OK, so here is where the pet-lovers vary all over the board. I am a confirmed and strong dog-person, always will be. But for a million reasons (of which very few are financial), I have never engaged in heroic or extraordinary medical care for my dogs. All the routine care, immunizations, visits for acute problems, of course.
But if it came to major trauma and life support, kidney, liver or heart failure, cancer, or any other major health event, I would do whatever was necessary for their comfort but not anything beyond that. Part of it is that I hate to see them suffer, and that kind of care can cause suffering. Also, I feel they are put in this world for their alloted 10-15 years and you quickly reach the point of diminishing returns with lots of time, worry, and cost for the remaining few years.
Mostly, though, they are not people. They may have a lot to teach us humans, but in the end, I always felt I owed to them peace and comfort, rather than staggeringly expensive and short-lived heroic measures with small returns and major sacrifices. I sometimes wonder whether the "dog approach" might be better in some ways for many of my patients, but that's another story.