FENO measurement
FENO values were measured according to American Thoracic Society/European Respiratory Society recommendations25 by using a nitric oxide chemiluminescent analyzer (Sievers NOA 280; Sievers, Boulder, Colo) with a sampling flow rate of 50 mL/s. FENO values were calculated on an end-expiration plateau sustained for at least 3 seconds. The mean value of 3 consecutive acceptable trials was computed.