Understanding Heart Murmurs in Dogs: Causes, Symptoms, and Care

A heart murmur can occur in any dog regardless of age, sex, size, or breed. Not all heart murmurs are problems for dogs. However, a heart murmur may be a sign of a serious problem in the body.

What is a Heart Murmur?

A heart murmur is an abnormal heart sound that resembles a whooshing noise when listening with a stethoscope. It occurs when there is turbulent blood flow in the heart. A heart murmur is not a disease in itself but it may indicate heart disease or another problem in the body.

The heart has four chambers: the right atrium, the right ventricle, the left atrium, and the left ventricle. In a normally functioning heart, oxygen-depleted blood from the veins enters the right atrium and is pumped into the right ventricle. The blood flows through the artery of the lungs, becomes oxygen-saturated, and enters the left ventricle. The oxygenated blood is pumped through the left atrium and then goes out to the body through the arteries. Valves between each chamber and the blood vessels keep blood moving in the proper direction. Any disruption to this blood flow may cause turbulence that can be heard with a stethoscope.

Heart murmurs are typically graded by intensity on a scale of one to six (I-VI). Grade I heart murmurs are the mildest and can be difficult to detect. These murmurs are quiet and may occur intermittently. Grade VI murmurs are the most severe. These are very loud when listening with a stethoscope and can usually be felt by placing the hands on the dog's chest. The higher the grade of a heart murmur, the greater the blood flow turbulence in the heart.

An incidental heart murmur may be found during a routine wellness examination even if no signs are present. This is why it is so important for your dog to see the vet at least once a year for a check-up.

Signs of Heart Murmurs in Dogs

Signs of Heart Murmurs in Dogs

• Coughing
• Lethargy
• Exercise intolerance
• Weakness
• Panting or abnormal breathing
• Pale gums
• Distended abdomen (bloated appearance of belly)
• Episodes of collapse

Dogs with heart murmurs may or may not exhibit other clinical signs of illness. Because a heart murmur is a sign of a possible heart problem but not an actual disease, the presence of other signs depends on the degree of underlying disease, if any.

If you know your dog has a history of a heart murmur and you notice one or more of these signs or others, be sure to contact your veterinarian for advice. An examination and further diagnostic testing may be necessary.

Causes of Heart Murmurs in Dogs

A heart murmur may occur for several reasons. Further diagnostics are typically needed in order to look for an actual cause, especially with higher-grade murmurs.

Heart Structure Abnormalities

Structural problems like leaky valves or holes in the chamber of the heart can cause improper blood flow. This creates turbulence which can be heard as a heart murmur. Some structural abnormalities are present at birth while others can develop later in life. Some of the better-known heart structure abnormalities include:

• Mitral valve disease
• Tricuspid valve disease
• Subaortic or pulmonic stenosis (narrowing of valves)
• Patent ductus arteriosus (extra vessel present at birth)
• Endocarditis (inflammation in the heart; sometimes caused by an infection)

Extracardiac Conditions

Certain problems in the body can affect the flow of blood through the heart, even if the heart itself is functioning normally.

• Anemia
• Heartworm disease
• Hyperthyroidism
• Hypoproteinemia
• Infections

Innocent Heart Murmurs

"Innocent" or physiological heart murmurs are benign, meaning they are not caused by a disease process. These murmurs are generally mild to moderate (grade I-II). Puppies may have innocent heart murmurs that go away as they grow.

If your vet detects a mild to moderate heart murmur when no other signs are present, your vet may want to simply recheck your dog periodically to monitor the murmur rather than recommending further diagnostics right away.

Treatment and Prevention

Because a heart murmur is not a disease itself, but a sign of potential disease, the treatment will depend on the underlying cause of the heart murmur, if any. An asymptomatic dog with a mild heart murmur may not need additional diagnostic tests or treatments. However, a moderate to severe murmur warrants further investigation, especially if clinical signs are noted.

After obtaining a detailed medical history from you regarding your dog, your veterinarian will perform a thorough physical examination on your dog. The vet will listen to the heart with a stethoscope. If a heart murmur is heard, it will be given a grade of intensity (from I to VI).

Several diagnostic tests are needed to assess heart function and overall body health. Your vet will recommend blood chemistry, complete blood count, and urinalysis to assess organ health and cell counts. Radiographs (X-rays) of the chest will be needed to look at heart size and details of the heart and lungs.

Specialized heart tests may be needed next to get a closer look at the heart. Your primary veterinarian may refer you to a veterinary cardiologist for this testing. An electrocardiogram (ECG or EKG) measures electrical activity in the heart by displaying waveforms that can be analyzed to look for abnormalities. An echocardiogram is an ultrasound of the heart that shows the heart beating as well as the blood flow through the heart. The sonographer can measure the chambers and valves of the heart as well as look for abnormal blood flow patterns.

Once a diagnosis has been made, treatment recommendations can be discussed. Some heart abnormalities will require surgery to repair. Other conditions may be treated with medications. Dogs with heart disease will need routine follow-up visits to the primary vet and/or cardiologist to monitor the response to treatment and evaluate the progression of the disease.

Heart murmurs cannot be prevented, but early detection can enable the vet to treat an underlying disease before it becomes serious. Always be sure to follow your vet's recommendations for treatment and follow-up. Talk to your vet if you are having trouble complying with recommendations. Your dog's life depends on it.