Actually both solutions are right since both are a hypotonic solution. But your pt is also hypovolemic so in order to help replenish fluids, and keep Na and Cl down then the MD is right using 1/2 NS solution. Sorry that nurse treated you like that, she is of no help at all! She probably said that you should already know because she did not know how to answer your question herself. Good luck in ICU it can be a lot of fun.