Review of variable quality trials finds that warming local anaesthetic to body temperature reduces self-reported pain of infiltration by 11 mm on a 100 mm rating scale
- Correspondence to Jawad Sultan
ST4 Trauma and Orthopaedics Royal Oldham Hospital Rochdale Road, Oldham, Manchester OL1 2JH, UK
Implications for practice and research
■ Warming local anaesthetic solution to body temperature reduces pain from infiltration.
■ In places where local anaesthetic is frequently used, for example in accident and emergency, warming local anaesthetic before infiltrating it is a simple and inexpensive way to reduce further pain in patients, using readily available equipment.
■ Future research should investigate the combined effect of warming and buffering the local anaesthetic solutions on the pain of its infiltration.
Local anaesthetic is frequently used in emergency settings as well as in elective surgical and dental procedures. The pain associated with injecting the local anaesthetic is not only due to the needle stick, but is also due to the injection of the local anaesthetic substance which causes burning and stinging sensations. It is a cause of distress to patient and doctor alike. Several methods have been described to reduce the pain of infiltrating local anaesthetics, including slower rate of injection, buffering the solution and warming it.1 Several studies have looked into warming local anaesthetics as a cheap and simple way to reduce pain of infiltration.2 However, most of these studies had a small sample size, and some had contradictory results. This systematic review and meta-analysis seeks to provide clear and strong evidence on warming local anaesthetics as a method to reduce the pain of infiltration.
Hogan et al have carried out a comprehensive search of reported databases, and identified 21 potentially relevant studies. Studies included were randomised or pseudo- randomised, single or double-blinded. The quality of studies was assessed by two authors using the Cochrane risk of bias assessment tool. Eighteen studies with 831 patients were included in the analysis. Three were excluded, two did not meet the inclusion criteria and one had missing data. Data were obtained from the published articles; no attempt was made to contact authors of the published papers. Where data were available in graphical forms only, two authors estimated the values from the graph. The primary outcome measure was the mean pain score on a scale from 0 to 100 mm, correlating to a visual analogue pain score. Further subgroup analyses were used to explore the potential effect of heterogeneities.
Warming local anaesthetic to body temperature reduced the pain of infiltration by a mean of −11 mm on a scale of 100 mm (95% CI −14 to −7 mm). Further subgroup analysis showed this to be true for buffered and unbuffered solutions, with or without epinephrine.
This study addresses a simple but important question; does warming local anaesthetic reduce the pain of its infiltration? This is a relevant clinical question, as local anaesthetic is commonly used to carry out simple surgical and dental procedures, in elective and emergency settings.
The search strategy used by the authors was adequate, and inclusion and exclusion criteria were clear. Data were pooled from published results, and no attempts were made to contact the authors of these studies. This is a limitation particularly that the authors estimated values from graphs when numerical results were not available, and imputed means and other measures when not available.
The majority of the studies included had small samples, were not double-blinded and tested ‘unclear’ for risk of bias. The population studied was heterogeneous and included adults and children, patients and healthy volunteers, facial, limb and dental procedures. However, the authors accounted for heterogeneities in the studies in their data analysis, through adequate and comprehensive subgroup analysis. These included analysis for the effect of buffering, using epinephrine and site and rate of infiltration. Where outliers were found and excluded, the overall results of the analysis did not change thus supporting the final conclusion.
Despite the above limitations, the balance of evidence clearly shows a clinically meaningful3 reduction in pain associated with infiltrating local anaesthetics when warmed to body temperature. Warming local anaesthetic solutions can be done easily and inexpensively using readily available equipment such as thermostatically controlled water baths, incubators, fluid warmers, baby-food warmers, warming tray or a syringe warmer.
Therefore, this study indicates that warming local anaesthetic solution before infiltration should become routine practice. Further research should investigate the combined effect of warming and buffering local anaesthetic solutions on the pain of infiltration.