In Neon Genesis Evangelion, Keel Lorenz is always seen "seated", and we never see any part of him below the torso.
http://projectenthalpy.com/lorenz.jpg
Looking more closely at Keel's remains after he liquifies, it's clear that, by 2015, he did not look human below the waist.
Whatever the reason for his cyborgization, it was so extensive that it appears he was immobile. No legs are visible, and his spinal cord keeps going long past the point at which a human spine should end. For all we know, his spine and the other tubes keep going for yards past the edge of the screen.
All of those countless gray and blue tubes must be plugged into something, meaning he is connected to another machine of some sort.
A machine which may or may not have looked something like this.
This sort of surgery seems insanely complex even for the Eva universe. It would involve removing all of his body below the torso, all of his internal organs below the head, including his lungs and his heart, and all of his bones except those in his head and arms.
Keel's skin was still organic, but absolutely everything in his torso below the skin and maybe some muscle was scooped out. What an insane procedure that would be, scooping out all the bones and organs from inside the torso. So insane, in fact, that I don't believe it ever happened.
Removing the heart and lungs would result in a quick death. Keel would be dead by the time they got all his insides out of his torso just to make room for the mechanical parts. Remember, this wouldn't be a simple orthotopic procedure: everything from his ribcage on in would be removed. Surviving that would be impossible.
I don't think an operation of that sort ever happened. An entirely different procedure occurred.
Not quite like this one, but closer to this than to traditional surgery.
Here's what probably happened: Sometime after Second Impact, Keel Lorenz was either dying or he actually died. Whatever caused this, most of his body was destroyed. He was too important to Seele to lose, however, so his body was "regrown". His head may have been his original head, and his arms may be his original arms as well, but the rest of him was regenerated around his mechanical parts. Or maybe what we see of him was entirely regenerated and his soul was harvested from his old body, which is now rotting somewhere.
Regardless of which way it happened, Keel spent some time dead. Even if the preposterous scooping-out-the-insides-and-hooking-the-mechanical-parts-up-piece-by-piece process did occur, Keel must have been dead at some point during the operation as he would have lacked a heart and lungs for a good length of time before his new mechanical insides were attached in a manner that could sustain him.
In case anyone cares or it becomes relevant, Grigorievich Rodonaia apparently holds the record for being dead the longest time before returning to life. He was dead for three days. I can't find any source on this information or proof that whoever declared him dead was just incompetent, however, so I'm going to assume he was never quite dead.
