Towards a practical clinical use of fractioned exhaled nitric oxide levels in chronic cough

Fractioned exhaled nitric oxide (FeNO) is the amount of nitric oxide (NO) present in the airways detectable with chemiluminescent or electrochemical methods in the exhaled air. The exhaled NO measured is the sum of the constitutive gas produced by the neuronal and the endothelial isoforms of the NO synthases and the amount produced after activation of the inducible isoform of this enzyme (1). The inducible isoform produces a high quantity of NO, nM range, compared to the constitutive isoforms, which produce pM amount of the gas. The inducible isoform is mainly present in macrophages and epithelial cells in the airways and it accounts for most of the FeNO detected. One interesting characteristic of the inducible production of FeNO is that it is quite well down-modulated by corticosteroids compared to the NO produced by the constitutive isoforms (1).