Ph-adjustement of commercially prepared solutions of mepivacaine causes a time-dependent decrease in the concentration of mepivacaine

Introduction: The pH adjustment of commercially available solutions of mepivacaine by adding ImEq of NaHCO3 per 10 ml of mepivacaine (MEPI) is often suggested in order to decrease pain on injection and accelerate the onset of neuronal blockade . Since the addition of NaHCO3 to mepivacaine has been reported to cause a visible precipitation, we measured the effects of this adjustment on the concentration of mepivacaine in the resulting solution. Methods: All solutions were brought to room temperature, and 1 ml of 8.4% (ImEq) NaHCO3 was added to 10 ml of 1.5% or 2% MEPI (Polocaine or Carbocaine; Astra Pharmaceutical Co., Westborough, MA), respectively. The pH of the resulting solutions were measured 5 minutes after mixing using a calibrated pH meter (Model PH 32, Beckman Instruments, Inc., Fulerton, Ca). Using High Pressure Liquid Chromatography (HPLC) with a reversed phase column and electrochemical detector, concentration of MEPI was determined at the following intervals: 0,1,2,3,4, and 5 hours after addition of NaHCO3 The samples were also inspected for visual precipitation at the same intervals. All experiments were performed in triplicates and the data are reported as mean ±SD. Results: The addition of 1 mEQ of NaHCO3 to 10 ml of either 1.5% or 2% MEPI (pH 5.59 ±0.04) resulted in a near-physiologic pH (7.21 ±0.01). No visible precipitations were noted in any of the solutions during the five hour observation period. Measured concentrations of MEPI after the pH adjustment are shown in Figure 1. Concentration of MEPI did not change when the HCO3 was not added. Discussion: The pH-adjustement of 1.5% and 2% of MEPI to near-physiologic values causes a timedependent decrease in the concentration of mepivacaine. Since no precipitation was observed, we postulate that a soluble end-product or metabolite was formed. While the effects of these changes on anesthetic potency of MEPI are not known, our results suggest that pH adjusted solutions of MEPI should be used soon after their preparation.