What To Know About Heart Murmur in Dogs

Medically Reviewed by Amy Flowers, DVM on December 09, 2021
3 min read

A heart murmur is an abnormality of the heartbeat. Heart murmurs are usually identified when a veterinarian uses a stethoscope to listen to your dog's heart. 

There are several types and degrees of heart murmurs in dogs. Some are not serious and may not lead to symptoms of heart disease in dogs, but others may require medical care. 

A dog's heart works just like a human's. It pumps blood throughout their bodies to keep them alive and functioning.

Generally, heart murmurs are not worrisome. Some puppies may have soft heart murmurs in their developmental phase. But, some heart murmurs indicate the presence of an underlying heart condition. 

Many inherited heart defects can cause this condition, but in some dogs, heart murmurs can also develop later in life.

The primary cause of heart murmurs in dogs is rapid blood flow in the heart due to narrowed or leaky valves. When the normal blood flow within the heart gets disturbed, it creates an unusual, audible noise, called a murmur. 

A vet can easily distinguish a heart murmur from a regular heartbeat with a stethoscope.

The causes of heart murmurs can be divided into three categories depending upon the blood flow disturbances:

  • Blood flow disturbance due to abnormal heart valves or unusual vibrations
  • Blood flow disturbance due to leaky, narrowed, or diseased heart valves
  • Blood flow disturbance due to regurgitant or backward flow

Veterinarians also divide murmurs into different types based on their timings. There are a few major types of murmurs.

Systolic murmurs. In this most common type of murmur, the heart muscles contract. The primary cause of systolic murmurs is pulmonic or subaortic stenosis, the condition related to the dilation of blood vessels that causes a restriction in the blood flow. 

Other conditions leading to systolic murmurs can be:

  • Hyperthyroidism, when the thyroid glands produce excessive thyroxine (a hormone)
  • Anemia or low hemoglobin in the blood
  • Heartworm disease, a lethal condition caused by a parasitic worm
  • Systolic anterior mitral motion (SAM), a life-threatening condition in which the distal part of the anterior leaflet of the mitral valve displaces toward the left ventricular outflow area
  • Aortic valve insufficiency (AVI), when the aortic valve is damaged
  • Cardiomyopathy, a condition in which the heart struggles to pump blood into your body
  • Mitral and tricuspid heart failure that could be due to backward blood flow
  • Endocarditis of the mitral and tricuspid valve or a lethal inflammation of the valves

Diastolic murmurs. This is a rare type of murmurs in dogs that happen when the heart muscle is relaxed between heartbeats. The primary reasons for diastolic murmurs are:

  • Aortic insufficiency 
  • Mitral and tricuspid valve stenosis, narrowing in the valves that prevent the blood flow
  • Endocarditis of the aortic and pulmonic valve, a life-threatening inflammation of the valves

Continuous murmurs. These murmurs happen throughout your dog's heartbeat cycle regularly. The common causes of continuous murmurs are:

  • Patent ductus arteriosus (PDA), a firm opening between the two primary blood vessels leading from your dog's heart 
  • Aortic regurgitation, in which the aortic valve doesn't close properly, leading to the backward flow of blood to the left ventricle
  • Aortic stenosis, or a narrowing in the aortic valve

Innocent murmurs. These are mild types of heart murmurs found in younger dogs that often resolve with time.

These grades are located on a scale of one to six:

  • Grade 1 murmurs. This is the initial phase of the murmurs, which are barely heard with a stethoscope. It is the least serious grade.
  • Grade 2 murmurs. These are more audible but soft murmurs that can be heard with a stethoscope.
  • Grade 3 murmurs. This is the problematic grade with loud murmurs.
  • Grade 4 murmurs. These murmurs are louder, which the vet can hear easily through either side of the chest.
  • Grade 5 murmurs. These are super-loud heartbeats that the vet can hear with a stethoscope and feel with bare hands easily.
  • Grade 6 murmurs. These murmurs are even louder than grade 5. This is the most severe phase of murmurs. 

To diagnose whether these murmurs are due to any underlying condition, the vet will perform additional tests. These include radiographs, echocardiograms, and electrocardiograms. 

You can't treat heart murmurs. Instead, you can manage the underlying heart condition causing the murmurs. 

The symptoms of heart murmurs in dogs vary, depending on the types and grades of the condition. If your dog shows any signs of heart issues, such as exercise intolerance, coughing, or weakness, you should visit a vet immediately.