Heart Murmurs in Small Dogs: What the Grade Means and When to Act

Small breed dogs carry a disproportionate burden of heart disease. Cavalier King Charles Spaniels, Dachshunds, Chihuahuas, Pomeranians, and Shih Tzus develop myxomatous mitral valve disease, the most common acquired cardiac condition in dogs, at rates far higher than large breeds, and many are diagnosed before they show a single outward symptom. The murmur your vet mentions at a routine check is not a death sentence. It is a data point, and knowing how to read it changes everything about how you respond.

The grading system runs from 1 to 6 and describes the loudness and character of the turbulent blood flow the vet hears through a stethoscope. A Grade 1 murmur is so faint it takes a quiet room and an experienced ear to catch. A Grade 6 is loud enough to feel as a vibration, called a precordial thrill, through the chest wall without a stethoscope.

Grade does not directly equal severity of disease. A dog with a Grade 3 murmur and a heart that has not yet enlarged is in a fundamentally different position from a dog with a Grade 3 murmur and significant left atrial enlargement. This is why the American College of Veterinary Internal Medicine's 2019 consensus guidelines on myxomatous mitral valve disease divided the condition into stages, A through D, based on structural changes and clinical signs rather than murmur loudness alone. Grade tells you something. It does not tell you everything.

What grade does predict reasonably well is progression speed. Dogs with louder murmurs tend to reach the point of cardiac remodelling sooner, which is why a Grade 4 or above at an annual exam typically prompts a cardiologist referral for echocardiography rather than continued watchful waiting.

Stage A covers dogs at elevated breed risk who have no detectable murmur yet. Stage B1 is a murmur with no cardiac enlargement on X-ray or ultrasound. Stage B2 is the point at which the heart has begun to remodel, the left atrium and left ventricle are measurably enlarged, but the dog still shows no symptoms. Stage C is symptomatic heart failure, and Stage D is heart failure that no longer responds adequately to standard medication doses.

The distinction between B1 and B2 is the one that changed small-dog cardiology most recently. The EPIC trial, published in the Journal of Veterinary Internal Medicine in 2016, demonstrated that starting pimobendan, a cardiac drug, in asymptomatic dogs at Stage B2 delayed the onset of heart failure by a median of fifteen months compared to placebo. That finding moved the treatment window significantly earlier and is now the basis for the ACVIM recommendation that B2 dogs begin medication before any cough or breathing change appears. Your vet or cardiologist will use chest X-rays and echocardiography measurements to determine whether your dog has crossed from B1 into B2.

A cough in a small dog with a known murmur is not automatically a cardiac cough, and the distinction matters because the treatments differ. Dogs develop coughs from tracheal collapse, bronchitis, reverse sneezing, and respiratory infections, all common in small breeds, and treating a collapsing trachea with diuretics intended for heart failure will dehydrate a dog without helping it breathe.

A cardiac cough typically appears at night or in the early morning, when the dog has been lying down long enough for fluid to shift. It often sounds soft and moist rather than the harsh honking of tracheal collapse. The dog may seem to be trying to clear something from the throat and may appear restless before settling. Alongside the cough, watch for exercise intolerance that is new or worsening, a dog that used to walk a full block now stops halfway, and a resting respiratory rate that has climbed above thirty breaths per minute. Counting breaths for one minute while the dog sleeps, once daily, is the single most useful monitoring habit an owner can build. Several veterinary cardiology centres have published free apps for this purpose.

An elevated resting respiratory rate, consistently above thirty breaths per minute across multiple readings, is an earlier and more reliable warning sign of fluid accumulation than coughing alone. Many dogs in early congestive heart failure never cough prominently at all.

Some presentations require a call to an emergency clinic, not a note for the next scheduled appointment. A dog breathing with its elbows pushed out from its body and its neck extended is working hard to move air and is in distress. Blue or grey gums, cyanosis, indicate that oxygen is not reaching the tissues and is an immediate emergency. Sudden collapse or extreme weakness, a dog that cannot stand or responds only weakly, is another. Rapid, shallow breathing that does not settle after a few minutes of rest belongs in the same category.

Dogs in acute heart failure often stand rather than lie down because lying flat makes breathing harder. They may refuse food and appear anxious without an obvious cause. If your dog has a known cardiac diagnosis and you see any combination of these signs, do not wait to see if it passes. Emergency stabilisation for acute congestive heart failure, typically oxygen, injectable diuretics, and monitoring, is time-sensitive in a way that most veterinary conditions are not.

The most practical thing an owner of a small dog with a cardiac diagnosis can do is establish a baseline before anything changes. Count the resting respiratory rate on three or four consecutive calm nights and write the numbers down. Note the distance your dog walks comfortably before slowing. Photograph or video any coughing episode so your vet can assess the character of it rather than relying on your description alone.

Schedule cardiac recheck intervals according to your vet's or cardiologist's recommendation rather than waiting for symptoms. Stage B1 dogs are typically rechecked every six to twelve months. Stage B2 dogs on medication may be seen more frequently. Echocardiography gives a clearer picture of disease progression than a stethoscope alone, and knowing whether the left atrium is growing is more useful than knowing the murmur is still a Grade 3.

Myxomatous mitral valve disease is progressive and has no cure, but the window between diagnosis and the first episode of heart failure is often measured in years, not weeks. Dogs managed carefully through that window, with medication started at the right stage and owners who know which changes to act on immediately, reach Stage C with more reserve than those whose disease goes unmonitored. The murmur grade is where the conversation starts. What you do with the information after that is what shapes the years that follow.