I've had Cavaliers since the 1980s. Charlie, Harry, Timmy, Sophie and now Puppers.
Puppers was a rescue -- a beautiful tri-color bitch. My others all had MVD (except for Sophie who lived to the ripe age of 17!), but we caught it early and they all lived to around 11 or so.
Puppers seemed "clear." About a month ago she began coughing. We were moving and I thought she might have a bug. Two weeks ago (in the new house) she became distressed, panting heavily. I took her to the vet and sure enough -- very enlarged heart and lungs full of fluid.
Puppers is 8 (we think) and we had no clue of the MVD until we discovered it at a fairly advanced state. She is now on Vasotec (5 mg) and Lasix twice a day (2.5 mg) as well as some antibiotics as her white blood cell count was elevated. Her thyroid is also low.
She has done fairly well the last week, but now she is back to sleeping a lot, barely moving, coughing when she does and now she farts when she coughs!
My vet just got Vetmedin in (it wasn't around when my other Cavaliers had MVD). I hope it helps.
Poor little Puppers, she is such a sweet little girl and my son (11) adores her. Sigh.