Effects of dexmedetomidine on preventing sleep disturbance and delirium during high flow oxygen therapy in patients with acute respiratory failure

INTERVENTION: Dexmedetomidine 0.2‐0.7mcg/kg/h for nocturnal sedation No sedation or sedation without dexmedetomidine during night time. CONDITION: Adult patients with acute respiratory failure treated with high flow oxygen therapy PRIMARY OUTCOME: Sleep quality and quantity (total sleep time, the rate of each sleep stage: non‐REM sleep stage 1‐3, REM sleep) and the incidence rate of delirium. SECONDARY OUTCOME: successful rate of liberation from high flow oxygen therapy, the length of ICU stay, and mortality. INCLUSION CRITERIA: Adult patients with acute respiratory failure equipped with high flow oxygen therapy in the intensive care units.