Yes aim low at heart . Because when they react they drop down for more power to get out of there...But it's best to wait til they turn head forward obviously.
If your sure that's the spot you hit then it's probably a dead Deer. But, just above that is a void area.
Also, the spine like you mentioned is above not below the shot.

I thought that, but if you look diagram of deer's anatomy the spine is far below arch of back by shoulders, further back spine is up there top edge.

Either way no spine, no lungs. Either above or below the spine.