Effect Of Local Anesthetic Adjuvant Dexmetomidine On Blood Concentration Of Ropivacaine During Sciatic Nerve Block In Rabbits

Objective:To investigate the changes of plasma concentration of ropivacaine in sciatic nerve block when dexmedetomidine was administered as an adjuvant of ropivacaine.Methods:12 New Zealand rabbits（2–3 kg）,were divided into 2 groups randomly:ropivacaine group（group R,n=6）and ropivacaine combined with dexmedetomidine group（group RD,n=6）.After general anesthesia,the rabbits were indwelled catheter in right femoral vein and beside the left sciatic nerve.The rabbits in group R were injected with 0.375%ropivacaine 3 ml for sciatic nerve block while the rabbits were perineurally administered 0.375%ropivacaine 3 ml plus 1.5μg/kg dexmedetomidine in group RD.Blood samples were collected from femoral vein before sciatic nerve block（T0）,15min（T1）,30min（T2）,45min（T3）,60min（T4）,120min（T5）and 180min（T6）after sciatic nerve block.The plasma concentration of ropivacaine was determined by high performance liquid chromatography（HPLC）.The standard and internal standard for the determination of ropivacaine were ropivacaine mesylate and tramadol.The plasma concentration of ropivacaine for standard curve was 0.025、0.05、0.1、0.5、1.0、1.5、2.0、2.5、3.0、4.0mg·L^-1.The XPeonyx C18 column was used as stationary phase with UV detection wavelength of220nm,and the column temperature was maintained at 35℃.The mobile phase A was acetonitrile and mobile phase B was 0.02mol·L^-1NaH₂PO₄containing0.015mol·L^-1triethylamine（PH3.7）,with flow rate of 1.0ml/min.The injection volume was 20μl,and gradient elution condition in 0～10min was 21%A,10～16min was 21%～17%A,16～20min was 17%A,20～24min was 17%～21%A and 24～28min was 21%A.Results:The plasma concentration of ropivacaine in group RD was significantly lower than that in group R at T1,T2 and T3（P<0.05）,and there was no significant difference between group RD and group R at T4,T5 and T6（P>0.05）.The peak concentration of ropivacaine in group RD was significantly lower than that in group R（P<0.05）,but there was no significant difference in the peak time and the area under the concentration-time curve between the two groups（P>0.05）.Conclusion:Dexmedetomidine as an adjuvant of ropivacaine in rabbit sciatic nerve block can delay the absorption of ropivacaine and reduce the peak plasma concentration of ropivacaine.Dexmedetomidine slowing down the local absorption of ropivacaine may be one of the mechanisms for prolonging the blocking duration of ropivacaine.