I believe that all states now have what are called "Good Samaritan" laws, that provide some protection for professionals who respond to an emergency out in the field. However, what I have been told (and, no, I'm not going to research it myself in order to post the definitive answer here) is that you are covered for actions that a reasonable professional with your same training and skills would provide but not covered for something that would not be considered a typical intervention by your peers (so one can't just do any old crazy thing ...) So, BLS, first aid, that sort of thing would be covered.
However, it is also true that, unless you are employed in a first-responder job (fire department, EMS, etc.) you are not under any obligation to stop and render assistance to anyone in an emergency or accident situation. If, say, you're a nurse and you're grocery shopping and someone falls over dead in the next aisle, it is entirely your choice whether you offer assistance. You cannot get in trouble for not doing CPR, etc. on someone in public, just because you are a nurse.