Perineural dexmedetomidine added to ropivacaine causes a dose-dependent increase in the duration of thermal antinociception in sciatic nerve block in rat.
BACKGROUND
The current study was designed to test the hypothesis that dexmedetomidine added to ropivacaine would increase the duration of antinociception to a thermal stimulus in a dose-dependent fashion in a rat model of sciatic nerve blockade.


METHODS
Fifty adult Sprague-Dawley rats (10 rats/group) received unilateral sciatic nerve blocks with 0.2 ml ropivacaine (0.5%) or 0.2 ml ropivacaine (0.5%) plus dexmedetomidine (2.7 microm [0.5 microg/kg], 11.7 microm [2 microg/kg], 34.1 microm [6 microg/kg], or 120.6 microm [20 microg/kg]) in a randomized, blinded fashion. Time to paw withdrawal latency to a thermal stimulus for both paws and an assessment of motor function were measured every 30 min after the nerve block until a return to baseline.


RESULTS
Dexmedetomidine added to ropivacaine increased the duration of dense sensory blockade and time for return to normal sensory function in a dose-dependent fashion (P < 0.005). There was a significant time (P < 0.005), dose (P < 0.005), and time-by-dose effect (P < 0.005) on paw withdrawal latencies of the operative paws. There were no significant differences in paw withdrawal latencies of the control paws, indicating little systemic effect of the dexmedetomidine. The duration of motor blockade was also increased with dexmedetomidine. High-dose dexmedetomidine (120.6 mum) was not neurotoxic.


CONCLUSIONS
This is the first study showing that dexmedetomidine added to ropivacaine increases the duration of sensory blockade in a dose-dependent fashion in rats. The findings are an essential first step encouraging future efficacy studies in humans.
Department of Anesthesiology, Division of Pain Medicine, University of Michigan, Ann Arbor, USA. cbrummet@umich.edu
Chad M
CM
Brummett
Amrita K
AK
Padda
Francesco S
FS
Amodeo
Kathleen B
KB
Welch
Ralph
R
Lydic
D000316:Adrenergic alpha-Agonists / Q000008:administration & dosage*; D000577:Amides / Q000008:administration & dosage*; D000779:Anesthetics, Local / Q000008:administration & dosage*; D000818:Animals; D003000:Clonidine / Q000008:administration & dosage; D020927:Dexmedetomidine / Q000008:administration & dosage*; D004305:Dose-Response Relationship, Drug; D006993:Hypnotics and Sedatives / Q000008:administration & dosage*; D008297:Male; D009407:Nerve Block / Q000379:methods*; D059408:Pain Management; D051381:Rats; D017207:Rats, Sprague-Dawley; D000077212:Ropivacaine; D012584:Sciatic Nerve / Q000187:drug effects
D000316:Adrenergic alpha-Agonists; D000577:Amides; D000779:Anesthetics, Local; D006993:Hypnotics and Sedatives; D020927:Dexmedetomidine; D000077212:Ropivacaine; D003000:Clonidine