EFFECT OF PROLONGED NERVE BLOCK ON INFLAMMATORY HYPERALGESIA IN RATS - PREVENTION OF LATE HYPERALGESIA

Background: Recent evidence suggests that the duration of the nocicept ive block may be an important factor in determining the effect of the block an injury-induced hyperalgesia after block resolution. The autho rs examined whether a tonicaine nerve block lasting for 12 to 16 h cou ld prevent late inflammatory hyperalgesia. Methods: Inflammatory hyper algesia was induced by injection of carrageenan into the rat paw. A th reshold of motor response to increasing pressure was determined for th e injected paw, contralateral pan, and tail. The development of edema of the pan and an increase in pan-temperature also were determined. Th e block was achieved by simultaneous percutaneous injections of tonica ine (a new long-acting anesthetic agent) or lidocaine at the sciatic n erve (greater trochanter level) and the saphenous nerve (midthigh leve l), Results: Carrageenan without nerve block caused a profound primacy (injected paw) and secondary (contralateral paw and tail) hyperalgesi a that lasted for 3-5 days. Tonicaine nerve block administered before carrageenan completely prevented primary and secondary hyperalgesia. T onicaine block administered 5 h after carrageenan injection reversed s econd hyperalgesia and prevented the development of late (greater than or equal to 24 h) primary and secondary hyperalgesia. Edema and tempe rature of the paw were not significantly affected by the nerve block a dministered before or after carrageenan, Conclusions: A prolonged nerv e block (12-16 h) can prevent the development of long-lasting (3-5 day s) inflammatory hyperalgesia. Prevention of late hyperalgesia can he p rovided not only by the preinjury block but also by the postinjury blo ck administered when hyperalgesia is already well established.