IMHO, he's actually a cyborg, hence the organic looking eyes and the heart/lungs that Obi Wan ends up shooting him in.
At the end of the second series of Clone Wars (the animated series on Cartoon Network), you get to see when Palpatine is kidnaped by Grievous. None of the 'lesser' Jedi seem to be able to do anything to stop him and just before he escapes with the Chancellor, Mace Windu uses some sort of Force crush on Grievous' chest - hence the breathing difficulty and coughing.
At the end of the second series of Clone Wars (the animated series on Cartoon Network), you get to see when Palpatine is kidnaped by Grievous. None of the 'lesser' Jedi seem to be able to do anything to stop him and just before he escapes with the Chancellor, Mace Windu uses some sort of Force crush on Grievous' chest - hence the breathing difficulty and coughing.