Abstract： Objective To compare the effects of dexmedetomidine on the respiration and circulation with midazolam in mechanically ventilated patients.Methods A total of 62 patients supported with mechanical ventilation in medical intensive care unit were randomly (random number) divided into two groups:midazolam group ( group A,n =38 ) and dexmedetomidine group ( B group,n =24).The patients in group A were intravenously administered with midazolam in bolus dose of 0.05 mg/kg given over 30 - 60 s,and then maintained with 0.05 -0.15 mg/(kg ·h) by venous pump.The patients in group B were intravenously administered dexmedetomidine in bolus dose of 1.0 μg/kg given in more than 10 min,and then maintained with 0.2 - 0.7 μg/( kg · h) by venous pump.When the patients' sedation degree were required to reach Ⅲ - Ⅳ Ramsay Grade,the heart rate,mean arterial pressure,respiratory rate and SpO2 before sedation were compared with those 10 min,30 min and 2 hours after sedation between two groups.Results Respiratory rate or heart rate after sedation significantly decreased compared with those of presedation ( P ＜ 0.05 ).Blood pressure after sedation did not decrease significantly compared with that of presedation in Dexmedetomidine group.In midazolam group,blood pressure was (83.11 ± 12.95 ) mm Hg before sedation,and there was a significant decrease to (74.13 + 12.50) mm Hg in 10 min after sedation (P ＜ 0.05 ),while there were no significant difference between ( 80.53 + 12.93 ) mm Hg 30 min after sedaton and (82.47 ± 12. 15 ) mm Hg 2 hours after sedation.Conclusions There is no difference in effects on respiration of mechanically ventilated patients between dexmedetomidine and midazolam,and the effect of dexmedetomidine on circulation was less than that of midazolam.