Are you billing for the physician or the facility? You can only assign codes for physician services from the documentation of the provider who performed the service. The RNs in an ED are facility employees and their notes wouldn’t count toward the physician’s service.
If billing for the hospital then you need to follow your facility’s internal guidelines. ED code assignment in a facility doesn’t follow the usual CPT rules. In my own opinion though, I wouldn’t rely on RN notes because RNs usually aren’t trained on critical care coding and don’t know what time can or cannot be included in that service.