Identification of bronchodilator responsiveness by forced oscillation admittance in children

The upper airway wall motion may be responsible for significant error when measuring respiratory resistance (Rrs) with the forced oscillation technique (FOT), particularly in young children with airway obstruction. Assessing the response to methacholine from the change in respiratory admittance (Ars, the reciprocal of respiratory impedance, Zrs) avoids the artifact. The aim of the study was to explore the possibility of assessing the response to a bronchodilator from the change in Ars. FOT measurements were performed at 12 Hz in 36 young children before and after salbutamol. Transrespiratory pressure was varied in two ways: directly at the mouth (standard generator, SG) and around the head (head generator, HG), a variant nearly free of upper airway artifact. Salbutamol induced significantly lower Rrs and Zrs change with SG than HG (respectively, p = 0.0003 and 0.05). The relative change in Ars was not significantly different with SG and HG. Both estimates were significantly correlated (p < 0.0001). The change in Ars may thus be useful to avoid the upper airway artefact when assessing the response to salbutamol using the FOT in young children.