Pulmonary function test
Spirometry (BTL-08 spiro, Germany) was performed to determine the lung function measurements and bronchodilator reversibility. Post-bronchodilator FEV1/FVC% and FEV1 were measured 15 min after inhalation of 400 μg salbutamol.
Fractional exhaled nitric oxide (FENO) was measured by the NIOX system (BEDFONT SCIENTIFIC limited 2009) by use of a single-breath on-line method according to European Respiratory Society/American Thoracic Society guidelines [13]. Briefly, the subject inhaled NO-free air to total lung capacity and exhaled through a dynamic flow restrictor with a target flow of 50 mL/s for 10 s. No nose clip was used. The NIOX system was calibrated according to the manufacturer’s instructions. FENO (ppb) < 25 means airway inflammation unlikely, FENO (ppb) 26–49 mild airway inflammation, FENO (ppb) > 50 means significant airway inflammation.