Following the discovery in 1987 that the previously uncharacterized endothelium derived relaxing factor was nitric oxide, there has been an explosion of interest in this molecule. This free radical gained further importance in respiratory medicine following the observation by Gustafsson, et al.1 that endogenous nitric oxide can be measured in exhaled air; subsequently, it was found that levels are high in asthma2 and fall after steroid use.3 Since then over 1600 articles have been published on nitric oxide and asthma, yet we still do not know whether using measurements of the fraction of nitric oxide in exhaled air (FENO) is of practical benefit.
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