Duration of experimental nerve block by combinations of local anesthetic agents

The effects of bupivacaine-prilocaine and meperidine-lidocaine combinations (as compared with those of the agents used alone) on the duration of peripheral sensory nerve block were studied with the infraorbital nerve block model (IONB) in the rat, and those on motor block with spinal anesthesia (SA) in the mouse. The duration of bupivacaine-induced IONB was invariably prolonged when prilocaine was included in the solution. When included in 0.125% bupivacaine, 1.0% prilocaine had a slightly less pronounced enhancing effect than 0.5% prilocaine (24-57% vs. 74%-104%, respectively). The duration of IONB with 1.0% prilocaine was significantly reduced (14-37%) by inclusion of 0.125% bupivacaine. In SA, inclusion in 0.125% bupivacaine of prilocaine (0.5% or 1.0%) prolonged motor block by 128% and 192%, respectively. When included in 0.25% bupivacaine, both 0.5% and 1.0% prilocaine significantly reduced the duration of SA, by 42% and 37%, respectively. With one exception, the duration of IONB by meperidine was significantly shortened (< 44%) when lidocaine was included in the solution. In SA, inclusion of 2% lidocaine with 2% meperidine did not affect the duration of meperidine-induced motor block. The duration of SA obtained with the combination of 4% lidocaine and 4% meperidine was 45% shorter than that induced by 4% meperidine alone. The reasons for these variable effects are not clear, but may be due to interaction or antagonism at any of multiple sites.