What Causes a Heart Murmur in Dogs?

Heart murmurs in dogs may be identified during routine exams when a veterinarian uses a stethoscope to listen to a dog's heart. If your dog has a heart murmur, instead of hearing the usual "lub-dub" sound, your vet may hear a "whoosh" sound. Not all causes of dog heart murmurs are a cause for concern; however, if your vet notices one in your dog, it's important to know what's causing the murmur and whether the condition needs to be treated. Here's more on what heart murmurs are, what causes them and how they can be treated.

What Is a Heart Murmur in Dogs?

When your veterinarian listens to your dog's heart with a stethoscope, part of what they listen for is a classic "lub-dub" sound that suggests that your dog's heart valves are opening and closing properly. These heart valves open and close in a synchronized manner that allows blood to be pumped to the lungs and the rest of the body without ever flowing in the wrong direction. However, whenever blood flows the wrong direction through the heart, which can be due to things such as impaired heart valve function or heart muscle defects, vets will hear this as a turbulent "whoosh" sound, also known as a heart murmur.

Vets identify heart murmurs based on the intensity of what they hear when they listen to the heart — a "1" rating would indicate the softest murmur while a "6" would be rated as the loudest. However, the volume of the murmur has no correlation with the severity of the condition causing the murmur; it's simply one method that vets use to record the murmur over time to see if it changes.

While heart murmurs may be indicative of heart disease or other conditions, they aren't always a cause for concern. For example, puppies may have an "innocent heart murmur" that they're born with. But, once they're about four months old, the murmur disappears without any treatment. In contrast, a heart murmur in a senior dog may be due to a valve disease, requiring vet treatment and monitoring. Depending on the cause of the heart murmur, your vet may elect to monitor your dog or may recommend additional testing and treatment.

Causes of Dog Heart Murmurs

Anything that causes abnormal blood flow within the heart can cause a murmur. Here are the most common causes of heart murmurs in dogs:

• Leaky heart valve: A dog's heart has four chambers: left and right atria, and left and right ventricles. In between these chambers are valves that open and close to let blood flow normally through the heart. Leaky valves in the heart are the most common cause of heart murmurs in dogs. Common breeds at risk for this disease include Cavalier King Charles Spaniels, cocker spaniels, poodles and chihuahuas.
• Heart Muscle Disease (Cardiomyopathy): In dogs, this is most often caused by a weakening of the heart muscle's walls, resulting in turbulent blood flow and a murmur. Breeds at risk for this condition include Dobermans, boxers and giant breed dogs.
  
• Cancer: A tumor anywhere inside the heart can cause turbulent blood flow and a murmur.
• Congenital heart defects: There are a wide variety of congenital heart defects in dogs that can result in heart murmurs. A few of the more common congenital heart defects include: patent ductus arterious (PDA), which is seen in many breeds including German Shepherds, Maltese, poodles and Yorkshire Terriers; pulmonic valve stenosis (narrowing), which is commonly seen in English Bulldogs; and aortic valve stenosis (narrowing), which may be seen in breeds such as Golden Retrievers and Rottweilers.