what is a job in a big hospital look like

Tomorrow we need to attend classes concerning resuscitation. ACLS it was called.
What I want to know is this.... can we really save a person's live.Of course i have helped in some CPR , some people make it, at least for few days, some of the old people give up. some keep on, some became totally bed-ridden and dependent.make a lot of stresses on family.
what is better?
let a person go natural course?
or do everything we can to make him/her lives longer?
no one can predict the outcome, no one can know the consequences of our heroic measures. so what do we do?
when our loves one are involved.