Asthma exhaled nitric oxide

Overview
Measurement of fractional nitric oxide concentration in exhaled breath (FeNO)is a non-invasive method of assessing underlying airway inflammation. However, due to technical complexities associated with the procedure, it is not routinely used.

Exhaled Nitric Oxide Breath Test
Asthma is a chronic inflammatory disease with hyper-responsive airways. As a part of the inflammatory process, there is an associated up-regulation in the nitric oxide synthase observed in the respiratory mucosal epithelium, which contributes to the increased concentrations of nitric oxide released in exhaled breath.

Benefits of measurement of exhaled nitric oxide in asthmatics:

 * Asthmatics are associated with higher concentrations of expired nitric oxide and hence expired nitric oxide evaluation may be a useful index to assess the disease severity.
 * Exhaled NO has also shown to be associated with other asthmatic clinical parameters such as airway inflammation, sputum eosinophilia and viral infection.
 * Exhaled nitric oxide closely correlates with the percentage of eosinophils in induced sputum and therefore may be a useful non-invasive marker to assess airway inflammation.
 * Exhaled nitric oxide levels reduced with the administration of steroids and leukotriene receptor blockers suggesting that exhaled nitric oxide measurement may be a beneficial marker for assessing the airway inflammation in asthmatics.

Supportive trial data:

 * A prospective randomized study demonstrated that inhaled steroid therapy when adjusted to control the fraction of exhaled nitric oxide measurement was beneficial as opposed to controlling the standard clinical parameters of asthma. The study further reported a significant reduction in the cumulative maintenance dose of inhaled steroid without worsening asthmatic symptoms.
 * However, two other studies did not demonstrate any beneficial effect with the use of eNO-guided therapy in comparison to conventional-based therapy. One of the studies, involving 546 patients demonstrated good symptomatic control with conventional management; however, the use of fraction of exhaled NO as an indicator of asthmatic control, resulted in higher doses of inhaled steroids, without clinically improving symptomatic control. Another randomized trial that assessed 118 asthmatics, demonstrated that the measurement of exhaled nitric oxide did not cause a significant reduction in the frequency of exacerbations or in the total amount of inhaled corticosteroid therapy used over 12-month period when compared to the current asthma guidelines.