At the hospital I work in, CRNAs are supervised by an Anesthesiologist. When the patient is being put to sleep and being reversed, the MD is paged and stops by the room to make sure everything is going smoothly. CRNAs have the same drug privileges as MDs and can give medication as they see fit throughout a case. They generally do smaller, less complicated cases such as choles, hernias, resections, OB, etc. Not to say that they don't take care of ICU patients, but the complex and potentially "problem cases" are run by an anesthesiologist. Sometimes they help the MD place lines(A-line, SWAN, etc), but don't normally float them themselves. The CRNA helps with pre-op H and Ps and some pages from floors. Generally, the CRNA will work an 8 hour shift, but a few have started working 3 12 hours.