I read an article by a veterinarian about this topic and his thought was the aorta was the key. If you hit the heart, the circulatory system has valves to keep blood pressure up and the animal lives for a while and can run off.

This guy said, if you hit just above the heart and unhook the aorta and major plumbing, the animals blood pressure drops instantly and they " faint" . They then expire from massive blood loss while unconscious.

It make and interesting theory also.

KC