Skin infiltration of local anesthetics causes pain. In a double-blinded protocol, 22 volunteers received random intradermal injections to the volar surface of the forearm with each of the following solutions: normal saline solution 0.9% (NSS), lidocaine 1% (L), lidocaine 1% and sodium bicarbonate 8.4% (L+BIC), 2-chloroprocaine 2% (CP), 2-chloroprocaine 2% and sodium bicarbonate 8.4% (CP+BIC), and NaCHO(3) 8.4% (BIC). Initially, each volunteer received an open-labeled injection of NSS. A 100-mm visual analog scale (VAS, 1-100) was used to assess pain with each injection. The pH of each solution was stable for the length of the study. Repeated measures of variance were used for analysis. The VAS scores (mean +/- SD) for open-label and blinded NSS injections were 15.5 +/- 15.9 and 14.0 +/- 18.1, respectively. The scores for the studied solutions were as follows: BIC, 47.2 +/- 25.5; L, 25.8 +/- 27.6; L+BIC, 16.0 +/- 14.2; CP, 8.6 +/- 7.4; and CP+BIC, 6.8 +/- 6.7. No significant difference was found between CP and alkalinized CP, but the injection of both solutions was significantly less painful than that of all other solutions (P < 0.05). The pH of the solutions was not related to the pain score. We found that chloroprocaine caused less pain at injection than the more commonly used lidocaine.
Implications: Using 2-chloroprocaine can diminish pain caused by the intradermal injection of lidocaine. pH variations of the solution did not relate to the pain profile of the local anesthetic.