Delirium is a common syndrome in intensive care unit (ICU) patients. Those experiencing delirium may suddenly feel confused, have trouble thinking clearly, struggle to pay attention, or see and hear things that are not real. Delirium is associated with worse long-term outcomes such as cognitive impairment, depression, and PTSD (post-traumatic stress disorder). This study examines whether an investigational medical-grade ketone supplement drink (ketone monoester \[brand name: Ultrapure Ketone Monoester\]) is safe and feasible to use in ICU patients, and to look for signals that it might reduce delirium or shorten its duration compared to a volume-, taste-, and calorie-matched placebo.