Crystallization of short-acting and intermediate-acting local anesthetics when mixed with adjuvants: a semiquantitative light microscopy analysis.

The addition of adjuvants to short-acting local anesthetics (LA) is common practice in clinical routine to speed up block onset and decrease pain on injection. In a previous study, we observed the development of microscopic crystal precipitations after bupivacaine or ropivacaine were mixed with adjuvants; this follow-up study is intended to clarify whether crystallization (A) also occurs in short-acting or intermediate-acting LA-adjuvant mixtures, (B) changes over time, and (C) is associated with the solutions' pH. Lidocaine 2%, prilocaine 2%, mepivacaine 2%, procaine 2% and chloroprocaine 2% were individually mixed with clonidine, dexamethasone, dexmedetomidine, epinephrine, fentanyl, morphine or sodium bicarbonate 8.4% in clinically established ratios. For each mixture, we measured initial pH and recorded crystallization patterns at 0, 15, 30 and 60 min using a standardized, semiquantitative light microscopy approach. Lidocaine 2% and mepivacaine 2% plus sodium bicarbonate 8.4%, and mepivacaine 2% plus dexamethasone developed delayed grade 5 crystallization over 1 hour. Prilocaine-based, procaine-based and chloroprocaine-based mixtures showed much less pronounced crystallization, with a maximum of grade 2. Initial pH and grade of crystallization showed weak monotonic relationships at time points t

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Competing interests: None declared.