Please confirm that an ethics committee approval has been applied for or granted: Not relevant

Background and Aims Local anesthetics (LAs) are commonly prepared in acidic solutions for stability. Alkalinization with sodium bicarbonate may enhance onset, duration, and reduce pain.^1-3 We assessed the pH effects of normal saline and sterile water on LA preparations at different dilution ratios, an aspect currently unexplored in the literature.

Methods Approved by the department, this service evaluation project was conducted in an accredited lab. Baseline pH measurements were taken for each solution. LA preparations were mixed with diluents at ratios of 1:1, 1:2, and 1:3 using a calibrated micropipette. Three pH measurements per dilution were averaged.

Results Table 1 depicts significant pH increases in bupivacaine with both diluents, notably higher with normal saline. Lignocaine diluted with normal saline showed non-significant pH fluctuations. Significant pH drops were noted with 2% lignocaine diluted with sterile water at 1:2 and 1:3 ratios. In table 2, normal saline yielded more favorable pH levels for lignocaine and bupivacaine, particularly evident with 2% lignocaine.

Conclusions This study is the first to focus on pH measurement when diluting local anesthetics with normal saline and sterile water.While some emphasize alkalinization, caution against sodium-containing solutions exists due to increased competetion at sodium channels.^{4 5} We believe pH and the unionized fraction of local anesthetic are deemed clinically crucial. We propose using normal saline for diluting local anesthetics as it typically yields a better pH change. However, patient trials are required to confirm pH’s impact on onset and effectiveness.

References

1. DiFazio CA, Carron H, Grosslight KR, Moscicki JC, Bolding WR, Johns RA. Comparison of ph-adjusted lidocaine solutions for epidural anesthesia. Anesth Analg. 1986 Jul;65(7):760.

2. Kim JH, Ryu SJ, Yu SB, Kim K Han, Chang TH, Kim SH. Effect of normal saline and distilled water used for dilution of 4% plain lidocaine in epidural anesthesia. Korean J Anesthesiol. 54(3):256-60.

3. Momsen OH, Roman CM, Mohammed BA, Andersen G. [Neutralization of lidocaine-adrenaline. A simple method for less painful application of local anesthesia]. Ugeskr Laeger. 2000 Aug 14;162(33):4391-4.

4. Best CA, Best AA, Best TJ, Hamilton DA. Buffered lidocaine and bupivacaine mixture - the ideal local anesthetic solution? Plast Surg. 2015;23(2):87-90.

5. Dhir S, Tureanu L, Bouzari A, Masood A, Francispragasam M, Ganapathy S. Reduction in sodium content of local anesthetics for peripheral nerve blocks: a comparative evaluation of saline with 5% dextrose--a randomized controlled double-blind study. Anesth Analg. 2012 Jun;114(6):1359-64.