Rack-Attack, you looked at the heart but you did not say whether or not the sack that surrounds the heart was intact when to preformed your field autopsy?
Only guessing but I' d venture that you lacerated the sack that surronds the heart or some veins or arteries(http://www.kerrlake.com/deer/white2.htm......scroll down to circulation) that are below and forward of the heart.
A Deer can travel some distance as long as it is getting some oxygen to the muscle. Lost of blood depletes the carring vehicle but as long as the lungs are still functioning it can travel a good didtance. It all depend on the rate of blood loss. Once the blood isn' t there to carry the oxygen it crashes.
Just a thought.