What exactly is happening inside the heart when a vet hears a murmur?

A heart murmur is turbulent blood flow that produces an audible whooshing, swishing, or rumbling sound instead of the crisp two-part lub-dub of a normal heartbeat. The Merck Veterinary Manual explains that murmurs arise when blood moves abnormally fast, through a narrowed opening, backward through a leaking valve, or between chambers through a hole that shouldn't exist, and that the sound's timing and location help identify which structure is involved (Merck Veterinary Manual, 2023). Veterinarians grade murmurs on a standard scale from I to VI, where a grade I murmur is barely audible even with careful listening and a grade VI is so loud it can be felt as a vibration through the chest wall, a system described in cardiology references used across general practice (Merck Veterinary Manual, 2023).

The location where a murmur is loudest, called the point of maximal intensity, points toward a specific valve. A murmur heard best over the left side of the chest near the fifth intercostal space often traces back to the mitral valve, the most frequent site of trouble in small-breed dogs, according to cardiology guidance from Cornell University's veterinary teaching hospital resources on canine heart disease (Cornell Riney Canine Health Center, n.d.). Congenital murmurs detected in puppies, by contrast, are frequently loudest higher up near the heart base and often relate to malformed valves or connections present from birth.

How do owners usually first notice something might be wrong with their dog's heart?

Most heart murmurs are found incidentally, not because an owner spotted a symptom, but because a veterinarian heard it during a wellness exam. Many dogs with early-stage murmurs show no outward signs at all and can live for years before disease progresses, a pattern well documented in dogs with degenerative mitral valve disease, the most common acquired heart condition in the breed group most affected — small and toy breeds over age eight (ACVIM Consensus Statement, 2019). When physical signs do appear, they tend to be subtle at first: reduced stamina on walks, more frequent panting at rest, or a cough that owners initially blame on allergies or age.

As disease advances, signs become harder to dismiss. A soft, moist cough that worsens at night, labored or rapid breathing, a swollen abdomen, or fainting spells during excitement or exercise all suggest the murmur has progressed toward congestive heart failure. The American College of Veterinary Internal Medicine notes that resting respiratory rate above 30 breaths per minute measured at home is a practical red flag owners can track between vet visits, since a rising rate often precedes visible breathing distress by days (ACVIM Consensus Statement, 2019).

What actually causes a dog to develop a heart murmur?

Congenital murmurs are present from birth and stem from structural defects such as patent ductus arteriosus, where a fetal blood vessel fails to close, or pulmonic and subaortic stenosis, where a valve or vessel is abnormally narrow. The Merck Veterinary Manual lists these three defects as the most frequently diagnosed congenital heart abnormalities in dogs, with certain breeds like the Cavalier King Charles Spaniel, Chihuahua, and Boxer overrepresented for specific defects (Merck Veterinary Manual, 2023).

Acquired murmurs develop later in life, most often from myxomatous mitral valve disease, a progressive degeneration of the valve tissue that eventually prevents it from closing fully. This condition accounts for roughly three-quarters of all canine heart disease diagnoses and is especially prevalent in small-breed dogs such as Dachshunds, Poodles, and Cavalier King Charles Spaniels, per consensus data compiled by veterinary cardiologists (ACVIM Consensus Statement, 2019). Large-breed dogs are more prone to dilated cardiomyopathy, a disease of the heart muscle itself that can also produce a murmur as the heart chambers stretch and valves stop sealing properly, a link that prompted the FDA to investigate a possible connection between certain grain-free diets and cardiomyopathy in dogs (U.S. Food and Drug Administration, 2019).

What tests will the vet run to figure out what's causing the murmur?

A murmur detected on physical exam typically leads to staged diagnostics rather than a single test. Chest X-rays let veterinarians measure heart size using the vertebral heart score, a standardized method comparing heart width to vertebral length that helps flag early enlargement before clinical signs appear (Cornell Riney Canine Health Center, n.d.). Blood pressure measurement and bloodwork, including a cardiac biomarker called NT-proBNP, can indicate whether the heart is under strain even when X-rays look borderline.

The definitive test is an echocardiogram, an ultrasound of the heart that lets a cardiologist directly visualize valve motion, measure chamber dimensions, and estimate blood flow velocities across each valve using Doppler imaging. The American College of Veterinary Internal Medicine's consensus guidelines recommend echocardiography to stage mitral valve disease into categories A through D, a classification system that directly guides when medication should start (ACVIM Consensus Statement, 2019). An electrocardiogram may also be added if the vet suspects an arrhythmia is complicating the picture.

Does every dog with a heart murmur need to start medication right away?

Not necessarily. Dogs in ACVIM Stage B1 — meaning a murmur is present but the heart has not yet enlarged on imaging — are typically monitored with rechecks every six to twelve months rather than started on cardiac drugs, since no medication has been shown to delay disease progression at that stage (ACVIM Consensus Statement, 2019). Once imaging shows the heart chambers enlarging (Stage B2), the consensus guidelines recommend starting pimobendan, a drug that improves the heart's pumping strength and has been shown in the EPIC trial to delay the onset of congestive heart failure by around 15 months on average compared to placebo (ACVIM Consensus Statement, 2019).

Once a dog develops overt congestive heart failure (Stage C), treatment typically expands to include a diuretic such as furosemide to clear fluid buildup, alongside pimobendan and often an ACE inhibitor like enalapril or benazepril. Congenital defects such as pulmonic stenosis or patent ductus arteriosus are handled differently and may be candidates for a minimally invasive balloon valvuloplasty or catheter-based occlusion procedure performed at specialty cardiology centers, interventions the Merck Veterinary Manual notes can produce near-normal outcomes when done early in life (Merck Veterinary Manual, 2023).

What can owners do day to day to support a dog living with a heart murmur?

Weight management matters more than many owners expect, since extra body weight forces the heart to work harder with every beat; keeping a dog at a lean body condition score reduces that workload measurably. Sodium intake is another lever — prescription cardiac diets formulated for dogs in later heart failure stages restrict sodium to lessen fluid retention, though the ACVIM panel cautions against restricting sodium in early-stage dogs without a cardiologist's guidance, since overly aggressive restriction can activate compensatory hormone systems that worsen disease (ACVIM Consensus Statement, 2019).

Home monitoring of resting respiratory rate has become a cornerstone of caring for dogs with known heart disease, since a sleeping dog's breathing rate is one of the earliest and most sensitive indicators of fluid building up in the lungs. Owners are typically asked to count breaths for 30 seconds and double it once daily, logging the trend rather than a single reading, and to call the vet if the rate consistently exceeds 30 breaths per minute at rest (ACVIM Consensus Statement, 2019). Regular but not strenuous exercise is generally encouraged for dogs in earlier stages, adjusted based on the cardiologist's recommendations as disease stage changes.

What's the realistic long-term outlook for a dog diagnosed with a heart murmur?

Outlook depends heavily on the underlying cause and stage at diagnosis rather than the murmur itself. Dogs found to have an innocent, or physiologic, murmur — common in puppies and often fading by six months of age — need no treatment and carry a normal life expectancy, a pattern noted in veterinary cardiology teaching materials on pediatric murmurs (Cornell Riney Canine Health Center, n.d.). Dogs diagnosed early with degenerative mitral valve disease at Stage B1 can often live years with only periodic monitoring, while those who progress to congestive heart failure have a median survival that the EPIC trial data placed at roughly 13 to 15 months on appropriate therapy, though individual dogs vary widely (ACVIM Consensus Statement, 2019).

Congenital defects carry the most variable prognosis of all. A dog with mild pulmonic stenosis may never need treatment, while one with severe subaortic stenosis faces a meaningfully shortened lifespan without intervention and remains at risk of sudden death even with treatment, per descriptions in the Merck Veterinary Manual's congenital cardiology sections (Merck Veterinary Manual, 2023). This range is exactly why an echocardiogram, rather than the murmur grade alone, is what actually determines a dog's individual prognosis.