Not sure which book I read it in, probably a Plummer, I recall reading about risking blowing your dogs if you give them too much stick early on.
Now, is there actually such thing as over exertion which permanently damages the lungs?
I've read about sighthounds sometimes suffering from a condition called rhabdomyolysis but that usually occurs in regions of high temperature and high humidity.
I've heard that a blown dog may be called so because of damage to the diaphragm but has anybody actually seen this.