No advice I'm afraid, I just wanted to say how sorry I am that Cookie is having such a bad time.
I am struggling with the same problem with Tyson, my 7 year old Japanese Chin. He is also on frusemide, fortekor and vetmedin.
On Sunday he collapsed after racing my other dogs through the back gate.
It is something I have experienced before with cavaliers with MVD. Careful management of your pet's lifestyle, & trying to prevent too much excitement & too much exercise is probably the best you can do.
Tyson has been my 'middle child'..... Not one of the youngest, not an oldie, and such a good little dog that he was sometimes a little overlooked.
He is getting lots of fuss now.
I do hope that you get better news for Cookie tomorrow.
Cavaliers......Tommy Tuppence 2000-2012 and a half share of Faith.
Japanese Chins.... Dandy, Benny, Bridgette and Hana.