To observe the effect of different doses of dexmedetomidine (Dex) on the minimum alveolar concentration of sevoflurane to blockade adrenergic response to surgical incision in 50% of children (MAC_BAR).

Eighty-one children, 2-6 years old, with ASAⅠ or Ⅱ undergoing lower abdominal surgery, were randomly allocated into three groups, receiving saline only (27 cases) or a bolus dose of 0.5 μg/kg and maintenance dose of 0.5 μg·kg^-1·h^-1 Dex (27 cases) or a bolus dose of 1 μg/kg and maintenance dose of 1 μg·kg^-1·h^-1 Dex (27 cases) before induction. The initial concentration of sevoflurane was 4% for each group after intubation. The "modified up-and-down" method (Sympathetic responses to surgical incision was considered positive if the MAP or HR increased≥15%) was used, and the MAC_BAR of sevoflurane and its 95% confidence interval were calculated.

The differences in the three groups were significant (P<0.01). The MAC_BAR of sevoflurane was respectively (3.95±0.1)% (95%CI=3.88%-4.02%) in group D₀, (3.05±0.1)% (95% CI=2.98%-3.12%) in group D_0.5, and (1.90±0.0)% (95% CI=1.90%-1.90%) in group D₁. The MAC_BAR reduction of sevoflurane was 23% in group D_0.5 and 52% in group D₁ (P<0.01).

A bolus dose of 0.5 μg/kg and maintenance dose of 0.5 μg·kg^-1·h^-1 dexmedetomidine results in a 23% decrease in the MAC_BAR of sevoflurane. Increasing the dose of dexmedetomidine to 1 μg/kg and 1 μg·kg^-1·h^-1 produces a further 29% decrease in the MAC_BAR values of sevoflurane.