Objective  To investigate the effect of dexmedetomidine on the median effect concentration (EC₅₀) of ropivacaine during sciatic nerve block combined with femoral nerve block in patients undergoing lower extremity surgery.

Methods  Patients with sciatic nerve block combined with femoral nerve block anesthesia who underwent lower extremity surgery from November 2021 to November 2023 were selected as the study objects. They were randomly divided into control group (0.9% saline), group D₁ (0.50 μg·kg^-1 dexmedetomidine), group D₂ (0.75 μg·kg^-1 dexmedetomidine) and group D₃ (1.00  μg·kg^-1 dexmedetomidine). The stress response, serum pain mediators, vital signs and visual analogue scale (VAS) of patients at different time points during operation were analyzed by repeated measures ANOVA. ropivacaine EC₅₀ was measured by sequential method, and the relationship between dexmedetomidine dose and ropivacaine EC₅₀ was analyzed by Logistic regression.

Results  A total of 208 patients were include and each group was 52 patients. Compared with the same group before surgery, the stress response level of the 4 groups after surgery and 1 h after surgery was significantly decreased, and the serum pain mediators level was significantly increased (P<0.05). Compared with the control group, the stress response and serum pain mediators levels in groups D₁, D₂ and D₃ were more normal after surgery and 1 h after surgery, among them, group D₃ was most close to the normal value (P<0.05). There were no significant differences in blood oxygen saturation and bifrequency index of EEG among the four groups at each time point (P>0.05). At T1 and T2, the heart rate (HR) of the control group was significantly higher than that of the group D₂ and D₃ (P<0.05). At T1, the control group had a significantly higher mean arterial pressure (MAP) than the other three groups, at T2, the control group had a significantly higher MAP than the group D₂ and D₃, and at T3, the control group had a significantly higher MAP than the group D₃ (P<0.05). VAS scores in 4 groups were significantly lower after surgery and 1 h after surgery than before surgery (P<0.05). The VAS score in group D₃ was significantly lower than that in group D₁ and D₂ (P<0.05). Repeated measurement ANOVA showed that the effects of time on stress response and serum pain mediators were different with different anesthesia methods. The influence of time on HR, MAP and VAS scores varied with different anesthesia methods. Sequential assay results showed that the EC₅₀ of ropivacaine in control group, group D₁, group D₂ and group D₃ was 5.985, 5.631, 5.329 and 5.125 μg·mL^-1, respectively. Logistic results showed that the dose of dexmedetomide was a protective factor for ropivacaine EC₅₀ in sciatic nerve block combined with femoral nerve block in limb surgery patients (P<0.05).

Conclusion  The ropivacaine EC₅₀ can be significantly reduced by 1.00 μg·kg^-1 dexmedetomidine. This is a protective factor for sciatic nerve block combined with femoral nerve block in patients undergoing lower limb surgery, and it can be applied clinically.

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