There was a neighbor child several years ago who died (fell unconscious and could not be revived) after a kicked soccer ball hit him in the chest. The autopsy showed no sign of a heart defect, but clearly, the blow to the chest had caused his sudden death.
It appears now that if the heart receives a hard enough blow at a certain point in its cycle, it will just stop entirely. It has to happen at just the right time with just the right force, but it can kill a person.
My prayers and thoughts go out to the families involved.
There was also a softball player that got hit in the chest and didn't survive.