This is a prospective, randomized, controlled, multiple cross-over trial evaluating the efficacy of ketamine in the suppression of CSDs. This multiple crossover design was chosen in order to be able to develop preliminary data which could guide implementation of future multicenter trials. Because of the significant variability between patients, a study randomized by patients would be subject to a large amount of potential bias. Because factors such as time of day or hospital day also are known to affect CSD, a brief crossover period of 6 hours was chosen. The study will be registered with clinicaltrials.gov prior to enrollment of patients. Patients with severe traumatic brain injury or subarachnoid hemorrhage who fit the inclusion/exclusion criteria will be approached by either research coordinators or study investigators who will consent the LAR for the study prior to clinically indicated craniotomy. It is not expected that patients will be able to independently consent given the severity of the condition, however, if the patient is conscious, attempts will be made to discuss the study with him or her as well.