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Mitral Valve Disease and Heart Failure, Canine
Affected Animals: Dogs. The disease is extremely rare in cats.
Mitral valve disease more commonly affects middle-aged and older
small-to-medium sized breeds. Breeds with a high prevalence of this
disease include the cavalier King Charles spaniel, the Chihuahua,
the miniature poodle, the miniature pinscher, fox terriers, Boston
terriers, and miniature schnauzers. Males are 50 percent more likely
than females to be affected. Cavalier King Charles spaniels may have
symptoms at an earlier age than other breeds.
Overview:
Mitral valve disease is a serious heart condition
caused by the abnormal function of the valve that separates the
upper and lower chamber of the left side of the heart. Dogs with
mitral valve disease typically have difficulty exercising and a
cough that increases in frequency as the disease progresses towards
congestive heart failure and pulmonary edema.
The heart is made up of four chambers: the left and the right atria
and left and right ventricles. Each upper chamber, or atrium, is
separated from a lower chamber, or ventricle, by an atrioventricular
valve. The valve on the left side of the heart is called the mitral
valve, and the valve on the right side is called the tricuspid
valve. In a normal dog, the leaflets of each valve open to allow
blood to move from the atria into the ventricles and then close
completely when the heart pumps the blood out of the ventricles and
into the blood vessels.
Courtesy of: Gale Mueller
A close-up view of a diseased mitral valve, showing thickening of
the valve leaflets.
Right Atrium
Right Atrioventricular Valve (Tricuspid Valve)
Right Ventricle
Left Atrium
Left Atrioventricular Valve (Mitral Valve)
Left Ventricle
Papillary Muscle
Chordae Tendinae
Diseased Mitral Valve Leaflets
Mitral valve disease is caused by the
accumulation of the plaque within the heart valve, although the
cause of this plaque is unknown. The plaque results in the
deformation of the valve leaflets and regurgitation of blood back
into the atrium. This leakage in turn causes a decrease in blood
flow from the heart. A valve that fails to close completely will
allow blood to leak back into the atrium, resulting in a heart
murmur.
In order to compensate for this deficiency, the heart dilates and
the volume of blood pumped increases. This compensation allows a dog
to remain free of symptoms for some time, but simultaneously worsens
the disease, the eventual result of which is congestive heart
failure.
Mitral valve disease is more common in middle-aged and older
small-to-medium breeds. While the prognosis for dogs with mitral
valve disease is poor, some dogs may be managed with medications for
a period that varies from case to case. Dogs may live for years
after manifesting symptoms of mitral valve disease.
Clinical Signs: Dogs with mitral valve disease have a diminished
capacity for exercise and a tendency to cough during exertion. As
the heart disease progresses to pulmonary congestion and edema, the
frequency of coughing and the resting respiration rate increase. The
cough typically is worse at night, in the morning, and during
exertion. Physical exam findings may include an audible heart
murmur, an increased heart rate, and a decreased or weak pulse.
Symptoms:
See clinical signs.
Description:The normal heart is made up of four
chambers: the left and right atria and left and right ventricles.
Each upper chamber, or atrium, is separated from a lower chamber, or
ventricle, by an atrioventricular valve. The valve on the left side
of the heart is called the mitral valve, and the valve on the right
side is called the tricuspid valve. In the normal dog, the leaflets
of each valve open to allow blood to move from the atria into the
ventricles and then close completely when the heart pumps the blood
out of the ventricles and into the blood vessels.
Mitral valve disease is caused by the accumulation of plaque within
the heart valve. The plaque causes the valve to become thickened,
shrunken, distorted, and hence unable to function properly. The
muscles that control the valve, or chordae tendineae, may also
become weak and thickened. This degeneration occurs only
occasionally in the tricuspid valve.
The repeated leakage of blood through the distorted valve leaflets
causes a secondary dilation of the left atrium and ventricle, which
in turn causes the mitral valve opening to dilate as well, worsening
the disease. Blood that leaks back into the atrium usually moves at
a high velocity, and may cause fibrous lesions or scar tissue where
it collides with the muscle wall. Because of these lesions, the
atrial wall may become weak and susceptible to tears. A valve that
fails to close completely will allow blood to leak back into the
atrium, resulting in a heart murmur.
The dilation of the left side of the heart initially causes it to
become strained by the increased volume of blood. This work overload
causes changes in the heart that make it less effective at moving
blood forward into the blood vessels. The body compensates for this
deficiency by providing a larger volume of blood for the heart to
pump, which in turn dilates the heart even further. In most dogs,
these compensatory actions delay the onset of notable symptoms in
the early stages of the disease.
However, the compensatory dilation of the atrium and ventricle
continues to worsen the disease by further decreasing the amount of
blood the heart can effectively move into the blood vessels. When
the compensatory mechanisms are no longer able to provide for
adequate cardiac function, congestive heart failure and pulmonary
edema develop. Pulmonary edema is an accumulation of fluid within
the lung tissues that greatly impairs the animal's ability to
breathe. In the late stages of the illness, the heart muscle's
ability to contract is diminished, further decreasing the amount of
blood that moves out of the heart.
Although mitral valve disease typically advances slowly, there are
complicating factors that may acutely exacerbate symptoms. Such
complications include heart arrhythmias, rupture of the chordae
tendineae, a high sodium diet, and atrial muscle tears. Any cause of
an increased workload for the heart, such as anemia, exercise, high
blood pressure, or other organ failure, also can cause severe
complications. Episodes of acute dysfunction usually require
emergency treatment.
The cause of mitral valve disease is not well understood, but a
genetic predisposition is suspected in certain small breeds of dog.
The disease typically affects dogs that are middle-aged to older.
Although there is no cure for valvular degeneration, there are
medications available that can improve the function of the heart and
relieve some of the symptoms of heart failure in order to allow the
dog a more comfortable life.
Diagnosis: Common diagnostic procedures for mitral valve
disease include a CBC, or complete blood count, a serum chemistry
panel, and urinalysis. X-rays of the chest are used to evaluate
heart size and shape, and the status of lung fields. An
electrocardiogram, or reading of the electrical impulses of the
heart, may suggest heart enlargement and identify any arrhythmias
that may be present. For a more accurate assessment of cardiac
function and the severity of the disease, a dog may be referred to a
veterinary cardiologist for an echocardiogram, or ultrasound
examination of the heart.
Prognosis: The prognosis for a dog with mitral valve disease
is poor. However, some dogs may be managed with medications for a
period that varies from case to case. The long-term prognosis
depends on how early the disease is diagnosed and how an individual
dog responds to medication. Some dogs may live as long as four years
or more after first showing signs of heart disease. The prognosis
for a dog in an acute crisis depends on the severity of its
condition and the speed with which the animal is treated.
Transmission or Cause: As an animal ages, its heart valves become
thickened, distorted, and stiff due to the accumulation of
polysaccharides, or complex carbohydrates. The abnormal functioning
of the valves results in the regurgitation of blood back through the
closed valve, increased pressure within the atria of the heart,
decreased blood pumped out of the heart, activation of compensatory
mechanisms, and congestive heart failure. The resulting volume
overload within the heart leads to dilation of the ventricles and
impaired ventricular function. All of these problems worsen and lead
to even further dysfunction of the heart. It is also possible for
the atria to tear, resulting in acute symptoms such as weakness and
acute collapse.
Treatment:Valvular replacement surgery is not
usually an option for dogs, as it is for humans with mitral valve
disease. Consequently, veterinarians usually prescribe medications
to reduce the symptoms of heart disease and improve the function of
the dog's ailing heart. But even with long-term treatment, most dogs
will require frequent reassessment of their illness and periodic
adjustments to their medication regimen. Acute destabilization can
cause severe symptoms, but often these may be treated successfully
with aggressive techniques.
Common treatment regimens include a lower sodium diet, which may
become increasingly restrictive as the disease progresses. Exercise
is restricted until symptoms of heart failure are controlled, and
then only mild to moderately intense activity is recommended.
There are numerous medications available to treat heart
abnormalities. Typically, dogs with congestive heart failure
secondary to mitral valve disease are treated with multiple drug
types simultaneously in order to improve the forward movement of
blood flow, decrease the backflow of blood into the atrium, and
moderate the compensatory mechanisms. These medications include
diuretics, such as Lasix, which promote urine formation and
excretion and thereby decrease the amount of excess fluid in the
body. The veterinarian may also prescribe drugs that dilate the
blood vessels in order to prevent high blood pressure. These include
enalapril, an angiotensin-converting enzyme inhibitor, as well as
other specific arteriolar and venous dilating drugs. Medications
called positive inotropic agents, such as digoxin, may also be
prescribed to promote the heart muscle's ability to contract.
The examining veterinarian or veterinary cardiologist will determine
the appropriate time to start medical therapy, as well as what drugs
are necessary for the individual patient. Because this heart
condition is progressive, patients will require frequent
reassessment in order to appropriately adjust the treatment as the
disease worsens. A dog with heart failure will require long-term
medication.
Animals that have an acute episode of congestive heart failure and
pulmonary edema will require more aggressive types of therapy in
order to be stabilized. In such cases, a veterinarian will prescribe
cage rest and the avoidance of stress and anxiety, and diagnostic
procedures should be kept at a minimum until the animal is
stabilized. The dog may be given oxygen in order to reduce the load
on its heart and lungs. In some cases, free fluid will accumulate
around the lungs of a dog with congestive heart failure, limiting
the space in which the lungs can expand. This fluid can be removed
by means of a procedure called a thoracocentesis. High doses of
fast-acting drugs may be administered in order to promote the
improved functioning of the heart.
Prevention: There are no preventive measures for mitral valve
disease. Early detection and appropriate treatment of the disease
may improve the prognosis.
