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Whilst there are a number of disease processes that affect the canine heart, mitral valve disease (MVD) is by far the most common.
MVD is an acquired disease, which develops in adult, small breed dogs as they age. Some breeds, in particular the Cavalier King Charles Spaniel, are markedly predisposed to MVD.
The mitral valve is situated in the left side of the heart, between the left atrium and the left ventricle. The normal function of the mitral valve is to ensure that blood flows in the right direction. It acts as a one-way valve, allowing blood to move from the left atrium to the left ventricle and, when the left ventricle contracts, the mitral valve also ensures that blood is pumped forwards to the body by blocking any movement backwards into the left atrium.
When dogs are affected by MVD, for reasons that are not fully understood, the valve tissue degenerates, becoming distorted and unable to fully close. Functionally, this means that when the left ventricle contracts, some blood leaks through the misshapen valve, moving backwards into the left atrium. This leak causes an audible murmur, which can be detected by listening to the heart with a stethoscope as part of a clinical examination.
MVD is a progressive disease and gets worse with time, however the rate at which the disease advances varies a lot between individuals. In a majority of dogs this rate is slow and not all individuals will develop symptoms within their lifespan. In cases where the disease does progress, MVD may bring about secondary changes to the shape, size and function of the heart. Dogs with advancing disease may go on to develop congestive heart failure, showing symptoms such as an increased respiratory rate when resting, breathlessness, coughing and a reduced tolerance for exercise. Dogs that are in congestive heart failure may die as a result of their heart condition.