Rio, I think a dog can be "blown", from many causes. One of the finest looking dogs I've seen, collapsed a couple of years ago after taking a massive amount of rabbits. A very well known guy off the sites was there and tryed everything;mouth to mouth, chest massage no avail. The emergency vet saved it's life but it was blind and paraplegic, so was obviously PTS. The diagnosis was an anueism on the brain; basicaly a stroke. Some dogs will run 'till they kill themselves, some owners will run their dogs'till they kill them ! I'ts all about knowing when enoughs enough.
Cheers. BANG ON..