No more common then in cocker spaniels, nearly all the cocker puppys that get returned to breeders are down heart murmers from what ive notticed. is it true to the spaniel types or mostly the king charles?? They seem very prone to getting heart murmers is this down to problems associated with age? Or is it more to do with poor breeding?? Almost all the King charles spaniels I know have had a heart murmer at some point in their life it seems fairly simple to treat depending on the level/grade??