Taken from a website in reference to human spinal injury, but basically the same;
"If the spinal cord is bruised or partially damaged due to swelling or laceration, then quadriplegia or other neurological problems may occur. If the spinal cord is severely injured and this injury is at or above the fifth cervical vertebra (C5), then breathing may be effected and the person may die from asphyxiation. In order to help someone breath, a hole is cut into the windpipe (trachea), and a tube inserted which is connected to a ventilator to supply the lungs with oxygen.
The portion of the cord that controls breathing via the diaphram is about C3 through C5. If the damage is below C6, then paralysis may occur, but breathing would be left intact although at a reduced capacity."
I stand by my belief that a a blow to the back of the scull to render it unconsious followed by severing of the spinal cord high up the spine is the most humane way of dispatch. If it were me, I'd rather not know about it, rather than being aware that I was about to die due to asphyxiation.
I don't doubt that people have sucessful results, but I remain of the opining that being unaware and dying is preferable to being aware of dying.