Researchers Address Effect of Sodium Bicarbonate on Peripheral Nerve Block Performed with Lidocaine in Rats.Sinnott et al. (page 1045)

Using five lidocaine solutions prepared from crystalline lidocaine Hcl (0.5%) or commercial lidocaine at concentrations of 0.5% and 1.0%, Sinnott et al.  performed sciatic nerve blocks on 15 groups of 9 to 10 rats. An additional six groups of four rats each were administered percutaneous injections of 100 ml of one of six control solutions. The authors’ goal was to assess the effects on duration of block when lidocaine was alkalinized with sodium bicarbonate (BC). Analgesia was measured every 2 min in the ipsalateral limb for up to 40 min after injection and every 10 min thereafter. Analgesia was reported as the mean withdrawal response to deep pinch ± SD. Duration of block was defined as the time until response to pinch returned to a value of 3. Time of onset was the time for the response to pinch to reach a value of 2 from a normal response of 4.

In part 1 of the experiments, researchers compared the effects of alkalinization by BC versus  sodium hydroxide (NaOH) to pH 7.8 on 0.5% lidocaine, with and without epinephrine, prepared from crystalline salt. They used 0.5 and 1.0% commercial lidocaine solutions, with and without epinephrine, in part 2 of the experiments, comparing effects of unalkalinized solutions with those of solutions alkalinized with BC or NaOH. In solutions alkalinized with NaOH, pH was adjusted to 7.8.

Results of tests in part 1 showed that adding NaOH or BC to 0.5% lidocaine without epinephrine produced a faster onset than did unalkalinized lidocaine, but that degree or duration of block was not affected. Solutions containing epinephrine produced no differences in onset, degree, or duration of block with BC- or NaOH-alkalinized lidocaine. In part 2, using commercial lidocaine solutions, researchers noted that adding BC decreased the degree and duration of block by 25% compared with unalkalinized lidocaine, and by more than 50% compared with lidocaine alkalinized with NaOH. In the lidocaine solutions containing epinephrine, BC hastened onset of block without affecting degree and duration, compared with unalkalinized solution.

Although sodium bicarbonate may reduce onset time, as shown in this study, it also decreases the degree and duration of analgesia. The authors call for further clinical investigation to illuminate the role of sodium bicarbonate alkalinization of plain lidocaine and lidocaine containing epinephrine in the quality of peripheral nerve block.