Understanding Heart Murmurs in Dogs

The contraction and relaxation of the heart muscle delivers blood throughout the body by way of the blood vessels. A heart murmur occurs when the normal pumping of the heart is disturbed by an abnormal flow of blood. This disturbance may stem from a defect, disease, or infection of the valves or vessels of your dog's circulatory system, or the blood itself.

The veterinarian can usually diagnose a heart murmur during your dog's annual exam. In more severe cases, your dog may begin to exhibit symptoms of a murmur that prompt you to make an appointment. These symptoms include trouble breathing, loss of appetite, weight loss, pale gums, and lethargy. If the vet discovers a murmur, he or she may perform some tests to determine the location of the murmur and any underlying diseases or conditions. Tests include echocardiograms, electrocardiograms, and blood tests.

Treatment depends on the underlying cause of your dog's murmur. The vet may prescribe medication for treatable conditions such as hypothyroidism, heartworm, and anemia. Surgery can correct certain structural heart defects. Often, innocent murmurs that do not cause harm to your dog require no treatment at all and may resolve on their own, though they should be regularly monitored.

Abnormalities in blood flow result from structural or functional abnormalities of the dog's heart, vessels, or blood. Structural defects include improperly functioning valves, diseased valves or vessels, and obstructions. Diseases and infections affecting the blood are functional irregularities. Causes often depend on the type or murmur, which may be classified as systolic, diastolic, or continuous.

In systole, the heart contracts and pumps blood through the arteries that supply the body with oxygenated blood. This is the first heart sound in a normal heart beat. A systolic heart murmur, then, occurs when the heart contracts and pumps blood, and it is the most common type of murmur in dogs. Systolic heart murmurs most often result from a narrowing of the heart's left ventricle or pulmonary artery in subaortic or pulmonary stenosis, respectively. Other causes include some types of heart failure, endocarditis, anemia, hypothyroidism, and heartworm.

Diastole refers to the relaxation of the heart, when the veins empty deoxygenated blood into the organ. A diastolic heart murmur occurs when the heart relaxes. Aortic insufficiency is the most common cause of a diastolic murmur in dogs and occurs due to an improperly functioning aortic valve, which results in the heart leaking blood. Narrowing of the mitral and triscupid valves might also cause a diastolic murmur.

A continuous heart murmur occurs in both systole and diastole, occurring throughout the normal contraction and relaxation cycle of your dog's heart movements. The most common cause is patent ductus ateriosus, a congential defect. Aortic regurgitation and aortic stenosis with aortic regurgitation might also lead to a continuous heart murmur.

Heart sounds in a murmur or heart murmur qualities vary depending on underlying factors. Your veterinarian will listen for the particular quality in your dog's murmur to make a judgement on the type and cause. There are four common qualities:

• Plateau murmurs are uniformly loud
• Crescendo-decrescendo murmurs rise and then fall in volume
• Decrescendo murmurs begin loud and then decrease in volume
• Continuous murmurs are machine-like in volume and pattern

Your veterinarian will grade your dog's heart murmur according to its severity, on a scale from I to VI. Grades I and II can be heard only through a stethoscope and are typically the least concerning. Most serious heart murmurs are graded III or higher. Grade IV murmurs are audible on either side of the chest. The most concerning and potentially threatening murmurs, grades V and VI are the loudest murmurs and can be felt through your dog's chest without instruments.

Your dog's prognosis depends on the underlying cause and her overall health, age, and breed. Due to the variability of a heart murmur's outcome, only your dog's veterinarian can provide an accurate prognosis. Murmurs can often be managed or, in some cases, reversed entirely through medication or surgery. A severe murmur might indicate a defect, infection, or disease that has progressed toward heart failure. Depending on the stage of heart failure, the likelihood of survival may be low. In such cases, treatment may be confined to relieving symptoms and ensuring your dog is as comfortable as possible.