Objectives: To estimate the overall efficacy of caffeine upon lung funtion and to identify whether there is a need to control for caffeine consumption prior to lung function testing.Search strategy: A search was carried out within the Cochrane Airways Group Register of Asthma and Wheeze randomized controlled trials (RCT's). The bibliography of each RCT was searched for additional trials. Authors of identified RCTs were contacted for other published and unpublished studies.
Selection criteria: Thirty-nine references were obtained from the literature search and independently assessed by the two reviewers. Full texts of potentially relevant trials were obtained. From these, six matched the inclusion criteria for this review.
Data collection and analysis: Data were extracted independently by the two reviewers. Since lung function outcomes were measured at different times and following different doses of caffeine, data for each outcome were grouped according to three time frames: 'short' - less than or equal to 2 hours; 'medium' - greater than 2 hours and less than or equal to 4 hours; 'long' - greater than 4 hours. Analyses were carried out using all caffeine doses and two subgroups based on the median dose: 'low' dose - 5mg or less of caffeine per kg of body weight; 'high' dose - greater than 5mg of caffeine per kg of body weight.
Main results: Caffeine, even at a low dose, compared to placebo was found to significantly improve lung function, measured in terms of FEV1, FEF25-75 and specific airway conductance for up to 2 hrs post ingestion. This effect was sustained for FEF25-75 for over 4 hrs. Improvement was also seen in FEV1 up to this time, however this effect did not reach statistical significance. No data were available for specific airway conductance after 2hrs.
Conclusions: It is recommended that patients be advised to withhold caffeine for at least four hours prior to lung function testing. Subjective patient benefits and clinical implications have not yet been thoroughly investigated. Caffeine improves airway function by a modest amount in asthmatic patients for up to four hours.