Discussion
In this moderate to severe asthmatic schoolchildren, RI was found in a
high proportion of the sample (70%). This percentage is similar that we
found in a previously published study with high number of participants
(2). Results of both studies are concordant and confirm that RI is
frequent in schoolchildren with moderate to severe asthma and represents
a real peripheral airway condition and not a technical artefact.
Interestingly, there were asthmatic (group 2) in whom RI disappeared
after the bronchodilator test, indicating that in these cases it is due
to reversible bronchial obstruction; while in others asthmatic (40% of
the patients) the RI persisted after the bronchodilator test (group 3).
These last group of patients could have a compromised peripheral airway
function for different causes, such as increased inflammation,
remodeling, or diminished lung function present from birth.
There were also another differences in lung function between these three
groups. Patients in group 1 with no prematurity or low birth weight
record, had better values in spirometry and in IOS parameters reflecting
the peripheral airway function even in X5c. In contrast, lung function
was lower in groups 2 and 3. Resistance was increased and the parameters
reflecting peripheral airway obstruction were higher in children from
groups 2 and 3 than in group 1. Group 3 had highest total airway
resistance (R5) and greater small airway compromise (X5c and DR5-R20)
than group 2. Besides, group 3 had a lower RBD average in X5c and
DR5-R20 than in group 2, which could indicate that group 3 corresponds
to asthmatics with a narrow and/or a more collapsible airway associated
with prematurity that it persisted until school age.
It has been described that the increase in expiratory resistance and
reactance compared to inspiratory may be useful to differentiate
asthmatic children from healthy ones or premature children from those
born at term and reflects the narrowing of the airways during expiration
(5,6). In real life, both conditions may or may not coexist, and in this
situation the evaluation of the clinical history, and spirometry becomes
important. In the study by Tsukahara et al. (1), the clinical history
was not reported, nor spirometry was performed; and an alternative
explanation for their finding could be that children with RI were also
asthmatic.
Finally, it is important to highlight that in the lung, not only a
series resonant structure, but a parallel resonant structure is
incorporated. The parallel resonant effect exists in every subject. In
children with obstructive disease, the accessed lung chamber becomes so
small that a superimposed parallel resonance is seen. The greater the
degree of obstruction, the smaller the pulmonary chamber accessed and
the higher the specific parallel resonance frequency. At low frequencies
(5 Hz), this will cause the RI phenomenon. In young patients with
moderate to severe asthma, obstruction and small peripheral airway may
explain why RI occurs. In these cases, the X5 reactance is no longer
representative; and the use of ”X5 c” would be the most indicated (2).
This was also seen in the present study where the basal alterations and
bronchodilator response were significant with X5c and not with X5Hz.