Effect of prolonged nerve block on inflammatory hyperalgesia in rats: prevention of late hyperalgesia.

BACKGROUND: Recent evidence suggests that the duration of the nociceptive block may be an important factor in determining the effect of the block on injury-induced hyperalgesia after block resolution. The authors examined whether a tonicaine nerve block lasting for 12 to 16 h could prevent late inflammatory hyperalgesia.
METHODS: Inflammatory hyperalgesia was induced by injection of carrageenan into the rat paw. A threshold of motor response to increasing pressure was determined for the injected paw, contralateral paw, and tail The development of edema of the paw and an increase in paw temperature also were determined. The block was achieved by simultaneous percutaneous injections of tonicaine (a new long-acting anesthetic agent) or lidocaine at the sciatic nerve (greater trochanter level) and the saphenous nerve (midthigh level).
RESULTS: Carrageenan without nerve block caused a profound primary (injected paw) and secondary (contralateral paw and tail) hyperalgesia that lasted for 3-5 days. Tonicaine nerve block administered before carrageenan completely prevented primary and secondary hyperalgesia. Tonicaine block administered 5 h after carrageenan injection reversed secondary hyperalgesia and prevented the development of late (> or = 24 h) primary and secondary hyperalgesia. Edema and temperature of the paw were not significantly affected by the nerve block administered before or after carrageenan.
CONCLUSIONS: A prolonged nerve block (12-16 h) can prevent the development of long-lasting (3-5 days) inflammatory hyperalgesia. Prevention of late hyperalgesia can be provided not only by the preinjury block but also by the postinjury block administered when hyperalgesia is already well established.