Enhancement of Bupivacaine Sensory Blockade of Rat Sciatic Nerve by Combination With Phenol

We sought to determine whether the addition of phenol would enhance a bupivacaine nerve block. The effects on nerve conduction of bupivacaine (0.125%) and phenol (0.5%), singly and combined, were evaluated in vivo on the rat sciatic nerve. Three groups of 10 animals each were used. The left sciatic nerve was infiltrated with 0.125% bupivacaine, 0.5% phenol, or a solution that contained 0.125% bupivacaine and 0.5% phenol. The right limb served as control (saline injected). Motor deficits (visual assessment) and sensory blockade (hot-plate assay) were evaluated at 30-min intervals after injection. Phenol injected alone produced no motor blockade. The incidence of motor blockade at 30 min for 0.125% bupivacaine was 70% (P = 0.003), and for the combination treatment, 80% (P = 0.001). The analgesia score derived from the hot-plate test was more and persisted longer for the combination treatment than for either 0.125% bupivacaine or 0.5% phenol given singly; e.g., the average sensory block score after 150 min for the combination treatment was 1.0 compared with 0.1 for either bupivacaine or phenol given alone (P = 0.003). Analysis of the areas under the sensory score-time curves also demonstrated enhanced blockade from the combination treatment, which would be consistent with a synergism of the separate Na -channel blocking effects of charged and uncharged local anesthetics. These findings may suggest other candidates for clinically useful combinations of amine and neutral local anesthetics.