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
LISTENING TO LEARN
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."
THE BIG PICTURE
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
"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
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
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"
DANGERS OF ANESTHESIA
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