Effects of intrathecal dexmedetomidine on ropivacaine spinal block in rats

Objective To investigate the effects of intrathecal (IT) dexmedetomidine on analgesia and neurotoxicity produced by ropivacaine spinal block .Methods Male SD rats weighing 250-300 g were used in this study. The animals were anesthetized with intraperitoneal 10% chloral hydrate 300 mg/kg. IT catheter was placed according to the technique described by Yaksh and Rudy. The tip of the IT catheter was positioned at lumbar region. Thirty-six SD rats in which IT catheter was successfully placed without complication were randomly allocated into 6 groups (n = 6 each): group Ⅰ received normal saline IT (group C); group Ⅱ received 0.5% ropivacaine 20 μl IT (group R); group Ⅲ received dexmedetomidine 3 μg/kg IT (group D ); group Ⅳ, Ⅴ , Ⅵ received 0.5% ropivacaine 20 μl + dexmedetomidine 1, 2 and 3 μg/kg IT respectively (group DR1, DR2, DR3). Tail-flick test, paw withdrawal threshold to yon frey stimuli and incline plate test were performed at 5, 30, 60, 120 and 240 min after IT drug administration. Two weeks later, the animals were sacrificed and the lumbar segment of the spinal cord was removed for microscopic examination. Results The duration of spinal block was significantly longer and the effect stronger in group DR1, DR2 and DR3 than in group R. Electron microscope showed that the injury to the myelin sheath of axon was the most severe in group DR3. Little or no damage to the axon was found in the other 5 groups (pathological score = 0). Conclusion Dexmedetomidine IT can enhance spinal block produced by 0.5 % ropivacaine, and there is celling effect. Key words: Dexmedetomidine;  Amides;  Anesthesia, spinal;  Injections,spinal