Exhaled Nitric Oxide



Measurement of fractional exhaled nitric oxide (FENO) is a noninvasive, safe,  method of quantifying airway inflammation. 

Exhaled nitric oxide can be measured in a breath test for asthma and other respiratory conditions characterized by airway inflammation. 

Nitric oxide (NO) is a gaseous molecule produced by certain cell types in an inflammatory response. 

The fraction of exhaled NO (FENO) is a promising biomarker for the diagnosis, follow-up and as a guide to therapy in adults and children with asthma. 

It is produced by cells lining the airways, specifically epithelial cells and inflammatory cells, such as eosinophils. 

NO is involved in various physiological processes in the respiratory system.

In the context of respiratory health, the measurement of exhaled nitric oxide can be useful in assessing airway inflammation, particularly in conditions such as asthma. 

Inflammation in the airways leads to increased production of nitric oxide by certain cells involved in the immune response.

By measuring the level of exhaled nitric oxide,  information about the degree of airway inflammation can be obtained.

It serves as a non-invasive biomarker to assess the severity of asthma and monitor the response to treatment. 

Higher levels of exhaled nitric oxide are often associated with increased airway inflammation, while lower levels may indicate reduced inflammation.

Nitric oxide is produced from L-arginine by three enzymes called nitric oxide synthases (NOS): inducible (iNOS), endothelial (eNOS), and neuronal (nNOS). 

Endothelial (eNOS), and neuronal (nNOS) are constantly active in endothelial cells and neurons respectively, whereas iNOS’ action can be induced in states like inflammation.

In inflammation, several cells use iNOS to produce NO, including eosinophils. 

Fractional exhaled nitric oxide has been dubbed an inflammometer.

 The lower airways contribute most of the exhaled NO.

Patients with asthma have higher eNO levels than other people. 

In conditions that trigger inflammation such as upper respiratory tract infections or the inhalation of allergens or plicatic acid, eNO levels rise.

Drugs used to treat asthma, inhaled glucocorticoids or leukotriene receptor antagonists, also reduce eNO levels.

Factors other than inflammation can increase eNO levels, for example airway acidity.

The fraction of eNO has been found to be a better test to identify asthmatics than basic lung function testing for airway obstruction.

A positive eNO test might be useful to rule in a diagnosis of asthma; 

Exhaled NO is minimally increased in chronic obstructive pulmonary disease, but levels may rise in sudden worsenings of the disease or disease progression. 

ENO has a possible role for eNO in predicting the response to inhaled glucocorticoids and the degree of airway obstruction reversibility.

Children with cystic fibrosis have low eNO levels. 

With bronchiectasis high levels have been found. 

The upper normal level of eNO in different ranges from 20 to 30 parts per billion. 

Men have higher eNO values than women. 

Smoking lowers eNO values, and even former smoking status can influence results. 

The levels are higher in people with allergies.

Nitric oxide is thought only to play a role the detrimental effects of air pollution on the respiratory tract.

It is an endothelium-derived relaxing factor, has role as a cell signalling molecule and neurotransmitter.

It is used in breath tests but also as a therapeutic agent for conditions such as pulmonary arterial hypertension and possibly for the acute respiratory distress syndrome.


FENO in asthma may help  make the diagnosis, monitor compliance with prescribed medications, and predicting pending exacerbations.


FENO is not diagnostic for asthma, nor does a normal FENO measurement exclude the diagnosis of asthma. 


FENO does have utility in predicting steroid responsiveness.


The FENO test use includes:


Assessment of cough, wheezing, and dyspnea


Identifying eosinophilic asthma


Assessing the potential response to anti-inflammatory agents, notably inhaled corticosteroids (ICS)


A baseline FENO should be established during a period of clinical stability for subsequent monitoring of chronic persistent asthma.


It is used to guide changes in anti-inflammatory medications, to assist in the evaluation of adherence to anti-inflammatory medications, and to assess whether airway inflammation is 


contributing to poor asthma control.


FENO measurements have been shown to be increased by nitrate-containing foods such as lettuce.


 FENO measurements may be transiently lowered by drinking coffee or water and smoking cigarettes. 


FENO measurements  should be done after patients refrain from smoking and the ingestion of food or beverages for 1 hour.


Upper and lower respiratory tract viral infections may increase levels of FENO.


Instruments for the measurement of exhaled nitric oxide are available.




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