Spinal sufentanil in rats: Part II: Effect of adding bupivacaine to epidural sufentanil.

Male Wistar rats were injected epidurally with various doses of sufentanil, bupivacaine or sufentanil combined with bupivacaine either 40 or 80 micrograms. Addition of 80 micrograms bupivacaine to sufentanil produced a 2.2- to 3-fold decrease of the lowest ED50 for a tail withdrawal reaction latency greater than 6 and greater than or equal to 10 s. The potentiation was already present at low doses. Whereas both 0.16 microgram sufentanil and 80 micrograms bupivacaine were inactive, the combination of the two drugs resulted in a deep analgesia in all animals. Because the addition of low doses of bupivacaine to sufentanil did not increase the ED50's for side-effects, higher specificity ratios were obtained as compared to sufentanil given alone. These results were discussed with regard to the potentiation of the analgesic properties of epidurally administered lipophilic opioids by local anaesthetics.