nice sketch. I like the design of the caterpillar alien. The scene is a classic and fun idea that you can really play with.
It looks like you're going for a distorted fish eye lens feel to emphasize the creepiness and whatnot. I think it could be even more distorted, have the caterpillar alien looming even more over the guy.
You have a few foreshortening problems on the guy being operated on, esp. in his legs.
And I think it might be cool to have even more pieces of machinery and technology crowding around him. Really nasty looking stuff to heighten the feeling of doom.