Lung function measurements and FeNO
IOS was performed in accordance with established guidelines.16The difference of the respiratory system resistance (Rrs) at 5 Hz and 20 Hz (Rrs5-20) has a high sensitivity for detecting small airway dysfunction.17Airway resistance was also measured at 1, 2, 3, 5, 10, 15, and 20 Hz for calculation of area of reactance (AX) as a function of frequency. The signals were analyzed for 30 s at each frequency. Oscillometric tests were performed using a Jaeger MasterScreen device (Jaeger Co., Wurzburg, Germany).
The FeNO concentration in exhaled breath in parts per billion (ppb) was measured using a portable nitric oxide analyzer (NIOX MINO®; Aerocrine, Solna, Sweden) according to the American Thoracic Society (ATS) guidelines.18