[Precipitation of pH-adjusted local anesthetics with sodium bicarbonate]

Sodium bicarbonate has been added to various local anesthetics for shortening the onset time. We often noticed the precipitate in the pH-adjusted local anesthetics several minutes after adding the bicarbonate. We confirmed that this drug interaction was caused by large changes in pH. We have analyzed the precipitates by mass spectrometry and infrared absorption spectrum and have revealed it as unionized base forms of local anesthetics. Since these forms tend to be insoluble, we must limit the amount of 7% sodium bicarbonate. This study proposes a guideline for adding 7% sodium bicarbonate to the various local anesthetics. The amount of 7% sodium bicarbonate which can be added to bupivacaine without precipitation is below 0.02 ml. In cases of mepivacaine and lidocaine, allowable amount of 7% sodium bicarbonate is 0.5 ml. It is not known whether this lower dose of 7% sodium bicarbonate compared with the recent reports is effective or not. Further clinical investigation may be necessary.