Does warming local anesthetic reduce the pain of subcutaneous injection?

The most frequent complaint noted with the use of lidocaine (or other amide local anesthetic) is stinging or burning pain associated with subcutaneous infiltration. The purpose of this study was to evaluate the efficacy of warming buffered lidocaine for reducing the pain of infiltration. Forty adult volunteers were entered into a randomized, crossover study conducted at a community teaching hospital. Blinded subjects underwent 1-mL subcutaneous injections of the study agent through 27-gauge needles over 30 seconds. Following a crossover protocol, "room temperature" buffered lidocaine (20 degrees C) was injected into one midvolar forearm and "body temperature" buffered lidocaine (37 degrees C) into the opposite arm. The order and the handedness of the two injections were randomized; an independent observer prepared the anesthetic solutions. Pain was assessed using a 100-mm visual analog pain scale and subjects' comparison of pain on injection. Twenty subjects (50%) reported that 20 degrees C buffered lidocaine was more painful and 17 (42.5%) reported that the 37 degrees C solution was more painful (sign test, P = .74). Similarly, a median pain score difference of 5.0 mm favoring 37 degrees C lidocaine was not statistically significant (sign rank test, P = .42). The order or the initial side of the injection did not influence the pain scores. The study had a power of 80% to detect a 10-mm difference between the two solutions at alpha = .05. These results suggest that warming buffered lidocaine to body temperature (37 degrees C) does not reduce the pain of subcutaneous infiltration.