Early onset of airway de-recruitment assessed using the forced oscillation technique in subjects with asthma.

De-recruitment of airspaces in the lung occurs when airways close during exhalation, and is related to ventilation heterogeneity and symptoms in asthma. The forced oscillation technique has been used to identify surrogate measures of airway closure via the reactance (Xrs) versus lung volume relationship. This study, used a new algorithm to identify de-recruitment from the Xrs versus lung volume relationship from a slow vital capacity maneuver. We aimed to compare two de-recruitment markers on the Xrs versus volume curve; the onset reduction of Xrs (DR1vol ) and onset of more rapid reduction of Xrs (DR2vol ), between control and asthmatic subjects. We hypothesised that the onset of DR1vol and DR2vol occurred at higher lung volume in asthmatic subjects. DR1vol and DR2vol were measured in 18 subjects with asthma and 18 healthy controls, and their relationships with age and height were examined using linear regression. In the control group, DR1vol and DR2vol increased with age (r2 =0.68, p<0.001 and r2 =0.71, p<0.001, respectively). DR1vol and DR2vol in subjects with asthma (76.58% of total lung capacity (TLC) and 56.79%TLC respectively) were at higher lung volume compared to control subjects (46.1%TLC and 37.69%TLC, respectively) (p<0.001). DR2vol correlated with predicted values of closing capacity (r=0.94, p<0.001). This study demonstrates that de-recruitment occurs at two points along the Xrs-volume relationship. Both de-recruitment points occurred at significantly higher lung volumes in subjects with asthma compared to healthy control subjects. This technique offers a novel way to measure the effects of changes in airways/lung mechanics.