On a heart-shot deer, sometimes the heart has a hole in it, and sometimes it is burst apart. I think the difference is whether the bullet passed through the chamber when it was fully inflated with blood (causing it to burst) or not. In both cases the deer was DRT.
I read a letter-to-the-editor of a hunting mag once, written by a cardiologist deer hunter. The good doctor theorized that the occasional instant-collapse of deer/elk is due to the bullet passing through the heart while the aeortic valve is open. This allows a hydraulic shock wave to travel to the brain and stun the animal. The shock wave is not fatal by itself, but by the time the animal recovers from shock, it has bled out. This theory always made sense to me.