Delirium is common in septic patients, especially those receiving mechanical ventilation in the intensive care unit (ICU). Dexmedetomidine is a highly selective α2 adrenoreceptor agonist with anxiolytic, sedative, analgesic, and anti-inflammatory effects. Use of dexmedetomidine in mechanically ventilated ICU patients is associated with less delirium and improved outcomes. However, dexmedetomidine infusion produces dose-dependent bradycardia and hypotension; these limited the use of dexmedetomidine in ICU patients. This study is designed to test the hypothesis that low-dose dexmedetomidine infusion can also reduce delirium in mechanically ventilated ICU patients with sepsis.