[Comparison of bupivacaine hydrochloride 0.5% and bupivacaine carbonate 0.5% in interscalene plexus anesthesia].

Carbonated local anesthetics are less acidic than the hydrochlorides and require less buffering by the tissues. Rapid buffering and diffusion of the carbon dioxide enables free base to be deposited in high concentrations on nerve fibres. Carbon dioxide increases the intracellular hydrogen ion concentration, thus increasing the amount of active cation at the receptor site. The interscalene approach to the brachial plexus was chosen for comparison of bupivacaine hydrochloride 0.5% and carbonated bupivacaine 0.5%. The interscalene route has many potential advantages, including anesthesia of the shoulder and reliable block of the musculocutaneous nerve, but large volumes of local anesthetic solutions are needed. The use of such volumes may exceed the limit of toxicity. Therefore, the plasma concentrations produced by 40 ml of local anesthetic solution were studied in 42 patients for whom brachial plexus block was considered a suitable technique. The latency of onset of sensory analgesia was shortest and with the least variability in the dermatomes C5-C7. The first analgesia was detected by pinprick 5.1 +/- 0.4 (SE) min and complete analgesia in 19 +/- 1 min after bupivacaine hydrochloride and 4.7 +/- 0.5 min and 15 +/- 1 min after bupivacaine carbonate. The more caudad nerves showed a significantly longer latency time than the more cephalad ones. The duration of sensory analgesia varied between 6 and 12 hours, the more caudad nerves showing the shortest duration.(ABSTRACT TRUNCATED AT 250 WORDS)