Hi -- I'm sorry this has happened, but the vet is right to suggest introducing some of the meds back in. With CHF, the dogs do tend to go through many ups and downs like this. It is sadly, part of the difficulty of this changeable, progressive condition (and why we've made campaigning for better health in the breed, and more responsible breeding using appropriate tests and protocols, a major part of Cavalier Talk). Too many cavaliers get heart problems, and most of them get them far too young, compared to other breeds.
Most of the time the gradual addition, as needed, of further meds adds weeks to months -- even sometimes years -- of life for a cavalier, just as they do for people. Most of these meds will be used shorter term (as these are late-stage additions, generally, not given for years and years) and there's not a high risk of any debilitating side effects. The treatment approach for CHF is very well established and understood.
It's important to do just as you have done -- at any sign of new or increased distress or difficulty, contact your vet or (ideally) vet cardiologist, as meds likely need adjustment or even just a little boost for a few days or weeks.
I don't think people always know when it is time -- this can be very hard to judge and I think sometimes people wait too long, looking for some major sign of pain, which can leave a dog to struggle at a very poor quality of life for very long. I've posted an article elsewhere on the site about trying to understand when it is time -- but I've debated this in my head, for every dog or cat I've had to let go.
Sometimes it is clear an animal is just listless, sad and so poorly that one can sense that keeping him or her going would be more for one's own comfort rather than for the loved pet.
I think we all need to be watchful and honest with ourselves to try and prevent a dog dying in distress at home from heart disease when it could have been released gently with the help of a vet. The more experienced one becomes, the easier it is to read signs that it's 'time', but the support of a vet whose judgement you trust is hugely beneficial. I always ask mine for their honest view.
I do think when the time comes when we are talking about days or weeks left, it is better to be a little early and give that gift of a gentle death, in the owner's trusted arms, to a companion, rather than leave him or her to suffer in distress because the 'time' was left too late.
If a vet is advising a dog be let go when you remain unsure you can always get a second opinion, as well. But I think some of the closest ties to our vets are formed by going through these decisions, in an informed way, together.
I would not give up hope yet, though. From your description this may just be a temporary setback needing a temporary boost in meds, or might be a point where she just needs more support from more meds but still has quite some time left. Talking to your vet should help you understand the process better. :flwr:
I'd also ask what timeframe they feel is left, as this can seem hard to know, but I think is useful to know for ourselves, and can help us to gauge what to be looking for, and when.
Keep in mind all dogs manage differently on heart meds and progress in their own way so what stands for one may not be the case for another. Personally, I found adding prilactone made a huge difference when my Lucy began collapsing from heart episodes -- stopped them entirely for several months during which she had quite a good quality of life, before she passed on.