Nitric oxide (NO) is an endogenous messenger and inflammatory mediator that is widespread in the human body. While the role of NO in asthma pathogenesis is still under investigation, individuals with eosinophilic asthma have been found to have high levels of fractional exhaled nitric oxide (FeNO), which decreases with treatment with corticosteroids.
In the gas phase, NO is fairly stable, permitting its measurement in exhaled air. FeNO is typically measured during single breath exhalations. First, the subject inspires NO-free air via a mouthpiece until total lung capacity is achieved, followed immediately by exhalation through the mouthpiece into the measuring device.