The Club
The Breed

The Tell-Tale Heart Murmur

Causes and controls of cardiac disease

by Kim Campbell Thornton © 

First published in "YOUR DOG", Tafts University School of Veterinary Medicine., Aug 2000. Reprinted w/ permission of the Author.

Lup-dup, lup-dup, lup-dup. That's the sound of a normal heartbeat. As most of us learned in school, the heart operates with a pumplike action. Divided into four chambers-right atrium, right ventricle, left atrium, left ventricle-the heart has four valves that work to keep blood flowing in one direction. The valves open and close, letting blood in and then pumping it out. That lup-dup sound is created from the movement of the valves and the flowing of the blood.

But what happens when valves become diseased or worn? Often, they fall to close completely with each heartbeat, resulting in a backwash of blood. The effect is an abnormal sound called a heart murmur.

The sound a murmur makes depends on when it occurs in the cardiac cycle. A murmur that occurs when the ventricles are beating is called a systolic murmur. A murmur that occurs when the ventricles are relaxed-a period called cardiac diastole-is called a diastolic murmur. Instead of going lup-dup, lup-dup, the heart with a systolic murmur goes lup-shh-dup; a heart with a diastolic murmur makes the sound lup-dup-shh.


Murmurs are graded in severity from 1 to 6, with 1 being the softest murmur that can be heard and 6 being loud enough that it's evident before the stethoscope even touches the chest. Murmurs graded at 4 to 6 can often be felt if the hand is placed at the right spot on the chest.

Most murmurs are diagnosed by auscultation, which simply means that the veterinarian listens to the heart with a stethoscope. Although auscultation can indicate the presence of a murmur, further tests are needed to determine its cause and severity. Your veterinarian is likely to refer you to a veterinary cardiologist for these tests.

Depending on the breed, age, and predispositions of your dog, the cardiologist will want to take thoracic radiographs (chest X rays), if the referring veterinarian hasn't already taken them, and run an electrocardiogram and an echocardiogram (ultrasound).

The radiographi indicate overall heart size and enable the cardiologist to see the pulmonary vessels in the lung. This allows him to judge the degree of normalcy or abnormalcy resulting from the heart disease. An electrocardiogram shows the heart's rate and rhythm and is useful if abnormal rhythms are detected through auscultation.

An echocardiogram provides a precise measure of the thickness of the various chamber walls of the heart and a look at each of the cardiac valves. This test allows the cardiologist to determine the state of the valves and to calculate overall cardiac performance, which is helpful in deciding whether therapy is necessary. Follow-up echocardiograms tell the veterinarian whether or not the therapy is working.

The cardiologist may also check your dog's kidney function. "Renal disease is the most frequent cause of high blood pressure in dogs and cats," says James Ross, DVM, professor of cardiology at Tufts University School of Veterinary Medicine. "The presence of renal disease adds a greater workload onto the heart and can make some cardiac conditions much worse much quicker unless the hypertension is brought under control.

"So it's often advisable to do some screening of blood for buildup of waste products, take a look at some urine, and make sure there's nothing going on there-and maybe check some enzymes that are associated with liver disease and congestion that sometimes develops as the heart falls."


Should you worry if your dog has a heart murmur? That depends on factors such as the dog's age and breed, and whether the breed in question is prone to heart disease. Some dogs with murmurs can go for years without showing any signs of heart disease. "One big problem with heart murmurs is that they don't correlate exactly to the type or severity of heart disease," says Seattle veterinary cardiologistjerry Woodfield.

"For instance, an owner of a dog with a quiet murmur might expect that the heart disease would be minimal compared to a loud murmur that should indicate more severe heart disease. It simply doesn't work that way. What's more important is the company the murmur is keeping. Are there clinical signs such as exercise intolerance, collapsing episodes, bluish mucous membranes? Does the dog have a rapid heart rate that's not stress related? The more of these signs, the more likely a heart murmur is significant."

"What's really important about a murmur is that you get an accurate diagnosis of what's causing the murmur and the underlying problems associated with it," Dr. Ross says. "Once the cause is known, then you make adjustments based on the animal's lifestyle, the family's lifestyle, and the severity of that particular condition."


Heart murmurs may be the result of a birth defect, or congenital malformation. Congenital heart murmurs are typically seen in young dogs and are hereditary. Common congenital heart defects are patent ductus arteriosus (PDA) and pulmonic stenosis.

PDA is most commonly seen in miniature Poodles and German Shepherds. It occurs when the ductus arteriosus-a vessel in fetuses that allows blood to bypass the as-yet nonfunctional lungs-remains open after birth. The result is a leak from the aorta through the open ductus into the pulmonary side of the heart, causing the left ventricle to work harder to maintain normal blood flow.

When listening to the dog's heart, a veterinarian suspects PDA if he hears a continuous heart murmur-one that is present during both the systolic and diastolic cycles of the heartbeat. Chest X rays and an echocardiogram can confirm the diagnosis. Left untreated, PDA leads to heart failure, although the condition can be repaired surgically.

Puimonic stenosis, which usually affects small breeds, is a narrowing of the connection between the right ventricle and the pulmonary artery, increasing the resistance to blood flow and making it more difficult for the right ventricle to pump blood. In response, the right ventricle enlarges and thickens, just as any muscle does when worked. A systolic murmur is usually the first hint of puimonic stenosis, a diagnosis that is confirmed with chest X rays and an electrocardiogram. In severe cases, pulmonic stenosis can be treated with a balloon valvuloplasty, a procedure with a success rate of about 70 percent.

Some cardiac murmurs, especially in puppies, are innocent, meaning they eventually disappear. This type of murmur, also common in children, is usually no cause for alarm. It's often not possible to determine whether a murmur is innocent until the dog reaches two or three years of age. To avoid passing on genetic defects to another generation, it's important to refrain from breeding a dog until it has been cleared of any problems. A dog with a hereditary heart problem, even one that has been repaired, should not be bred.


Some murmurs are caused by anemia. Anemia this the blood, making it more prone to turbulence. This is especially common m athletic dogs. As their hearts contract, blood is ejected into the vessels with great velocity. Severe anemia causes a rapid pulse and breathing rate, and overexertion can cause the dog to pass out.

Heart disease and severe anemia have similar signs and can sometimes be confused. "If the murmur is caused by anemia, the best thing is to explore what's causing the anemia," Dr. Ross says. "It may be something as simple as worms. If you get rid of the worms, you get rid of the heart murmur" Parasites, such as whipworms or hookworms, can cause anemia.

One of the most common conditions associated with heart murmurs is mitral valve disease (MVD), sometimes referred to as mitral regurgitation.

This condition occurs when the valve degenerates and begins to leak. MVD can also result from an infection of the valves (endocarditis), from a valve that's malformed at birth, or in response to dilated cardiomyopathy.

Valvular degeneration is most often seen in small breeds, especially as they age. Cavalier King Charles Spaniels are particularly prone to MVD, and it often earlier in that breed than in others, Dr. Ross says. Other breeds in which it tends to occur include Chihuahuas, miniature Poodles, miniature Pinschers, Fox Terriers, Boston Terriers, and miniature Schnauzers. It's more often seen in males than females.

When mitral regurgitation occurs, blood leaks back from the left ventricle into the left atrium. In response, the left atrium enlarges to make room for the extra blood. The left ventricle also increases in size so it can pump more blood to make up for the leak. Severe cases lead to congestive heart failure, indicated by fluid accumulation in the lungs (pulmonary edema). Signs of MVD are exercise intolerance and coughing or wheezing.


Carol Richards of Knoxville, Tennessee, has two Cavalier King Charles Spaniels with MVD. Savannah is 9 years old with a grade 3 murmur. Dinah (shown on page 1) is 11 years old with a grade 6 murmur.

"Savannah is not on medications at this time, but I supplement her daily with CoEnzyme Q10 (30 mg), vitamin E (200 IU), and fish-oil capsules," Richards says. "She has never exhibited any symptoms of MVD, but she's not very active." The supplements Richards gives are believed to have beneficial effects on the heart, although no studies as yet have confirmed this in dogs.

"Dinah was in heart failure when I got her [both dogs were rescues]," Richards continues. "My veterinarian started her on Lasix, and we made an appointment with the cardiologist at the University of Tennessee College of Veterinary Medicine. She took chest X rays and a color doppler ultrasound of Dinah's heart. Dinah is now on 12.5 mg of Lasix once a day and 2.5 mg of Vasotec twice a day. Both medications keep her symptoms under control. She's also on the same supplement regimen as Savannah."

When a dog with MVD has difficulty breathing, as Dinah did, and chest X rays show a buildup of fluid in the lungs, diuretics (like Lasix) are often prescribed to relieve the congestion and fluid, Dr. Ross says. For the same reason, a low-sodium diet is often prescribed.

Dietary therapy isn't always beneficial, however. Its effectiveness depends on the precise condition.

"There have been a number of studies done that would suggest that severe salt restriction early on in heart disease may do more damage than good," Dr. Ross says.


Other medications that may be prescribed for dogs with heart disease are afterload reducers, which lower blood pressure, 

decreasing the workload on the heart. "One of the most popular drugs now for that use is called an angiotensin converting enzyme inhibitor," Dr. Ross says. "Enalapril is the only one that's been approved for veterinary use. Interestingly, it's one of the few drugs we know of that has prolonged and improved the quality of life in human patients with heart failure."

When a dog with MVD should begin taking medication is a matter of some debate. It's not clear that early administration has any advantage, and there are side effects to consider. "Some people are advocating putting them on enalapril a lot earlier than we used to," Dr. Ross says. "There has been some human literature that would suggest that it's a helpful drug in terms of prolonging life, but there's no question that whenever you have to take any drug, there's a risk of some undesirable side effects that you'd just as soon not have. So we go by our past experiences and by new information that becomes available"

In Dr. Ross's experience, the time to begin medication is when the resting heart rate is consistently more than 100 beats per minute. The normal canine resting heart rate is 80 to 100 beats per minute.

"It depends on the breed, how fast the heart rate is, and how often it's up, but if it's constantly up over 100 even when they're resting, I think I can help," he says.

"When the heart rate goes up, that increases the work load on the heart a lot. It can go up that fast with a burst of energy without deleterious effects, but if it remains high for a long period of time, that damages it."

Some cardiologists recommend that their clients learn to take their animals' heart rate, especially for older dogs. Simply place your hand on the animal's chest, count the heart beats for 15 seconds, and then multiply the number of heart beats by four to get the beats per minute. A heart rate taken at home where the dog is comfortable can be more indicative of a true resting heart rate than one taken at the veterinarian's office, where the dog may be excited or anxious.

Drugs that strengthen heart contractions may also be prescribed-for example, digitalis, an extract of foxglove, used for centuries as a heart tonic. "Determining when it's best to give often requires an echocardiogram to determine how well the heart's performing," Dr. Ross says. "When its contractions begin to weaken, that's when we often will use something to boost it a bit"


Dogs with heart murmurs or MVD can lead relatively normal lives, but their owners must consider carefully the risks of anesthesia for elective procedures such as teeth cleaning. Dinah and Savannah both had poor dental health when they came to live with Richards.

"I made an appointment to have their teeth cleaned as soon as it was okayed by the cardiologist," she says. "The veterinarian used isoflurane anesthetic, with antibiotics before and after the procedure. Dinah had one tooth fall out before the appointment and had seven teeth pulled. Savannah had two teeth pulled."

Although it's long been suspected that periodontal disease can contribute to heart disease, Dr. Woodfleld says there is no evidence that dental disease causes, contributes to, or exacerbates heart disease in dogs. "The statement that dental disease causes heart disease is used a lot but is simply an extrapolation from human medicine," he says. "It has no foundation in veterinary medicine." For that reason, he is wary of anesthetizing dogs with heart murmurs, particularly before they've been evaluated by ultrasound. The risks increase for long anesthetic periods.

"To make a decision for a dental, people should weigh risk versus benefit," he says. "Is there significant disease-not just tartar-that warrants an anesthetic? Does a tooth need to be pulled? Surgery performed? If it's simply scaling tartar off, maybe some home care and oral hygiene should be considered an alternative. If a time-consuming procedure such as a root canal is being considered, I think it's appropriate to consider a shorter procedure such as extraction instead of keeping the dog under anesthesia longer to save a tooth that it can live without."

To provide safety insurance during the anesthesia, Dr. Woodfleld recommends that the veterinarian use drugs that are short acting or reversible, and place an intravenous catheter prior to anesthesia so that CPR and emergency drugs can be administered without delay should the dog have an adverse reaction. Thorough monitoring during anesthesia is important-a pulse oximeter to measure the percentage of oxygen in the blood, an ECG monitor to measure heart rate and type of beats occurring, a trained technician to monitor pulse quality, and a respiratory monitor to indicate any decrease or arrest of respirations.


Although there is no way yet to prevent heart murmurs and the diseases associated with them, responsible breeders are working to eliminate carriers from their breeding programs. According to a report in the newsletter of the Cavalier Health Foundation, the results of the British CKCS Club heart-screening program show an overwhelming confirmation that murmurs in the parents influence the development of murmurs in offspring. Similar results were found in 1996 in a Swedish study. Both studies strongly support the conclusion that selective breeding should be able to reduce the incidence of chronic valvular disease in the breed.

The unraveling of the canine genome may also provide an answer some day. Until then, be aware of the possibility if you own one of the breeds that's prone to heart murmurs, and make sure your veterinarian is familiar with the breed's predisposition, too. The earlier a heart murmur is detected, a cause for it established, and treatment is begun, the better your dog's prognosis.

Kim Campbell Thornton is a freelance writer in Lake Forest, Ca1ifornia.

© 2000 Kim Campbell Thornton

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