^^This^^This is my observation also. A deer can run along ways on adrenaline without a heart. Only way to ensure a DRT is either a spine or brain shot and your odds are not as good on the smaller target then.
Rosewood
Heart-shot a 110lb doe with a .270Win 140gr silvertip. She made it about 80 yards leaving a blood trail that I could almost follow with me eyes closed. Exit hole seemed nearly big enough to put a Coke can in. No discernible heart could be found when I field dressed her. Adrenaline is amazing!
A couple of weeks earlier I heart-shot a 211lb nine-pointer standing in the same spot. Legs immediately collasped under him and he was DRT. Go figure.