Compare the Effects of Remifentanil and Fentanyl on the Duration of Mechanical Ventilation of ICU Patients

Inclusion Criteria: * (1) Oral endotracheal intubation requiring mechanical ventilation; and (2) 18-85 years old; and (3) expected to require mechanical ventilation for longer than 24 hours. Exclusion Criteria: * (1) Deep sedation is required (PaO2/FiO2 ≤100, loss of consciousness during treatment with muscle relaxants, status epilepticus, surgery or surgical conditions require absolute immobilization, severe brain injury with intracranial hypertension, therapeutic hypothermia, etc. And other clinical assessments need to maintain RASS\<-2); (2) The time of using sedatives or opioid analgesics is longer than 1 week before enrollment; (3) ICU stay ≥ 1 week before enrollment; (4) Invasive mechanical ventilation time ≥ 48 hours before enrollment; (5) According to the condition of the disease, it is estimated that mechanical ventilation will be needed for too long, such as the accumulation of respiratory muscles in peripheral neuromuscular diseases, brainstem damage and respiratory center involvement; (6) Delirium, alcohol withdrawal symptoms or mental illness or use of antipsychotic drugs; (7) Severe abnormal liver function (Child-Pugh grade C); (8) Renal insufficiency requires renal replacement therapy; (9) Surgical treatment is required during mechanical ventilation (except for minor operations, such as lumbar puncture, ventricular drainage, etc.); (10) Allergy to study drugs or other contraindications; (11) Pregnant or lactating women; (12) Has been selected for other RCT tests; (13) The patient or the legally authorized person is unwilling to participate in the trial; (14) The investigator judged that the patient was not suitable for selection (such as severe hypotension; potential disputes, etc.)