Stability of the pH of local anesthetics after bicarbonation: an electrochemical assay of clinical mixtures.

BACKGROUND

Raising the pH of local anesthetics shortens their time to onset, and this is commonly done in clinical practice via addition of sodium bicarbonate. A prior study found that the pH of local anesthetics decreases over time after a factory-sealed vial has been opened, presumably due to hydrolysis of atmospheric carbon dioxide. We hypothesized that the alkalinization effect achieved by adding bicarbonate to local anesthetics may similarly be unstable, potentially affecting the drug's time-to-onset if not used immediately.

METHODS

We prepared solutions of chloroprocaine 3% and lidocaine 2% each buffered with sodium bicarbonate 1 M at a 10:1 vol ratio. Mixtures were stored at room temperature, and to assess the effects of atmospheric exposure, we compared syringes sealed with a rubber cap versus a blunt needle. We recorded electrochemical pH measurements at timepoints of 0, 1, 2, 4, 6, 12, and 24 h.

RESULTS

Adding bicarbonate to lidocaine raised pH from 6.5 ± 0.02 to 7.4 ± 0.04. Adding bicarbonate to chloroprocaine raised pH from 3.5 ± 0.03 to 7.6 ± 0.03. We found that the pH of each bicarbonated local anesthetic solution was stable to within 0.2 pH units across 24 h, regardless of type of seal.

CONCLUSIONS

These findings indicate that the faster onset time achieved by adding bicarbonate is likely to be preserved for at least 24 h when these solutions are stored in a syringe at room temperature. From the standpoint of pH stability, these findings support the practice of preparing buffered local anesthetic mixtures before they are needed, in order to minimize delay of administration (as for emergency indications); however, safety standards for drug compounding and storage must always guide practice.