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Background: Oral clonidine, an alpha2-adrenergic receptor agonist, reduces the dose of propofol required for laryngeal mask airway (LMA) insertion. Target-controlled infusion (TCI) is becoming increasingly popular for propofol infusion. There is no information, however, on the propofol blood concentrations required for LMA insertion and the effect of oral clonidine premedication on these values.Methods: Propofol at target effect-site concentrations from 4.0 to 12.0 microg/ml were randomly administered using TCI in three groups of healthy male patients (n=35 each) who were undergoing elective orthopedic surgery: control, 2.5 microg/kg clonidine, and 5.0 microg/kg clonidine groups. Nothing was administered to the control group. Clonidine(2.5 microg/kg or 5.0 microg/kg) was administered orally 90 min before arrival at the operating room in the clonidine groups. After equilibration between the blood- and effect-site for 15 min, insertion of the LMA was attempted. The EC50 for LMA insertion (measured propofol serum concentration in equilibrium with the effect-site at which 50% of patients do not respond to the insertion of the LMA) was determined by logistical regression.Results: EC50+/-standard error values in the control, 2.5 microg/kg clonidine, and 5.0 microg/kg clonidine groups were 8.72+/-0.55, 7.76+/-0.60, and 5.84+/-0.58 microg/ml, respectively. The EC50 in the 5.0 microg/kg clonidine group was significantly lower than that in the control group (P < 0.01).Conclusions: The propofol concentration required for LMA insertion in healthy male patients is reduced by premedication with 5.0 microg/kg oral clonidine. [ABSTRACT FROM AUTHOR]