It is very, very important that you only purchase a puppy where the breeder tests their breeding dogs on a yearly basis by a CARDIOLOGIST not a regular vet.  It has been proven that early onset MVD can be bred away from by only using heart clear parents.  It is also helpful to know about the heart history of the grandparents.  My foundation girl is now ten years old and clear, my other foundation girl is eight  and clear, their mother had an excellent heart health history as well.  I only use males with clear hearts who's parents had clear hearts well into middle age.  The is no guarantee that you will not get MVD with a puppy whose parents are clear, it is present in ALL Cavalier gene pools and anyone that tells you differently is not being truthful or just ignoring the problem.  However that being said if all the appropriate heart checks are being  done, you stand a much better chance of never having a problem with MVD  All of my dogs show no signs of SM, however if we do produce a puppy that does develop SM we do offer a life time gurantee and the puppy will be replaced