Exercise Induced Collapse is a medical syndrome that is being studied in Labs. Apparently the Lab is not the only breed that can have it-is also occurs in other active dogs like the Malinois. Here is the link to read more about it. It is genetic and can be fatal.  To read more go to the following link  
http://www.cvm.umn.edu/vdl/ourservices/canineneuromuscular/taylor2008/home.html