I feel very badly for you about losing your dog. It is crushing when more than anything you just want an explanation.
The tears will still come, anger and all the things we know comes with grief. Hopefully as a well educated and well healed health professional I can offer you a few terms used with people to use when looking for information about what was likely occurring with your dog. It is tough to find animal specific disease when we have no idea where to start. I would suggest terms like CHF (Congestive Heart Failure), Chronic Pulmonary Hypertension and COPD (Chronic Obstructive Pulmonary Disease).
Keep in mind when you read about these things your Missy would have over-time known she had to take it easy sometimes if maybe too many goodies (take that back never too many goodies!!!! ) Once you and your family have taken sometime to share maybe make a gift to you local animal shelter or plan to plant a tree this spring look into some of my suggestions, we all know what it's like to be human, dog's bodies operate in this instance almost the same.
Again, I offer you my deepest sympathy and please share my ideas with your daughter to help her out if she needs it or you feel it will benefit her.
Product or Service Mentioned: Banfield Pet Hospital Pet Medical Service.
Reason of review: Poor customer service.