Relating small airways to asthma control by using impulse oscillometry in children.

BACKGROUND Previous reports suggest that the peripheral airways are associated with asthma control. Patient history, although subjective, is used largely to assess asthma control in children because spirometric results are many times normal values. Impulse oscillometry (IOS) is an objective and noninvasive measurement of lung function that has the potential to examine independently both small- and large-airway obstruction. OBJECTIVE We sought to determine the utility of IOS in assessing asthma control in children. METHODS Asthmatic and healthy children (6-17 years) were enrolled in the study. Spirometric and IOS (resistance of the respiratory system at 5 Hz [R5] and 20 Hz [R20], reactance of the respiratory system at 5 Hz [X5], resonant frequency of reactance [Fres], and area under the reactance curve between 5 Hz and Fres [reactance area {AX}]) values were collected in triplicate before and after a bronchodilator was administered. The physicians were blinded to the IOS measurements and assessed asthma control using American Thoracic Society guidelines. RESULTS Small-airway IOS measurements, including the difference of R5 and R20 [R5-20], X5, Fres, and AX, of children with uncontrolled asthma (n = 44) were significantly different from those of children with controlled asthma (n = 57) and healthy children (n = 14), especially before the administration of a bronchodilator. However, there was no difference in large-airway IOS values (R20). No differences were found between children with controlled asthma and healthy children in any of the end points. Receiver operating characteristic analysis showed cut points for baseline R5-20 (1.5 cm H(2)O · L(-1) · s) and AX (9.5 cm H(2)O · L(-1)) that effectively discriminated controlled versus uncontrolled asthma (area under the curve, 0.86 and 0.84) and correctly classified more than 80% of the population. CONCLUSION Uncontrolled asthma is associated with small-airways dysfunction, and IOS might be a reliable and noninvasive method to assess asthma control in children.