In the acute inflammatory response, there is a rapid but short-lasting influx of cells and mediators to the site of injury. The role of these substances is to limit infection and initiate tissue repair processes. By contrast, in the chronic inflammatory process, levels of cells such as eosinophils and mast cells, as well as lymphocytes are persistently elevated. In the airways, this causes cell damage, tissue swelling, mucus secretion and contraction.

Asthma is a chronic inflammatory disease which leads to airflow limitation and an increased sensitivity and response to allergens. This type of inflammation, eosinophilic inflammation, is also a hallmark of bronchial asthma [Bosquet 1990].

There is now good evidence that levels of exhaled NO correlate with the extent of eosinophilic inflammation in the airways [Berry 2005, Payne 2001, van den Toorn 2001, Lex 2006].

Exhaled NO is only produced at elevated levels when inflammation is present, so it can be used to differentiate between non-inflammatory conditions, and to indicate the presence and level of inflammation in the airways. While there are other methods for doing this, such as biopsy or bronchial alveolar lavage (BAL) these are invasive, time-consuming and require specialist facilities. Measurement of exhaled NO is easy, fast and non-invasive. 

Normal levels of exhaled NO have now been established for children and adults (around 20 to 25 parts per billion), and deviations from baseline values can be used to diagnose airways inflammation. Exhaled NO also reacts rapidly in response to treatment or worsening of the disease and can therefore be used routinely to monitor progress.

More information can be found on the measurement and interpretation of exhaled NO following this link.