Bronchial obstruction and reversibility in children: inspiratory or expiratory resistance?

Assessing bronchial obstruction and reversibility is of help in diagnosing asthma. The forced oscillation technique (FOT) has gained popularity in children since minimal cooperation is required. Owing to the fact that measurements are performed during tidal breathing, the upper airway may significantly impact on the respiratory resistance (Rrs) [1, 2]. The glottic aperture narrows during tidal expiration [3], contributing to Rrs being larger than in inspiration [2, 4, 5]. Acute bronchial obstruction promotes further laryngeal narrowing [6–8], which is expected to impact the Rrs measured during expiration. It is not known to what extent the mechanism is present in children with stable asthma, or whether the ability of Rrs to diagnose bronchial obstruction and reversibility is impeded in expiration. With a single excitation frequency, Rrs may be described along the respiratory cycle and computed in expiration (Rrs,e) and inspiration (Rrs,i). The aim of this study was to compare Rrs,i and Rrs,e, their response to salbutamol and respective ability to separate asthmatics from controls. The hypothesis was that the diagnostic value of Rrs,e and its response to bronchodilator inhalation is impeded compared with Rrs,i.