Systematic review and meta-analysis of the effect of warming local anesthetics on injection pain

Study objective: Local anesthetics are the main class of analgesics used for pain management during laceration repair and other minor surgeries; however, they are administered by injection, which is painful. Warming local anesthetics has been proposed as a cost-free intervention that reduces injection pain. A systematic review of the effectiveness of this technique has not yet been undertaken. We determine the effectiveness of warming local anesthetics to reduce pain in adults and children undergoing local anesthetic infiltration into intradermal or subcutaneous tissue.

Methods: We used published articles from MEDLINE (1950 to June 2010), EMBASE (1980 to June 2010), CINAHL (1982 to June 2010), the Cochrane Library (second quarter 2010), International Pharmaceutical Abstracts (1970 to June 2010), and ProQuest Dissertations and Theses database (1938 to June 2010). We included studies with randomized or pseudorandomized designs and healthy subjects or patients receiving subcutaneous or intradermal injection of local anesthetics that were warmed (body temperature) or not (room temperature). Studies of regional anesthesia and intraarticular, spinal, or periorbital administration of local anesthetics were excluded. Data were extracted onto predesigned forms and verified by 2 reviewers. Quality was assessed with the Cochrane risk of bias tool. The primary outcome was self-reported pain as assessed by a visual analog or numeric rating scale. Data were combined with mean differences with 95% confidence intervals (CIs) by using a random-effects model.

Results: Twenty-nine studies were retrieved for close examination and 19 studies met inclusion criteria. A total of 18 studies with 831 patients could be included in a meta-analysis. Seventeen studies had an unclear risk of bias and 1 had a high risk of bias. A mean difference of -11 mm (95% CI -14 to -7 mm) on a 100-mm scale was found in favor of warming local anesthetics. Subgroup analysis of 8 studies investigating the effect of warming on buffered local anesthetics yielded similar results: -7 mm (95% CI -12 to -3 mm).

Conclusion: Warming local anesthetics leads to less pain during injection and therefore should be done before administration.