Evidence-based emergency medicine/review article
Systematic Review and Meta-analysis of the Effect of Warming Local Anesthetics on Injection Pain

https://doi.org/10.1016/j.annemergmed.2010.12.001Get rights and content

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.

Introduction

Local anesthetics are frequently used to anesthetize the skin or oral cavity before painful medical procedures. Injection of local anesthetics, however, is itself painful. Local anesthetics cause a burning or stinging sensation while being infiltrated into tissue, and this pain has been reported to cause people to avoid dental procedures.1 Some strategies to reduce pain from infiltration include slowing the rate of injection,2, 3 avoiding the use of epinephrine, and raising the pH with buffering agents such as sodium bicarbonate.3

Warming local anesthetics was first described as a pain-reducing measure by Boggia4 in 1967, but the mechanism of action is still unknown. One theory suggests that colder solutions stimulate nociceptors to a greater degree than warmer solutions.5 Another possible explanation is that as temperature increases, more uncharged local anesthetic passes through cell membranes, resulting in a faster onset of effect.6, 7, 8, 9 A survey in the United Kingdom found that 34% of maxillofacial surgeons and 8% of general surgeons regularly warmed local anesthetics to reduce pain.10

Many studies have investigated warming local anesthetics to reduce pain from infiltration, but at present the overall effectiveness has not been determined. The purpose of this systematic review and meta-analysis was to determine the effectiveness of warmed local anesthetics as an analgesic strategy for reducing local anesthetic infiltration pain.

Section snippets

Study Design

The Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines for reporting meta-analyses of randomized controlled trials were followed.11 A protocol was developed but not published in advance; post hoc analyses are indicated in the “Results” sections.

We accepted randomized controlled trials or pseudorandomized trials with adults or children. The following were our inclusion criteria: (1) randomized or pseudorandomized design, (2) healthy subjects or patients requiring local

Results

The search strategy (Appendix E1, available online at http://www.annemergmed.com) identified 449 studies (Figure 1). Studies were screened by title or abstract, and 29 were retrieved for further assessment. From these, 6 were excluded because they injected local anesthetic into the periorbital area, and 1 used regional anesthesia23, 24, 25, 26, 27, 28, 29; 1 was excluded because it was a review.30 Two did not have comparisons that met the inclusion criteria,31, 32 and 1 did not provide a median

Limitations

Limitations to the meta-analysis may arise from the presence of heterogeneity among the study populations, interventions, outcome assessments, and potential bias within the individual studies. Variability in the types of subjects enrolled was investigated through grouping the studies by patients and volunteers. Heterogeneity of interventions was explored by grouping studies according to the pH of solution, type of tissue infiltrated, use of epinephrine, volume, drug, and needle size used during

Discussion

This systematic review and meta-analysis determined that warming local anesthetics before injection reduced pain compared with injecting at room temperature. The results were consistent when the following subgroup analyses were performed: buffered and unbuffered solutions, subcutaneous or intradermal infiltration, anchoring pain from needle insertion at the midpoint of the visual analog scale or not specifying an anchor, presence or absence of epinephrine, and using patients or healthy

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    Supervising editor: Michael D. Brown, MD, MSc

    Funding and support: By Annals policy, all authors are required to disclose any and all commercial, financial, and other relationships in any way related to the subject of this article as per ICMJE conflict of interest guidelines (see www.icmje.org). The authors have stated that no such relationships exist.

    Publication date: Available online February 12, 2011.

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