Input oscillometry and the forced oscillation technique for assessing lung function in preschool children with asthma

Preschool children with asthma present a challenge in lung function testing, as they cannot readily cooperate with spirometry. The forced oscillation technique (FOT) measures passive pressures and flows delivered by a loudspeaker to a facemask, at frequencies much higher than those occurring physiologically. This in turn allows for rapid collection of data from a spontaneously breathing child in a timespan of seconds. However, at very rapid oscillatory flow rates, the mechanical properties opposing flows into and out of the respiratory system (collectively termed the respiratory system impedance, and comprised of elastic, resistive and inertial components) are very different from at normal breathing frequencies, with elastic properties being less important and inertial properties being more important. An understanding of how the respiratory system behaves at high frequencies is essential to understanding the physiological basis of this technique. This article presents a way to understand these oscillatory mechanics of the respiratory system. It then describes studies of the FOT in normal preschool children and in children with asthma. The technique can also measure the separate contributions of the central and peripheral airways, as well as assess for changes after bronchodilator administration. The FOT holds promise for the objective measurement of lung function in children who are too young to reliably perform spirometry.