Exhaled nitric oxide

Exhaled Nitric Oxide (eNO) is a biomarker measured in the breath of individuals, indicative of inflammation in the respiratory system. This measurement is particularly significant in the diagnosis and management of asthma, as well as in monitoring the effectiveness of treatment for airway inflammation. Nitric oxide (NO) is a gaseous molecule produced by various cells in the body, including those in the airways. Its levels in exhaled air can reflect airway inflammation, making it a useful non-invasive tool in respiratory medicine.

Production and Function

Nitric oxide is synthesized from the amino acid L-arginine by the enzyme nitric oxide synthase (NOS). In the airways, it is primarily produced by the epithelial cells lining the respiratory tract. NO plays a crucial role in various physiological processes, including vasodilation, neurotransmission, and immune response. In the context of respiratory health, its production is often upregulated in response to inflammatory stimuli, making its measurement valuable in assessing airway inflammation.

Measurement of Exhaled Nitric Oxide

The measurement of eNO is performed using specialized instruments that analyze the concentration of nitric oxide in a breath sample. The procedure is non-invasive, quick, and relatively simple, requiring the individual to exhale into a mouthpiece connected to the analyzer. The results are typically reported in parts per billion (ppb). The American Thoracic Society and the European Respiratory Society have provided guidelines to standardize the measurement of eNO, ensuring consistency and reliability across different settings.

Clinical Significance

1. 1. 1. Asthma

In individuals with asthma, elevated levels of eNO can indicate eosinophilic airway inflammation, a common underlying mechanism in asthma exacerbations. Monitoring eNO levels can help in assessing the efficacy of anti-inflammatory treatment, such as corticosteroids, and in adjusting medication dosage to achieve optimal control of asthma symptoms.

1. 1. 1. Other Respiratory Conditions

While most commonly associated with asthma, elevated eNO levels can also be observed in other respiratory conditions, such as chronic obstructive pulmonary disease (COPD), allergic rhinitis, and cystic fibrosis. However, the interpretation of eNO levels in these conditions is more complex and requires careful consideration of the overall clinical context.

Limitations

While the measurement of eNO offers valuable insights into airway inflammation, it is not without limitations. Factors such as age, smoking status, and atopy can influence eNO levels. Additionally, eNO levels can vary throughout the day and in response to environmental exposures, necessitating standardized measurement conditions to ensure accuracy.

Conclusion

Exhaled nitric oxide is a valuable biomarker for the non-invasive assessment of airway inflammation, particularly in the management of asthma. Its measurement can aid in the diagnosis, monitoring, and treatment optimization for individuals with respiratory conditions. Despite its limitations, eNO remains a critical tool in the respiratory medicine arsenal, offering insights into the inflammatory status of the airways that are not readily obtainable through other means.