Excellent post Nicki and very helpful.
It is so important for people to get dogs to a vet if they are struggling in any way, finding it hard to breathe, etc.
If someone is wondering 'should I contact/get to the vet' --please put it in the context of what any of us would do if you saw your child, partner or parent in such a condition, or if it was yourself -- a dog is in just as much and often more distress as a person would be, as they cannot ask to be taken in for help before it gets really terrible for them. Imagine hardly being able to breather or move for dscomfort -- any of us would get ourselves to the doctor.
Generally a vet can greatly relieve such distress. And any such time one is seeing a dog in any kind of struggle or distress is probably the point, as Nicki advises, to have that serious talk about whether it is kinder to let them go -- as she says it is far kinder to let them go a little earlier than having a painful and distressing collapse. For anyone where cost is an issue in continuing care for a dog really at its final stages, then I would definitely advise that conversation be had as it just is not fair to the dog to be kept hanging on in an uncertain state where it might not be possible to give the continued relief it needs through meds/ongoing vet care, and where its passing as a consequencne might be in real distress.
And also there is always the quality of life issue. If a dog has little to no real daily joy left in its life and is really just weakly hanging on, we have the responsibility to be as courageous as that dog and give relief. It is better for any loved companion to have a kind passing with the help of a vet, in the presence of the person who has loved them and vice versa, than for that cavalier to pass away in pain and struggle and panic.