Mitral valve disease is a degeneration of the mitral valve, which is one of the valves that control the flow of blood through the heart. The mitral valve is located between the left-hand filling chamber (atrium) and pumping chamber (ventricle), and consists of two leaflets which allow blood to flow from the lungs to the heart.
The valve may eventually lose its flexibility, and when this occurs, it starts to leak. Initially, mild mitral valve insufficiency causes no distress, but a heart murmur may be heard. Later, as the leak becomes more pronounced, the heart enlarges to compensate, and heart failure and other serious complications may occur. The severity of the murmur is graded from 1 (mild) to 6 (severe) and can be measured by auscultation (stethoscopes) and Doppler echocardiograms (ultrasound scans).
In addition to the murmur, other symptoms may be coughing, build up of fluid in the lungs, etc., difficulty in breathing, exercise intolerance, and loss of appetite. Treatment in later stages of MVD may include the use of diuretics to remove excess fluid build-up in  the lungs and body cavities, and vasodilators and beta blockers to help control blood circulation and heart rate.
Premiere Cavaliers MVD information
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Further Information
It is believed that MVD is "polygenetic", which means that more than one gene may be the cause, and that it has an hereditary basis. Thus the more that can be done to achieve late onset of MVD in our Cavaliers, the better chance we will have of eradicating the problem, and both purchasers and breeders can help work towards this goal.
What can we do to help eradicate MVD in our Cavaliers?
However, research has shown that MVD is about 21 times more likely to occur in Cavaliers than in the typical dog, and the onset of the disease typically occurs much earlier in life. Also once diagnosed, MVD will progress much more rapidly in Cavaliers. Statistics indicates that by the age of five, around 50% of Cavaliers will be suffering from MVD, and a very large proportion of them will be suffering by the age of ten. 
Several breeds of dogs, mainly the smaller ones, are predisposed to suffer from MVD; but for most it does not result in heart failure and death,as it is usually a disease of old and tends to progress slowly.
Why is MVD a problem in Cavaliers?
What Is Mitral Valve Disease)?