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.