 The investigators plan to utilize conivaptan Vaprisol to promote isolated water loss, in combination with normal physiologic fluid replacement to maintain a normal blood volume status, in patients with severe TBI. The goal of this therapy is to raise blood sodium in a controlled fashion in subjects with severe TBI, and reduce the use of hypertonic saline infusion. We hypothesize that this therapy will maintain a stable state of high blood sodium, while decreasing the overall sodium load needed to achieve these goals. 