[Pitchwife]

But keep in mind that Beleg was a Sinda and had never been to Aman, whereas Maedhros had just returned from the Undying Lands- I think that makes a difference, not necessarily in terms of pure physique, but in terms of the "inner fire" (for lack of a better word).

Connected with this, I think, is another aspect which has been overlooked so far, namely that the Elves are not made (we might say, not designed) to die; they can be killed by irreparable damage to some vital organ, but barring that their fëa will keep the hröa alive in whatever pitiable state even under deprivation and torture, as long as their will is strong enough (which I think we can assume it was in Maedhros' case).