in general:
the anesthesiologist gives/chooses induction drugs and overall aneshetic plan, the crna is usually given the chance to intubate or manage the airway on the first attempt provided all is well, then the crna usually tapes the tube, does all the little stuff (turn on bair hugger, gas, ng to suction) until the case is completely in the maintenance phase, the attending might chime in and say, hey lets give this much fluid, this much narcotic, call me or not for wake up and leaves the room to do other things, run other rooms, be available for codes, etc.. the crna stays in the room and charts, gives narcs, monitors to ensure all looks well, call the anesthesiologist prn problems (ie sudden desats or changes during the case)
when there are advanced procedures to be done (ie not the usual IVs, alines, lma/ett placement, Og placement, etc) such as central lines, swanns, ultrasound guided blocks, difficult iv or aline, or difficult airway require an advanced device like a scope, thoracic neuraxial anesthesia, or when serious medical management needs to happen like giving blood products appropriately, electrolyte and volume replacement, pressor/narcotic/hypnotic drips.. the attending MD USUALLY does the procedure/management himself - now this varies institution to institution and a cardiac CRNA at some big center im sure places central lines routinely, and some probably like to do us guided blocks and manage electrolytes but it is all between what the anesthesiologist feels comfortable giving up control over - which is why it is such a heated topic
procedures like epidurals/spinals, ultrasound guided nerve blocks, central line/swann, really difficult airway, unit intubations/problems, vent setting/medical and fluid and product management - are what is being discussed as to in what setting its appropriate to have non MDs doing these procedures
