Look up these questions in the Cathoic Replies book by James J. Drummey.
From what I know there is a difference between preserving life and prolonging the death process. Everyone is responsible to feed and care for a person by normal standards. You don’t need to go as far as breathing for the sick. For example: Let’s say I’m in a coma. It’s up to my caregivers to feed me, bathe me, and so on until I die naturally. But let’s say my brainstem has been shot and the only thing keeping me alive is a machine that breathes for me. My caregivers are not obliged to keep the machine going forever, (unless there is a chance of my recovery). Once unplugged, I die, not because of starvation or lack of care, but because of my condition/illness.
This is a very quick overview and hopefully it helped out a little. Again look at the CATHOLIC REPLIES for a better explaination.