I don't think there is a need to elaborate on the responsibilities of a staff nurse. Anyone reviewing your resume will know what they are. I suggest something simple and general ... plus a brief description of the unit ... plus any "added" responsibilities you have assumed. If you served on any committees or worked on any special projects, I would list that in a separate section. Online applications might not have a section for such things, in which case I would add that information to your job description section.
Provided comprehensive nursing care to patient in a 16-bed CCU/ICU. Served as Charge Nurse and Preceptor on a regular basis for the past 3 years.