If you go to
www.aana.com you will be able to look at CRNA schools and see what they require. Each school has their own requirements, but majority of the schools do not accept ER experience as critical care experience. The schools that do will specify and they expect you to have exposure to sick patients who are on vents, gtts, and that require hemodynamic monitoring. You may be able to prove some of that through ER experience, but I think having ICU experience would really help.