Factors associated with elevated exhaled nitric oxide fraction in infants with recurrent respiratory symptoms.

Exhaled nitric oxide fraction (F(eNO)) has been proposed as a noninvasive marker of eosinophilic bronchial inflammation in active asthma, and supposed to reflect responsiveness to corticosteroid therapy. There are several factors influencing F(eNO), and its role in early childhood respiratory disorders needs to be established. Between 2004 and 2008, 444 children aged <3 yrs with recurrent lower respiratory tract symptoms were referred to a tertiary centre for further investigation. 136 full-term, steroid-free, infection-free infants, median age of 16.4 months (range 4.0-26.7 months), successfully underwent measurement of F(eNO), lung function tests, and a dosimetric methacholine challenge test. The median level of F(eNO) was 19.3 ppb (interquartile range 12.3-26.9 ppb). Elevated F(eNO) (≥ 27 ppb, the highest quartile) was associated with maternal history of asthma (adjusted OR 3.2, 95% CI 1.3-8.1; p=0.012), and increased airway responsiveness (the provocative dose of methacholine causing a 40% fall in maximal expiratory flow at functional residual capacity ≤ 0.30 mg) (adjusted OR 4.1, 95% CI 1.4-12.7; p=0.012). Atopy, blood eosinophilia and lung function were not associated with elevated F(eNO). In conclusion, maternal history of asthma, and increased airway responsiveness are associated with elevated F(eNO) in infants with recurrent lower respiratory tract symptoms.