Active_comparator: Propofol group

Patients will receive induction of sedation as follow:~Continuous propofol infusion will be given with syringe pump for maintenance, with the initial rate set at 25-75 mic/kg/min IV during the first 10-15 min. To avoid administering sedatives at rates greater than clinically necessary, infusion rates will be gradually titrated to 25-50 mic/kg/min and regulated with the clinical response, with an onset of peak drug action expected to occur within 2 min.

Experimental: Propofol combined with clonidine group

Patients will receive 2 μg/kg of clonidine for injection intravenously 30 min before induction of sedation, administered over 10 min.