There are 25 collectible med packs throughout the city. This equates to roughly five in each of the four districts along with a few on the central highway (and one in the ravine area). They will show up as smaller yellow circle inside a larger one. Also this must be done in one playthrough, or else you need to start over.
EDIT: TSG Dwaggie mentioned that this achievement NO LONGER requires to get all of them in one playthrough. It is still recommended however, that way you can tell which ones you need that are left.