Nitric Oxide Airway Measurements
When are nitric oxide airway measurements useful? Is it worth me buying a machine to measure it?
By Glenis K. Scadding MD, FRCP
Nitric oxide (NO) is produced from the amino acid arginine via the enzyme nitric oxide synthase.
It is a potent biological mediator which has a regulatory role in many cellular and tissue functions. In the upper and lower airways, NO is involved in regulatory, protective, defensive functions, but can have damaging effects. NO forms part of innate immunity, being constitutively generated at high concentrations in the paranasal sinuses.
Nitric oxide levels may reflect the inflammatory status of both the upper and lower airways. Measurement of exhaled bronchial nitric oxide is a useful, non-invasive tool in the diagnosis and management of eosinophilic asthma, since it is normally low unless nitric oxide synthase is induced by the inflammatory response. Normal levels indicate that lower airways eosinophilic inflammation is absent or controlled by therapy. NO levels may also help in the diagnosis of chronic cough.
Nasal nitric oxide is more complex since two factors are involved: high continually present sinus NO enters the nose via the ostiomeatal complex, but the nasal mucosa can also form inducible nitric oxide in an inflammatory response. Therefore nasal NO may be normal, raised or lowered in disease states. However measurement may be a useful tool in the diagnosis and management of patients with chronic rhinosinusitis, nasal polyps, and cystic fibrosis, as well as in the diagnosis of primary ciliary dyskinesia (PCD), where nasal NO is very low. In conditions where the ostiomeatal complex is blocked (such as nasal polyps) nasal NO tends to be low and to improve with nasal corticosteroids, correlating with the improvement in polyp grade. Further research is aimed at investigating the role of nitric oxide in allergic rhinitis, where results are variable, but tend to be elevated in active disease and lowered by nasal corticosteroids.
Measuring both bronchial and nasal nitric oxide may assist the combined management of upper and lower airways: for example in the coughing child normal exhaled NO (from the lower airways) suggests that the pathology is more likely to be in the upper respiratory tract.
Whether NO measurements are useful in your practice depends on the patients whom you are seeing. (FENO) values obtained using a portable analyzer correlate well with those obtained using an established laboratory analyzer and can be used to discriminate asthmatic from non-asthmatic patients. This may facilitate the measurement of asthmatic airway inflammation in primary care.
I have found NO very helpful in subjects with both upper and lower airway conditions, where they can obviate the need for a repeat CT scan, alert one to the likelihood of PCD or cystic fibrosis or to the fact that a patient is not using their inhaled corticosteroids in sufficient quantity.
There are guidelines for making the measurements accurately.
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