Comparison of hydrochloride and carbonated salts of lidocaine for epidural analgesia

A double blind comparison of lumbar epidural 1.73% lidocaine carbonate and 2% lidocaine hydrochloride was performed in 32 patients aged 19-75 years. Where possible, the injections were made at the third lumbar interspace; the remaining few were made at the second interspace. The local anesthetic was injected directly through the needle. Measurements of sensory and motor blockade were done at 1-2 minute intervals beginning after the completion of the injection. Blood samples were drawn for analysis of plasma lidocaine levels prior to the injection of local anesthetic, then at 5-minute intervals for the first 20 minutes and then at 30, 45, and 60 minutes. Carbonated lidocaine achieved faster onset and faster maximum spread than the hydrochloride salt. This effect was most dramatic in the L5-S1 dermatomes (p < 0.001). Motor block was significantly more profound with the carbonated lidocaine after the seventh minute. Also the plasma levels of lidocaine tended to rise more rapidly and slightly higher with the carbonated solution, although this difference never achieved statistical significance. It is concluded that carbonated lidocaine is superior to the hydrochloride salt for lumbar epidural analgesia.