MITRAL VALVE DISEASE
Endocardiosis of the canine mitral
valve is a chronic degenerative disease of the valve between the left atrium
and the left ventricle of the heart. This condition is more commonly referred
to as Mitral Valve Disease, or MVD. Deposition of mucopolysaccharide in the valve and its attached cords
causes the valve to become distorted, allowing blood to leak back into the
atrium during contraction of the ventricle. Most of the time, the mitral valve is the only valve affected by endocardiosis, but in approximately one-third of the
affected dogs, the tricuspid valve (between the right atrium and right
ventricle) will be affected as well.
Prevalence of canine MVD
is typically highest in small breeds, and is most common in Cavalier King
Charles Spaniels. Other breeds with a known predisposition are the
Diagnosis is most
typically made by auscultation. The murmur of MVD is typically heard best at
the left fifth intercostal (between-the-rib) space.
Murmurs are generally rated as grade I through grade VI, (one through six)
depending on the loudness of the murmur. As disease progresses, the murmur
typically becomes louder. In a very young dog with a very soft murmur, further
studies may be necessary to determine the source of the murmur. All murmurs are
not indicative of MVD; innocent flow murmurs sometimes appear in young, healthy
dogs. A Doppler study will document regurgitation into the left atrium, and may
be recommended in a young-breeding animal with a murmur. Doppler studies are
also being used in some areas, since regurgitation of blood will most likely be
seen on Doppler before it is heard as a murmur.
A dog with MVD may remain
symptom-less for years (the average longevity of a cavalier is about 11 years).
However, when the regurgitation of blood back through the valve gets to the
point that it interferes with oxygenation, the dog may begin to cough and have
shortness of breath with exercise. This point is termed "Congestive Heart
Failure", and is usually rated as mild, moderate or severe. When the heart
becomes very inefficient, other organ systems may also be affected. Poor blood
flow to the kidneys may lead to renal disease; liver congestion may also occur.
Long-term prognosis depends on the response to treatment and the stage of heart
failure, but heart failure secondary to MVD is the most common cause of death
in the Cavalier.
One other complication of endocardiosis is endocarditis. Endocarditis is a bacterial colonization of the diseased
valve that may cause fever and sepsis. For this reason, dogs with MVD should
receive antibiotic prophylaxis prior to surgical or dental procedures. There is
some controversy about the best time to begin treatment. Most commonly,
treatment is deferred until there is cardiac enlargement on xray.
The most common treatments include the vasodilator such as enalapril
(brand name Enacard), a diuretic such as furosemide (brand name Lasix),
and digoxin, a drug that may help the heart to beat
slower and stronger. A few surgical mitral valve
replacements have been successful, but it is not considered standard treatment
at this time.
The veterinary and genetic
communities agree that there is a genetic basis for MVD, and therefore the key
to decreasing the incidence of MVD lies in selective breeding. To this end, a
yearly heart exam is recommended, preferably by a cardiologist, and breeders
have been encouraged to use older stud dogs (over age 5) with healthy hearts.
Another recently proposed protocol suggests that all cavaliers used for
breeding be at least 2 and 1/2 years of age and free of murmur by a
cardiologist's auscultation. There are also ongoing studies utilizing selection
by Doppler examination. Hopefully, with these efforts, the incidence of MVD
will be decreased, or at least the prolong the age of onset, to the point that
MVD is no longer the limiting factor in cavalier longevity.