Local anesthetics. Is there an advantage to mixing solutions?

The advantages to using a 50/50 mixture of lidocaine and bupivacaine with respect to onset and duration of local anesthesia instead of using the solutions independently were evaluated. In a double-blind randomized experiment, 12 subjects, each volunteering both feet, were studied. One foot was injected with 1 ml of one of the following three solutions: 1% plain lidocaine, 0.25% plain bupivacaine (Marcaine), or a 50/50 mixture of 1% lidocaine and 0.25% bupivacaine; and in the other foot, a 1-ml injection of normal saline as a blinded control. A 5.07 (10 g) Semmes-Weinstein monofilament wire was used for testing for sensory blockade, and the onset and duration of anesthesia was recorded for each subject. It was determined that there was no significant difference in the mean onset times for the three solutions, and no significant difference between the durations of anesthesia of plain lidocaine and the 50/50 mixture. Additionally, it was determined that bupivacaine had a prolonged duration of anesthesia compared with the other two solutions. The results of this preliminary study suggest that there is no clinical advantage, with respect to onset and duration of local blockade, to using a 50/50 mixture of plain lidocaine and plain bupivacaine in place of their independent use.