hears it through the stethoscope as an extra sound layered on top of the normal heartbeat: sometimes a soft hiss, sometimes a loud, rushing roar.

Importantly, a murmur tells you that something is causing turbulence, but not what. The same loud murmur can mean a leaky valve in one dog, a congenital defect that went undetected for years in another, and severe anemia in a third. That is why a murmur, especially a new one in a senior pet, almost always deserves further investigation rather than a wait-and-see approach.

How veterinarians grade heart murmurs

Veterinarians use a standardized scale of I to VI to describe the loudness of a murmur. The grade does not by itself tell you how serious the underlying disease is, but it gives both clinicians and owners a shared language to track changes over time.

Grade I

Very soft. Audible only after careful listening in a quiet room, often only over one specific spot on the chest. Many grade I murmurs are missed at noisy or rushed appointments and discovered later by a particularly attentive ear.

Grade II

Soft, but consistently audible at the typical listening points. Slightly louder than grade I and easier to find. Many early-stage valve murmurs in small senior dogs first appear in this range.

Grade III

Moderate intensity. Easily heard, similar in loudness to the normal heart sounds themselves. Most murmurs in this range warrant a workup, even if your pet seems perfectly well.

Grade IV

Loud, but without a palpable thrill. (A "thrill" is a vibration that can be felt with the flat of the hand on the chest, like the buzz of a small motor.) A grade IV murmur is impossible to miss on exam.

Grade V

Very loud, with a palpable thrill. The murmur can be felt as a vibration through the chest wall.

Grade VI

The loudest grade. So loud that it can be heard with the stethoscope held just off the surface of the chest, and a thrill is easily felt.

The grade matters, but it is not the whole story. A grade II murmur from advanced cardiomyopathy in a cat can be more dangerous than a grade IV murmur from mild valve thickening in a small dog. The grade is one piece of a larger picture that includes timing in the heartbeat, location on the chest, the species and breed of the pet, and what an echocardiogram eventually reveals.

Why murmurs in dogs and cats are so different

One of the most important things for owners to understand is that a heart murmur means something fundamentally different in a dog than it does in a cat. The two species develop different diseases, hide their symptoms differently, and respond to murmurs in ways that often surprise people coming from the other side of the aisle.

Murmurs in senior dogs

In dogs, the loudness of the murmur tends to track reasonably well with the severity of the underlying disease. A loud, harsh murmur in a small senior dog usually means a substantial valve leak; a soft murmur often means mild disease. Over months and years, as a degenerative valve continues to deteriorate, the murmur usually grows louder and clinicians can use grade as one rough marker of progression.

The most common cause by far in senior dogs — particularly small and medium-breed seniors — is myxomatous mitral valve disease (MMVD). The mitral valve gradually thickens and stops sealing properly, blood leaks backward with each beat, and the resulting turbulence is heard as a left-sided systolic murmur. Cavalier King Charles Spaniels, Dachshunds, Chihuahuas, Yorkies, Poodles, and small mixed-breed seniors are especially prone, and many will eventually develop a murmur if they live long enough.

Murmurs in senior cats

Cats are an entirely different story. In cats, the loudness of a murmur correlates poorly with the severity of disease, and a quiet murmur — or even no murmur at all — does not mean the heart is healthy. Roughly 15 to 35 percent of apparently healthy cats have audible murmurs at routine exams; many of those murmurs are physiologic and harmless, while others are the only outward clue to hypertrophic cardiomyopathy (HCM), a serious disease in which the walls of the left ventricle thicken and the heart becomes a stiff, inefficient pump.

Conversely, plenty of cats with severe HCM do not have a murmur at all. The first sign of disease in those cats is sometimes a sudden, devastating event — a saddle thrombus that paralyzes the back legs, an episode of fluid in the lungs, or a collapse from an arrhythmia. This is why veterinary cardiologists treat any new murmur in a cat, no matter how soft, as a strong reason to investigate further. Cats simply do not give you the same gradual, telegraphed warning that dogs typically do.

What causes a murmur in a senior pet

The list of possible causes is long, but a relatively small number of conditions account for the great majority of murmurs in middle-aged and senior dogs and cats.

Myxomatous mitral valve disease (dogs)

MMVD is responsible for roughly three-quarters of all canine heart disease and the leading cause of new murmurs in senior dogs. The murmur is typically heard best on the left side of the chest, low and behind the elbow, and it occurs during the squeezing phase of the heartbeat (a systolic murmur). Many small-breed seniors live for years with a stable MMVD murmur and never develop heart failure; others progress through the staging system and eventually need medication.

Dilated cardiomyopathy (dogs)

DCM is a disease of the heart muscle itself, in which the chamber walls thin and stretch and the heart loses contractile strength. It is more common in large and giant breeds — Doberman Pinschers, Great Danes, Boxers, Irish Wolfhounds, and Newfoundlands — and unlike MMVD, the murmur it produces is sometimes soft or absent until the disease is advanced. A nutritional form of DCM has also been linked to certain grain-free, legume-heavy diets.

Hypertrophic cardiomyopathy (cats)

HCM is the most common heart disease in cats and affects an estimated 1 in 7 cats overall. The walls of the left ventricle thicken inward, leaving a smaller chamber that cannot fill or relax properly. The murmur in HCM is often a soft systolic sound, sometimes described as a "dynamic" murmur that comes and goes with the cat's heart rate and stress level. A gallop sound — a third heart sound, like a soft three-syllable rhythm — is another important finding in cats and is often more meaningful than the murmur itself.

Other cardiomyopathies (cats)

Restrictive and unclassified cardiomyopathies, in which the heart becomes stiff or behaves in mixed ways, are less common than HCM but cause similar murmurs and similar long-term consequences. Distinguishing them from HCM requires echocardiography.

Congenital defects discovered late

Some pets are born with subtle structural defects — small ventricular septal defects, mild pulmonic stenosis, or aortic stenosis — that go unrecognized through young adulthood and finally produce an audible murmur in middle or senior age, or that have been there all along and were simply missed. These are uncommon causes of new senior murmurs but worth ruling out.

Anemia

Severely thinned blood flows turbulently and produces a soft, often grade I to III murmur called a functional or physiologic anemia murmur. This kind of murmur disappears when the underlying anemia — from chronic kidney disease, immune-mediated blood disorders, gastrointestinal bleeding, or cancer — is treated. A senior pet with a new murmur and pale gums needs a complete blood count before anyone reaches for a cardiology referral.

Hyperthyroidism (cats)

Excess thyroid hormone in older cats drives the heart into a faster, more forceful rhythm and can produce a murmur, a gallop, or both. Hyperthyroid cats often develop secondary thickening of the heart walls that mimics HCM, and treating the thyroid disease frequently resolves both the murmur and the cardiac changes. Any new murmur in a senior cat should prompt a thyroid panel as part of the basic workup.

High blood pressure

Systemic hypertension — common in cats with chronic kidney disease and in dogs with Cushing's disease, kidney disease, or certain endocrine tumors — increases the workload of the left ventricle and can produce a murmur as the heart works harder to push blood against elevated pressure. Treating the underlying cause and lowering blood pressure often quiets the murmur.

Innocent and physiologic murmurs

Not every murmur reflects disease. Some otherwise healthy pets have soft, position-dependent, or stress-related murmurs that come and go and never prove to be anything at all. These are more common in young animals than in seniors, but they do occur, and an echocardiogram is the only reliable way to confirm that a murmur is truly innocent.

How veterinarians work up a murmur

A murmur on its own is a clue, not a diagnosis. The workup is the process of turning that clue into an answer specific enough to guide treatment.

History and physical exam

The story matters as much as the sound. Has there been a soft cough at night? Reduced exercise tolerance? Episodes of fainting or weakness? Increased breathing rate while sleeping? In cats, has there been weight loss, increased thirst, or sudden lameness? A careful history often points the way before a single test is ordered. The physical exam adds pulse quality, gum color, jugular vein distention, and the specific timing and location of the murmur on the chest wall.

Chest X-rays

Radiographs show heart size, the shape of individual chambers, and whether fluid is starting to accumulate in or around the lungs. They are the most direct way to tell whether a murmur is associated with congestive heart failure right now, and they provide a baseline for tracking changes over time. X-rays are particularly useful in dogs, where heart size on radiographs maps reasonably well to the staging system that guides treatment.

Echocardiogram

Cardiac ultrasound is the single most important test for a pet with a murmur. It shows valve leaflets in motion, measures wall thicknesses and chamber sizes, quantifies the strength and pattern of contraction, and identifies structural defects or tumors. An echocardiogram performed by a board-certified veterinary cardiologist is the gold standard for understanding what a murmur means, and for newly diagnosed senior pets it is worth seeking out whenever possible.

NT-proBNP blood test

NT-proBNP is a biomarker released by stretched heart muscle. In cats, it is especially useful as a screening tool: a normal NT-proBNP makes meaningful HCM unlikely, while an elevated value supports the case for an echocardiogram. In dogs, it is most helpful when distinguishing a cardiac cause of breathing difficulty from a respiratory one.

Electrocardiogram and blood pressure

An ECG identifies arrhythmias that sometimes accompany murmurs, especially in large-breed dogs with DCM. Blood pressure measurement is essential in any senior pet with a new murmur, since hypertension can both cause and complicate cardiac disease, and treating it can dramatically improve outcomes.

Blood work and thyroid panel

A complete blood count rules out anemia as a cause of a soft murmur. A chemistry panel checks kidney function — important both as a contributor to disease and as a baseline before any cardiac medications are started. In cats, a thyroid level should be checked routinely as part of the workup of any new murmur.

What happens after the murmur is explained

How a murmur is managed depends entirely on what is causing it, and on what the workup reveals about how far the underlying disease has progressed.

Stage B1 (dogs): murmur without heart enlargement

A dog with a known murmur from MMVD but no echocardiographic enlargement of the left atrium or left ventricle is in Stage B1. Most of these dogs need only periodic monitoring — typically a recheck and repeat imaging every 6 to 12 months — and no medication. Many remain in this stage for years.

Stage B2 (dogs): murmur with heart enlargement

Once echocardiographic measurements cross specific thresholds for left atrial and ventricular enlargement, the dog is in Stage B2, and the EPIC trial showed that starting pimobendan at this point delays the onset of congestive heart failure by an average of about 15 months. Identifying Stage B2 promptly is one of the most actionable reasons to investigate a murmur fully rather than simply listening at recheck visits.

Stage C and D (dogs): heart failure

Once fluid accumulation begins, the dog has reached Stage C and needs combination therapy — typically pimobendan, furosemide, an ACE inhibitor, and spironolactone — and the focus shifts from delaying disease to managing it day to day. Stage D is reserved for advanced disease that is no longer well controlled by the standard combination.

Cats with confirmed cardiomyopathy

Cats with HCM and no symptoms (preclinical) are typically monitored every 6 to 12 months without medication. Cats at higher risk of forming clots — especially those with severe left atrial enlargement — are often started on clopidogrel or aspirin to lower the risk of saddle thrombus. Cats who develop fluid in the lungs or chest move into the symptomatic group and require diuretics, often clopidogrel, and in some cases additional medications. Treating any underlying hyperthyroidism or hypertension is a critical part of care.

Treating reversible causes

When a murmur turns out to reflect anemia, hyperthyroidism, hypertension, or another systemic problem, the appropriate response is to treat the underlying disease. Many of these murmurs disappear or soften meaningfully once the primary issue is controlled, and no cardiac medication is needed at all.

Home monitoring: what owners can track

Whether your pet is being watched without medication or actively treated, your daily observations are the single most powerful early-warning system between vet visits. A few simple practices give you outsized power to catch problems early.

Sleeping respiratory rate

Counting your pet's breathing rate while she sleeps is the most useful single number you can track at home. Watch the chest rise and fall for 30 seconds, double the count, and you have her sleeping respiratory rate. A healthy resting dog or cat takes fewer than 30 breaths per minute, and most take far fewer. A sustained increase — especially a rate consistently above 35 to 40 over multiple readings — is often the very earliest sign of fluid backing up into the lungs, sometimes appearing days before any other symptom. Many cardiologists ask owners to log it nightly in a free phone app and call if the trend is rising.

Cough, energy, and appetite

A new soft cough, especially one that is worse at night or after lying down, is an important change in a dog with a known murmur. Reduced enthusiasm for walks, unwillingness to climb stairs that used to be easy, or new reluctance to play often signals worsening disease. In cats, watch for hiding, decreased appetite, weight loss, or any sudden change in jumping or activity.

Episodes of weakness or collapse

Brief loss of consciousness, hind-leg weakness, or sudden lameness — particularly in cats, where sudden hind-end paralysis can mean a saddle thrombus — should always prompt urgent veterinary attention. These events do not always accompany changes in the murmur itself.

Body condition and weight

Sudden weight gain over a day or two often reflects fluid retention rather than fat. Slow weight loss, by contrast, can reflect cardiac cachexia in advanced disease or progressing thyroid disease in cats. A simple weekly weigh-in catches both patterns early.

What to avoid

• Dismissing a new murmur as "just an old pet" finding. A murmur in a senior dog or cat almost always deserves at least a baseline workup, even if your pet seems entirely well.
• Skipping the echocardiogram. A murmur can be tracked by stethoscope alone, but only ultrasound can identify the underlying disease and, for dogs, distinguish Stage B1 from Stage B2 — the moment when starting pimobendan can buy more than a year of additional comfortable life.
• Heavily salted treats and table scraps. Sodium drives fluid retention and can push a marginally compensated heart into outright failure. Deli meats, bacon, cheese, jerky treats, and most table foods are surprisingly high in sodium for an animal of average size.
• Self-adjusting cardiac medications. Pimobendan, furosemide, ACE inhibitors, and clopidogrel are all dose-sensitive. Skipping doses or guessing at adjustments can trigger crises within a day or two.
• Over-the-counter cough or cold medications. Many human formulations are toxic to pets, and others mask worsening cardiac signs at exactly the moment when they should be most visible.
• Strenuous activity in heat. Hot weather and a heart with limited reserve are a dangerous combination, especially for small dogs with MMVD and cats with HCM.
• Ignoring an upward respiratory rate trend. A nightly count creeping from 22 to 28 to 36 is a warning worth a phone call, not a wait-and-see.

When to see the vet

A pet with a known murmur should generally be reassessed every 6 to 12 months while stable, more often if disease is progressing or medications have been changed. Between visits, a number of changes warrant earlier attention.

Call your vet soon if your pet shows:

• A sleeping respiratory rate consistently above 35 to 40 breaths per minute, or a rising trend over several days
• A new or worsening cough, especially at night or after lying down
• Reduced appetite, hiding, or noticeable weight loss
• Sudden weight gain that may reflect fluid retention
• Reluctance to walk, climb stairs, or play as far as usual
• New restlessness or difficulty settling at night
• A noticeably distended belly

Seek emergency care immediately if your pet shows:

• Labored, noisy, or open-mouth breathing at rest
• Belly-and-chest breathing with visibly increased effort
• Blue, gray, or very pale gums or tongue
• Sudden hind-leg weakness, paralysis, or unusual cries of pain in a cat (possible saddle thrombus)
• Collapse that does not resolve within seconds
• Continuous coughing, especially with pink or foamy fluid
• Severe lethargy combined with rapid breathing

Compassion and perspective

The first time a veterinarian mentions a heart murmur, most owners hear the word heart and feel the floor tilt. It helps to remember what a murmur actually is: a sound, a clue, an invitation to look more closely. Many murmurs in senior pets turn out to reflect mild disease that will be tracked, not treated, for years. Others reveal a condition that is genuinely serious but increasingly treatable — a small dog with Stage B2 MMVD who lives an extra fifteen months because pimobendan was started promptly, a cat with HCM whose owner counts breaths every night and catches a flare in time for diuretics to do their work, an elderly retriever whose murmur turns out to be hyperthyroidism and softens after a few weeks of methimazole.

Modern veterinary cardiology has changed what a murmur means in ways that the bare phrase cannot capture. Echocardiograms have become widely available, the staging systems are clearer than they were a decade ago, and the medications used to slow progression and manage failure have transformed expected outcomes. Most pets with a murmur live for years after the discovery, and many never go on to develop heart failure at all.

The work of love is also unchanged. You make the appointment for the echo. You learn to count breaths during the evening cuddle. You keep treats unsalted and walks gentle and the house cool in summer. You call early, not late, when something feels different. You let an aging companion be aging, without rushing her and without ignoring her. The murmur is not the whole story. It is a soft new sound under the steady, familiar one — a sound that tells you, more than anything, to listen carefully, to ask good questions, and to keep loving the animal whose heart you have been listening to all along.

This article provides general educational information about heart murmurs in senior dogs and cats. It is not a substitute for individualized veterinary advice. Every pet is different, and decisions about workup, monitoring, medication, and ongoing cardiac care should always be made in partnership with a veterinarian — and ideally, when significant disease is found, a board-certified veterinary cardiologist — who can examine your pet and interpret her specific imaging, blood work, and history. Never start, stop, or change cardiac medications without veterinary guidance. If your pet develops labored breathing at rest, blue or gray gums, sudden hind-leg paralysis, sustained collapse, or continuous coughing with pink or foamy fluid, seek emergency veterinary care immediately rather than waiting for a scheduled appointment.