Warming local anesthetics reduced injection pain

Warming local anesthetics before injecting them significantly reduces the pain of the injection, according to a study published Feb. 8 in Annals of Emergency Medicine.

The meta-analysis of 18 studies with 831 patients found a mean difference of −11 mm (95% CI, −14 to −7 mm; I²=90%) on a 100-mm scale in favor of warming local anesthetics to body temperature. All anesthetics appear to sustain efficacy after warming. Subgroup analysis showed the following:

• Eight studies investigating the effect of warming on buffered local anesthetics yielded similar results: −7 mm (95% CI, −12 to −3 mm; I²=81%).Patient-reported pain from unbuffered local anesthetics at body temperature showed a mean difference of −13 mm (95% CI, −20 to −6 mm; I²=92%) compared to anesthetics at room temperature in 14 studies.Patient-reported pain was reduced by warmed anesthetics when administered subcutaneously (n=14 studies: mean difference, −14 mm [95% CI, −19 to −8 mm]) and intradermally (n=3 studies: mean difference, −5 mm [95% CI, −9 to −1 mm]; P<0.05).The effect occurred regardless of whether epinephrine was in the solution, although it was larger when it was (mean difference, −20 mm; 95% CI, −32 to −8 mm; n=6 studies) than when it was not (mean difference, −6 mm; 95% CI, −9 to −4 mm; n=12 studies) (P<0.0001).

Among the sensitivity analyses conducted, warming injections prior to administering them consistently produced a clinically meaningful reduction in pain regardless of:

• needle gauge (range, 19 to 30, with all but two studies using between 25 and 30),rate of injection (five studies reported rates faster than 0.1 mL/s; five studies reported rates slower than 0.1 mL/s), andvolume of injection (range, 0.5 mL to 150 mL, with all but two studies using between 0.5 mL and 11 mL.)

Injections were warmed using controlled water baths, incubators, fluid warmers, baby food warmers, a warming tray and a syringe warmer. Warming anesthetics can be done in almost any clinical setting with already available equipment, the authors noted. Anecdotal reports suggest that some emergency physicians warm injections in their hands.