In lieu of an authoritative source which directly addresses this question I will offer the following hypothesis:
It seems to me that the status of a "כלי שמלאכתו לפיקוח נפש" (a utensil used for cases of pikuach nefesh) is dependent, in part, to the disagreement whether cases of Pikuach nefesh means Shabbos is הותרה (the action is permitted) or whether Shabbos is merely דחויה (pushed off).
If we say it is הותרה and the act is permitted then it would seem that the item would be a kli shemelachto l'heter since the activity is actually permitted when the item is in use.
Yet if we say that it is דחויה, pushed aside, then the act is still considered prohibited and it would seem that it would be a kli shemelachto l'issur.
There remains a few other considerations and it might help to compare a comparable case, a knife used for Bris Milah. While this case is similar, especially since there also seems to be discussion about whether Bris Milah is הותרה or דחויה, it is not exactly the same since a Bris Milah knife isn't used exclusively or even primarily for Brisim which over-ride the Shabbos (and whether it is הותרה or דחויה seems independent of whether pikuach nefesh is).
A milah knife is muktzeh mechamas chesron kis, due to monetary loss (O.C. 308:1). Since it retains this status even if it is stored in the general knife container (Rema, ibid) we can infer that not taking "chesron kis" into consideration it would be a kli shemelachto l'issur (see Graz 308:4) since the only kli shemelachto l'heter which can become muktzeh mechamas chesron kis is one which is given a special place to protect it (see SS"K 20:22). It would seem that it maintains this status even if Bris Milah is הותרה , but we have noted that unlike pikuach nefesh not every act of of milah is permitted on Shabbos. Even still we see that when a mohel has a bris scheduled for Shabbos then the milah knife is not muktzeh (Rema, Y.D. 266:2) because it wasn't muktzeh (set aside) during twilight since it was prepared for a permitted use.
From this we might infer that "כלי שמלאכתו לפיקוח נפש" equipment of a hospital or EMT where there is a reasonable expectation of its use would not be muktzeh (see also M.B. 318:15). This would seem to particularly true if the equipment was prepared (cleaned, stored, etc) in anticipation of Shabbos. It is possible that this consideration could even apply to an item stored in non-emergency locations for emergency use (such as a defibrillator in a shul). It would seem that in at least some circumstances a "כלי שמלאכתו לפיקוח נפש" would not be be muktzeh (even if we say Shabbos is only set aside because of pikuach nefesh) and there is need for a pesak to clarify when and if the likelihood it actually be needed has diminished enough that it can no longer be considered "muchan/prepared" for use.
