Dexmedetomidine may be useful as a sedative agent. However, it has been reported that dexmedetomidine decreases systemic blood pressure, heart rate, and cardiac output in a dose-dependent manner. The purpose of this study was to determine the appropriate dose of intravenously administered dexmedetomidine for sedation.

Forty-five American Society of Anesthesiologists physical status I-II patients under spinal anesthesia received dexmedetomidine 1 µg/kg intravenously as a loading dose. The patients were randomly allocated to one of three groups for maintenance dose: Group A (0.25 µg/kg/hr), Group B (0.50 µg/kg/hr), and Group C (0.75 µg/kg/hr). The hemodynamic variables and the Ramsay Sedation Scale (RSS) score were recorded for all patients. The numbers of patients who developed hypotension, bradycardia, or inadequate sedation necessitating further drug treatment were also recorded.

Systolic blood pressure, heart rate, and SpO₂ were decreased, and RSS score was increased significantly at both 20 min and 40 min after injection of dexmedetomidine in the three study groups compared to baseline, without significant differences between the groups. The prevalence of hypotension, but not that of bradycardia or adjunctive midazolam administration, exhibited a positive correlation with the dose of dexmedetomidine.

Intravenous injection of dexmedetomidine 1 µg/kg followed by continuous administration at infusion rates of 0.25, 0.50, or 0.75 µg/kg/hr produced adequate levels of sedation. However, there was a tendency for the incidence of hypotension to increase as the dose increased. To minimize the risk of hemodynamic instability, a dose of 0.25 µg/kg/hr may be the most appropriate for continuous administration of dexmedetomidine.