A heart murmur in a dog sounds like a “whooshing” or “shooshing” noise between the normal heartbeat sounds. A healthy heart produces two distinct sounds, often described as “lub-dub,” and a murmur creates an extra turbulent sound between or during those beats. You can’t hear it with your ear pressed against your dog’s chest. It requires a stethoscope and a trained ear to detect.
What the Vet Actually Hears
A normal dog heartbeat has two clear sounds: the “lub” (heart valves closing as the chambers contract) and the “dub” (a second set of valves closing as the chambers relax). A murmur is the sound of blood flowing abnormally, either leaking backward through a faulty valve, squeezing through a narrowed opening, or passing through an abnormal connection between heart chambers. The turbulence creates that characteristic whooshing quality layered over or between the normal heart sounds.
Not all murmurs sound the same. The three main types differ based on when they occur in the heartbeat cycle:
- Systolic murmurs happen during the contraction phase, between the “lub” and the “dub.” These are the most common type. Some rise and fall in intensity like a diamond shape on a sound recording, while others hold a steady volume throughout. The pattern helps the vet narrow down the cause.
- Diastolic murmurs occur during the relaxation phase, after the “dub.” These are less common and can sound like a grunting or dive-bomber noise that fades in intensity. They typically point to a leaky aortic valve.
- Continuous murmurs run through both phases of the heartbeat without stopping, rising and falling in a characteristic pattern sometimes called a “machinery murmur.” This type is most often caused by a birth defect called patent ductus arteriosus, where a blood vessel that should close after birth stays open.
Where on the Chest It’s Loudest
Vets don’t just listen in one spot. They slide the stethoscope across the chest, a technique called “inching,” to find where the murmur is loudest. That location reveals which heart structure is likely involved.
On the left side of the chest, there are three main listening zones. Higher up near the shoulder (the heart base), murmurs typically come from the pulmonic or aortic valves. Lower and further back (the heart apex), murmurs usually involve the mitral valve. On the right side of the chest, the vet listens for tricuspid valve problems or holes between the heart’s lower chambers. A continuous murmur from patent ductus arteriosus is typically loudest at the left base, near the armpit area.
The 1 to 6 Grading Scale
Once a vet hears a murmur, they grade its loudness on a scale from 1 to 6. This gives a rough sense of severity, though volume alone doesn’t tell the whole story.
- Grade 1: Barely audible, easy to miss. The vet has to listen carefully in a quiet room to catch it.
- Grade 2: Soft but immediately detectable with a stethoscope.
- Grade 3: Moderate volume, clearly heard.
- Grade 4: Loud, and the vet can sometimes feel a vibration (called a “thrill”) through the chest wall.
- Grade 5: Very loud with an obvious vibration felt through the stethoscope.
- Grade 6: So loud it can be heard with the stethoscope barely touching or even slightly off the chest, with a strong vibration.
A murmur intensity of 3 out of 6 or higher is one of the markers vets use to determine whether a dog’s heart disease has progressed enough to warrant treatment before symptoms appear.
What Causes the Sound
The most common cause of heart murmurs in adult dogs is myxomatous mitral valve degeneration, a progressive condition where the mitral valve (which separates two chambers on the left side of the heart) slowly deteriorates. As the valve weakens, it doesn’t close tightly, and blood leaks backward with each heartbeat. That backward flow is the whooshing your vet hears. Small breeds like Cavalier King Charles Spaniels are particularly prone to this condition.
In puppies, the most common cause of a murmur is pulmonic stenosis, a narrowing of the valve leading to the lungs. Other congenital defects include patent ductus arteriosus, subaortic stenosis, ventricular septal defects (holes between heart chambers), and malformed tricuspid or mitral valves. The prevalence of congenital cardiac defects in dogs ranges between 0.13% and 1.6%. Larger breeds are more likely to develop murmurs from cardiomyopathy, a disease of the heart muscle itself.
Not every murmur means heart disease. Puppies commonly have “innocent” murmurs caused by mild, normal anemia during growth. These typically disappear on their own. Functional murmurs can also crop up temporarily from fever, anemia, pregnancy, or an overactive thyroid, all of which increase the speed and volume of blood flow enough to create audible turbulence.
How Vets Figure Out What’s Behind It
Hearing the murmur is just the first step. Its sound, location, and grade tell the vet what might be going on, but confirming the cause requires further testing. A chest X-ray shows whether the heart is enlarged and whether fluid has built up in the lungs. An echocardiogram (an ultrasound of the heart) is the gold standard. It lets the vet watch the valves open and close in real time, measure chamber sizes, and see exactly where blood is flowing abnormally.
A blood test measuring a protein released by stretched heart muscle cells can help distinguish dogs with significant heart enlargement from those with mild, early-stage disease. Together, these tests place a dog into one of several disease stages, from dogs at risk but showing no changes (Stage A) through dogs with heart enlargement but no symptoms (Stage B), to dogs actively in heart failure (Stage C) or with advanced disease that’s difficult to manage (Stage D).
What You Can Monitor at Home
If your dog has been diagnosed with a murmur, one of the most useful things you can do is count their breathing rate while they sleep. A normal sleeping respiratory rate for a dog is under 30 breaths per minute. Dogs with well-controlled heart disease typically average around 20 breaths per minute during sleep, with little variation from day to day.
To count, watch your dog’s chest rise and fall while they’re sleeping soundly. Count for 30 seconds and multiply by two. If that number consistently creeps above 30 breaths per minute, it can be an early sign that fluid is starting to accumulate in the lungs, sometimes days before other symptoms become obvious. Keeping a simple log of this number gives your vet valuable information at checkups. Changes in exercise tolerance, new coughing, or a loss of appetite are other signs worth tracking.