Introduction
Asthma is one of the most prevalent chronic diseases in childhood,1 characterized by inflammation and obstruction in the small airways.2,3 This alteration in the small airways correlates with bronchial hyperreactivity, risk of exacerbations, incomplete response to corticosteroids, and persist asthma symptoms.4-6 Small-airway dysfunction could also be present in asthmatic patients with apparently good control of the disease; however, it is more common in severe asthma.7 Therefore, identifying small-airway alterations allows better characterization and management of asthmatic patients.8
Impulse oscillometry (IOS) has become relevant for lung function evaluation, because several studies agree that it can assess small-airway dysfunction and detect abnormalities in patients even when spirometry is normal.9-12 IOS measures the respiratory system resistance (R) and the reactance (X). Graphic representation of these elements allows building the resistance line between R5 (resistance of all airways) and R20 (resistance of the central airway) and the reactance line from reactance at 5 Hz [X5] to frequency of reactance [Fres], where the reactance is equal to cero. The following parameters, X5, Fres, reactance area (AX), and the difference between R5 and R20 (D5-D20) reflect changes in the obstruction of peripheral airways.
Usually, the reactance line should decrease hyperbolically as frequency decreases, from F Res to X5, but in some patients at low frequency the curve is inverted upward, tracing a curved trajectory. This is call “reactance inversion”. The reactance becomes less negative as the frequency decreases. In this case, X5 should be more negative. The solution for correcting this problem is to calculate the value of X5 that corresponds by projecting that line without the reactance inversion, generating a new parameter, “approximate X5” (X5 approx.), whose values are more negative than X5. This new parameter, X5 approx., is incorporated into some equipment, such as that of Jaeger-Carefusion, which provides X5 and X5 approx. These two values are equal when there is no inversion of the reactance curve. We postulate that X5 approx. correctly represents changes in the respiratory tract. Therefore, it should correlate better than X5 with alterations in the other IOS and spirometry parameters (i.e. R5, Fres, AX, D5-20, and FEF 25-75) that evaluate the function of small airways.
The aim of this study is to determine the prevalence of an inversion of the reactance curve in asthmatic children and whether X5 approx. more closely correlates with other IOS and spirometry parameters for small-airway evaluation than does X5.