The Air We Exhale: Deciphering the Role of Fractional Exhaled Nitric Oxide (FeNO) in Airway Diseases

Abstract

Fractional exhaled nitric oxide (FeNO) is an important indicator of type 2 inflammation, which is frequently utilized in the assessment and treatment of asthma. It is measured in exhaled breath and provides a simple, quick, and reproducible assessment of airway inflammation status. Elevated FeNO levels are strongly associated with eosinophilic airway inflammation. It represents the synthesis of nitric oxide in the airway epithelium, which is mainly caused by inducible nitric oxide synthase under the influence of cytokines like IL-4 and IL-13. FeNO is utilized in the assessment of symptoms in patients presenting with cough, wheezing, and dyspnoea. It plays a key role in phenotyping asthma, particularly in identifying the eosinophilic asthma phenotype, which is typically T2 high and steroid responsive. FeNO measurement is also valuable in predicting medication response, as it evaluates the possible reaction to anti-inflammatory drugs, mainly inhaled corticosteroids (ICS). Furthermore, it helps in guiding treatment changes in a step-wise manner, including stepping up or stepping down the ICS dose. FeNO is useful in assessing asthma control, especially in determining whether poor control is due to airway inflammation when other contributing factors such as rhinosinusitis, anxiety, gastroesophageal reflux, obesity, and ongoing allergen exposure are present. Additionally, variations in FeNO levels may serve as a helpful supplementary tool for identifying acute complications following lung transplantation. Emerging evidence in fractional exhaled nitric oxide (FeNO) has expanded its potential role beyond asthma, demonstrating its utility in evaluating conditions such as obstructive sleep apnoea, rhinoviral infection, and radiation pneumonitis. However, values may be impacted by things like smoking, illnesses, respiratory maneuvers, airway caliber and food. Standardized testing guarantees reliability. For the best possible care, FeNO should be read in conjunction with clinical results and other studies.