Effect of inhalation times on exhaled NO

Recent work suggests that exhaled nitric oxide (eNO) can be divided into airway and alveolar components, using varying expiratory flow rates. It is known that higher values of eNO are found with lower expiratory flow rates, thought to be due to prolonged contact of the expirate with the bronchial epithelium. However, whether a prolonged inspiratory time could contaminate the alveolar signal with bronchial derived NO, by the same mechanism, is unknown. We aimed to study the effect, including repeatability, of inhalation time on eNO measurements performed at three different flow rates. We measured eNO in 21 children with asthma of different severity (no steroids, n = 2; inhaled corticosteroid (IC) dose up to 200 mcg/day, n = 3; IC up to 500 mcg/day, n = 3; IC >500 mcg/day, n = 6; IC >500 mcg/day and oral steroid, n = 7) and 24 normal adult controls at 50, 100, and 200 ml/s expiratory flow rate. The effect of either a rapid or a slow inhalation on measured eNO was studied at each flow rate. Furthermore, 12 asthmatic children and 12 adults had repeated measurements 1 hr apart. Repeatability within 1 day was assessed by calculating the single-determination standard deviation (SD) and 95% range. Our results showed that repeatability was equally good for the three expiratory flow rates, and inhalation time had no influence on the results. The inhalation maneuver does not influence eNO measurements using the variable expiratory flow technique.