Issue Description The heart has four chambers. The upper
chambers are called atria (singular: atrium), and the lower chambers
are called ventricles. The heart is also divided into right and left
sides. Blood flows from the body into the right atrium. It is stored
there for a few seconds, then pumped into the right ventricle. The
right ventricle pumps blood into the lungs where it receives oxygen.
It flows from the lungs into the left atrium where it is held a few
seconds before going into the left ventricle. The left ventricle is
surrounded by the largest and strongest of the heart muscles. This
large muscle is necessary to pump blood to all parts of the body. Each
side of the heart has a valve to keep blood from going backward from
the ventricles to the atria. The valve between the left atrium and
left ventricle is called the mitral valve. Because of the very large
pressures created when the left ventricle contracts and the eventual
process of "wearing out," the mitral valve becomes leaky in many dogs.
Because this is a progressive disease, we assign each dog to one of
four stages based on clinical signs, historical findings, and x-ray
findings. Stage 1 is the earliest stage of mitral valvular disease.
Stage 4, the final stage, is the presence of life-threatening heart
failure. Other Names Degenerative Mitral Valve Disease
Causes The exact cause of the degenerative changes in
the mitral valve remains unknown. It is suspected that these changes
may be related to alterations in the connective tissue component of
the valve. The edges of the valve should be smooth and flat; instead,
they become thickened and knobby. When this occurs, the valve is
unable to provide an effective seal between the upper and lower
chambers of the heart. This results in an audible heart murmur when a
stethoscope is placed over the valve.
Breed Prevalence This is the most common cause of heart failure
in small dogs. Large breeds have relatively low incidence. Many small
dogs develop a mitral murmur as early as 6 years of age; most of them
will have a murmur by 10 years of age.
The breeds most
commonly affected include:
The Cavalier King Charles Spaniel has a high incidence of mitral
valve disease, and it tends to occur at a relatively younger age than
in other breeds.
Symptoms The earliest sign of a leaking mitral valve is
a heart murmur. The murmur is created by turbulent blood regurgitating
backward from the left ventricle into the left atrium. The murmur is
audible through a stethoscope and may progress in intensity as the
disease progresses. Another early sign of mitral valve disease is a
chronic dry, hacking cough. This occurs because the enlarging left
atrium puts pressure on the bronchus (a branch of the airway); this
compression leads to a cough.
Heart Failure The presence of a murmur does not mean that
heart failure is imminent. But, as time goes on, the leak becomes more
severe and more and more blood goes backwards. This results in reduced
pumping efficiency and, eventually, congestive heart failure. From the
time a murmur develops, it may be a few months to several years until
heart failure occurs.
When the heart begins to fail, it is
unable to deliver adequate oxygen to all the tissues of the body. This
sets into motion a series of compensatory events. In other words, the
body's cells become desperate and trigger a series of responses.
Various hormones are released by several organs in an attempt to
correct the problem. These hormones conserve fluid in an effort to
increase blood volume and the output of blood and oxygen by the heart.
For several months, these compensatory responses help the situation.
However, the increased fluid retention eventually becomes harmful.
Perhaps the most detrimental event occurs when this excessive fluid
leaks out of the pulmonary capillaries and into the air spaces
(alveoli) of the lung; this is called pulmonary edema. This fluid
collection in the lungs produces very obvious signs and may be one of
the first things an owner might notice. Noticeable signs include
weakness, coughing or gagging, fainting or collapse, and obvious
Diagnosis There are several tests that are used to look
at different aspects of the heart's structure and function:
Listening with a stethoscope (auscultation). This valuable tool
permits identification of murmurs, their location, and their
intensity. It also allows us to hear lung sounds so that we can better
understand what is happening within the lungs.
Blood and urine tests. These do not give direct information about
heart function, but they allow detection of other disorders in the
body that may have significance to heart function.
Chest radiographs (x-rays). The chest x-ray is useful for
examining the lungs and for viewing the size and shape of the heart.
It is also helpful in determining that the left atrium is enlarging
because of backward blood flow.
Electrocardiogram (ECG or EKG). This is an assessment of the
electrical activity of the heart. It allows accurate determination of
heart rate and rhythm. Abnormal rhythms (arrhythmias) can be detected
Ultrasound examination (sonogram, echocardiogram). This
examination uses sound waves that bounce off the structures of the
heart and are read on a TV-like monitor. It gives the most accurate
determination of the size of each heart chamber, and permits
measurement of the thickness of the heart walls. This is seen on the
monitor in actual time so the contractions of the heart can be
evaluated. Certain measurements can be taken which allow the actual
strength of the heart's contraction to be measured as a number and
compared to the normal animal. Ultrasound may not be available in all
private veterinary practices because of the additional training needed
to learn how to perform the examination and because of the cost of the
The combination of all of these tests give the best evaluation of the
dog and its heart function. However, if cost considerations prohibit
performing all of them, two or three will provide much valuable
Treatment A leaky heart valve can be replaced surgically
in people. However, this is rarely feasible in dogs. There are several
drugs that will improve heart function, even in the presence of a
leaky valve. In addition to the drugs described below, a low-salt diet
is usually indicated and can be obtained from your veterinarian.
Diuretics. These drugs stimulate the kidneys to remove excess
fluid from the body. Furosemide is most commonly used, although others
will be selected in certain circumstances.
Nitroglycerin. This drug is called a venodilator; it dilates the
veins throughout the body, especially the ones going to the heart
muscle. It decreases the amount of blood returning to the heart by
allowing some of it to "pool" in the veins. This temporarily reduces
the workload of the heart. This class of drugs can be very useful for
treating pulmonary edema.
Digitalis. This drug improves heart function in several ways. It
helps in control of certain arrhythmias, slows the heart rate, and
strengthens each contraction of the heart. It is only indicated in the
end-stages of mitral valvular disease.
Enzyme blockers. This is a relatively new class of drugs that can
help module the imbalance of hormones related to heart failure.
ACE-inhibitors, such as enalapril, are the most commonly used drugs.
Vasodilators. These drugs dilate the arteries and veins of the
body to permit better blood flow. They may be used long-term because
they continue to be effective, as opposed to the short-term effects of
Not all of these drugs are used in
each dog in heart failure. The results of the various tests will
determine which ones are appropriate.
Prognosis There are many factors that must be considered
before the outcome of treatment can be determined. The results of the
tests are important. Dogs in the early stages of mitral insufficiency
(Stage 1) can live months to years before failure begins. Dogs in
Stage 4 may survive only hours to weeks, but the prognosis is very
individualized for each dog.