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.