Brain injury causing coma after cardiac arrest can be fatal or may prevent patients from recovering. This study focuses on comatose, adult survivors of out-of-hospital cardiac arrest that have already been rapidly cooled per standard of care. Therapeutic hypothermia (cooling the body and brain a few degrees below normal temperature) is a standard treatment in which comatose patients are rapidly cooled to improve their chances of recovery. The purpose of the study is to determine whether different lengths of time for body cooling affect recovery; researchers want to learn more about a way to possibly lessen brain injury after cardiac arrest. Standard practice is to cool patients for about a day, but the best length of time is not known. After treatment, researchers will contact participants or their caregivers about once per month to see how the participant is doing.