Patients in the intensive care unit (ICU) often develop anxiety and agitation, sleep disturbances, and delirium. Delirium occurrence is associated with worse early and long-term outcomes. Dexmedetomidine and ketamine are recommended for sedation and analgesia in postoperative ICU patients, but each may induce side effects. The sedative effects of dexmedetomidine can help mitigate the neuropsychiatric side effects of esketamine. Recent studies showed that dexmedetomidine-esketamine combination improved analgesia and sleep quality without increasing side effects. This trial is designed to test the hypothesis that dexmedetomidine-esketamine combination for sedation and analgesia in postoperative ICU patients may reduce delirium.